The Project Gutenberg EBook of The Economic Aspect of Geology, by C. K. Leith This eBook is for the use of anyone anywhere at no cost and with almost no restrictions whatsoever. You may copy it, give it away or re-use it under the terms of the Project Gutenberg License included with this eBook or online at www.gutenberg.org Title: The Economic Aspect of Geology Author: C. K. Leith Release Date: January 19, 2009 [EBook #27842] Language: English Character set encoding: ISO-8859-1 *** START OF THIS PROJECT GUTENBERG EBOOK THE ECONOMIC ASPECT OF GEOLOGY *** Produced by Kevin Handy, Barbara Kosker, John Hagerson, Chrome and the Online Distributed Proofreading Team at http://www.pgdp.net THE ECONOMIC ASPECTS OF GEOLOGY C. K. LEITH UNIVERSITY OF WISCONSIN [Illustration] NEW YORK HENRY HOLT AND COMPANY COPYRIGHT, 1921 BY HENRY HOLT AND COMPANY _August, 1923_ PRINTED IN THE U. S. A. CONTENTS PAGE CHAPTER I. INTRODUCTION 1 SURVEY OF FIELD 1 ECONOMIC APPLICATIONS OF THE SEVERAL BRANCHES OF GEOLOGY AND OF OTHER SCIENCES 3 Mineralogy and petrology 3 Stratigraphy and paleontology 4 Structural geology 5 Physiography 6 Rock alterations or metamorphism 10 Application of other sciences 10 TREATMENT OF THE SUBJECT IN THIS VOLUME 11 CHAPTER II. THE COMMON ELEMENTS, MINERALS, AND ROCKS OF THE EARTH AND THEIR ORIGINS 13 RELATIVE ABUNDANCE OF THE PRINCIPAL ELEMENTS OF THE LITHOSPHERE 13 RELATIVE ABUNDANCE OF THE PRINCIPAL MINERALS OF THE LITHOSPHERE 14 RELATIVE ABUNDANCE OF THE PRINCIPAL ROCKS OF THE LITHOSPHERE 16 WATER (HYDROSPHERE) 18 SOILS AND CLAYS 18 COMPARISON OF LISTS OF MOST ABUNDANT ROCKS AND MINERALS WITH COMMERCIAL ROCKS AND MINERALS 18 THE ORIGIN OF COMMON ROCKS AND MINERALS 18 Igneous processes 19 Igneous after-effects 19 Weathering of igneous rocks and veins 20 Sedimentary processes 22 Weathering of sedimentary rocks 23 Consolidation, cementation, and other sub-surface alterations of rocks 24 Cementation 24 Dynamic and contact metamorphism 25 THE METAMORPHIC CYCLE AS AN AID IN STUDYING MINERAL DEPOSITS 27 CHAPTER III. SOME SALIENT FEATURES OF THE GEOLOGY AND CLASSIFICATION OF MINERAL DEPOSITS 29 VARIOUS METHODS OF CLASSIFICATION 29 NAMES 31 MINERAL DEPOSITS AS MAGMATIC SEGREGATIONS IN IGNEOUS ROCKS 34 MINERAL DEPOSITS WITHIN AND ADJACENT TO IGNEOUS ROCKS, WHICH WERE FORMED IMMEDIATELY AFTER THE COOLING AND CRYSTALLIZATION OF THE MAGMAS THROUGH THE AGENCY OF HOT MAGMATIC SOLUTIONS 36 Evidence of igneous source 37 Possible influence of meteoric waters in deposition of ores of this class 41 Zonal arrangement of minerals related to igneous rocks 42 The relation of contact metamorphism to ore bodies of the foregoing class 45 SECONDARY CONCENTRATION IN PLACE OF THE FOREGOING CLASSES OF MINERAL DEPOSITS THROUGH THE AGENCY OF SURFACE SOLUTIONS 46 RESIDUAL MINERAL DEPOSITS FORMED BY THE WEATHERING OF IGNEOUS ROCKS IN PLACE 50 MINERAL DEPOSITS FORMED DIRECTLY AS PLACERS AND SEDIMENTS 51 Mechanically deposited minerals 51 Chemically and organically deposited minerals 52 SEDIMENTARY MINERAL DEPOSITS WHICH HAVE REQUIRED FURTHER CONCENTRATION TO MAKE THEM COMMERCIALLY AVAILABLE 54 ANAMORPHISM OF MINERAL DEPOSITS 57 CONCLUSION 58 CHAPTER IV. MINERAL RESOURCES--SOME GENERAL QUANTITATIVE CONSIDERATIONS 60 WORLD ANNUAL PRODUCTION OF MINERALS IN SHORT TONS 60 WORLD ANNUAL PRODUCTION OF MINERALS IN TERMS OF VALUE 62 SIGNIFICANCE OF GEOGRAPHIC DISTRIBUTION OF MINERAL PRODUCTION 63 THE INCREASING RATE OF PRODUCTION 63 CAPITAL VALUE OF WORLD MINERAL RESERVES 64 POLITICAL AND COMMERCIAL CONTROL OF MINERAL RESOURCES 65 RESERVES OF MINERAL RESOURCES 65 CHAPTER V. WATER AS A MINERAL RESOURCE 67 GENERAL GEOLOGIC RELATIONS 67 DISTRIBUTION OF UNDERGROUND WATER 68 MOVEMENT OF UNDERGROUND WATER 71 WELLS AND SPRINGS 72 COMPOSITION OF UNDERGROUND WATERS 73 RELATION OF GEOLOGY TO UNDERGROUND WATER SUPPLY 75 SURFACE WATER SUPPLIES 76 UNDERGROUND AND SURFACE WATERS IN RELATION TO EXCAVATION AND CONSTRUCTION 78 CHAPTER VI. THE COMMON ROCKS AND SOILS AS MINERAL RESOURCES 80 ECONOMIC FEATURES OF THE COMMON ROCKS 80 Granite 82 Basalt and related types 82 Limestone, marl, chalk 82 Marble 83 Sand, sandstone, quartzite (and quartz) 84 "Sand and gravel" 84 Clay, shale, slate 85 The feldspars 86 Hydraulic cement (including Portland, natural, and Puzzolan cements) 86 GEOLOGIC FEATURES OF THE COMMON ROCKS 88 Building stone 88 Crushed stone 90 Stone for metallurgical purposes 91 Clay 91 Limitations of geologic field in commercial investigation of common rocks 92 SOILS AS A MINERAL RESOURCE 94 Origin of soils 94 Composition of soils and plant growth 96 Use of geology in soil study 97 CHAPTER VII. THE FERTILIZER GROUP OF MINERALS 99 GENERAL COMMENTS 99 NITRATES 101 Economic features 101 Geologic features 102 PHOSPHATES 104 Economic features 104 Geologic features 105 PYRITE 107 Economic features 107 Geologic features 108 SULPHUR 109 Economic features 109 Geologic features 110 POTASH 111 Economic features 111 Geologic features 112 CHAPTER VIII. THE ENERGY RESOURCES--COAL, OIL, GAS (AND ASPHALT) 115 COAL 115 ECONOMIC FEATURES 115 World production and trade 115 Production in the United States 117 Coke 118 Classification of coals 119 GEOLOGIC FEATURES 123 PETROLEUM 127 ECONOMIC FEATURES 127 Production and reserves 128 Methods of estimating reserves 134 Classes of oils 136 Conservation of oil 137 GEOLOGIC FEATURES 140 Organic theory of origin 140 Effect of differential pressures and folding on oil genesis and migration 142 Inorganic theory of origin 143 Oil exploration 144 OIL SHALES 150 NATURAL GAS 151 Economic features 151 Geologic features 151 ASPHALT AND BITUMEN 151 Economic features 151 Geologic features 153 CHAPTER IX. MINERALS USED IN THE PRODUCTION OF IRON AND STEEL (THE FERRO-ALLOY GROUP) 154 GENERAL FEATURES 154 IRON ORES 158 ECONOMIC FEATURES 158 Technical and commercial factors determining use of iron ore materials 158 Geographic distribution of iron ore production 160 World reserves and future production of iron ore 162 GEOLOGIC FEATURES 166 Sedimentary iron ores 166 Iron ores associated with igneous rocks 171 Iron ores due to weathering of igneous rocks 171 Iron ores due to weathering of sulphide ores 173 MANGANESE ORES 173 Economic features 173 Geologic features 176 CHROME (OR CHROMITE) ORES 178 Economic features 178 Geologic features 179 NICKEL ORES 180 Economic features 180 Geologic features 181 TUNGSTEN (WOLFRAM) ORES 182 Economic features 182 Geologic features 184 MOLYBDENUM ORES 185 Economic features 185 Geologic features 186 VANADIUM ORES 187 Economic features 187 Geologic features 188 ZIRCONIUM ORES 189 Economic features 189 Geologic features 189 TITANIUM ORES 190 Economic features 190 Geologic features 190 MAGNESITE 191 Economic features 191 Geologic features 192 FLUORSPAR 193 Economic features 193 Geologic features 194 SILICA 195 Economic features 195 Geologic features 196 CHAPTER X. COPPER, LEAD AND ZINC MINERALS 197 COPPER ORES 197 Economic features 197 Geologic features 199 Copper deposits associated with igneous flows 200 Copper veins in igneous rocks 201 "Porphyry coppers" 203 Copper in limestone near igneous contacts 204 Copper deposits in schists 204 Sedimentary copper deposits 205 General comments 206 LEAD ORES 209 Economic features 209 Geologic features 211 ZINC ORES 213 Economic features 213 Geologic features 216 CHAPTER XI. GOLD, SILVER, AND PLATINUM MINERALS 221 GOLD ORES 221 Economic features 221 Geologic features 226 SILVER ORES 231 Economic features 231 Geologic features 234 PLATINUM ORES 237 Economic features 237 Geologic features 239 CHAPTER XII. MISCELLANEOUS METALLIC MINERALS 241 ALUMINUM ORES 241 Economic features 241 Geologic features 243 ANTIMONY ORES 246 Economic features 246 Geologic features 248 ARSENIC ORES 249 Economic features 249 Geologic features 251 BISMUTH ORES 252 Economic features 252 Geologic features 252 CADMIUM ORES 253 Economic features 253 Geologic features 254 COBALT ORES 254 Economic features 254 Geologic features 255 MERCURY (QUICKSILVER) ORES 255 Economic features 255 Geologic features 258 TIN ORES 260 Economic features 260 Geologic features 261 URANIUM AND RADIUM ORES 263 Economic features 263 Geologic features 264 CHAPTER XIII. MISCELLANEOUS NON-METALLIC MINERALS 267 NATURAL ABRASIVES 267 Economic features 267 Geologic features 269 ASBESTOS 270 Economic features 270 Geologic features 271 BARITE (BARYTES) 272 Economic features 272 Geologic features 273 BORAX 274 Economic features 274 Geologic features 275 BROMINE 277 Economic features 277 Geologic features 278 FULLER'S EARTH 278 Economic features 278 Geologic features 279 GRAPHITE (PLUMBAGO) 279 Economic features 279 Geologic features 282 GYPSUM 283 Economic features 283 Geologic features 284 MICA 285 Economic features 285 Geologic features 287 MONAZITE (THORIUM AND CERIUM ORES) 288 Economic features 288 Geologic features 289 PRECIOUS STONES 289 Economic features 289 Geologic features 291 SALT 294 Economic features 294 Geologic features 295 TALC AND SOAPSTONE 299 Economic features 299 Geologic features 299 CHAPTER XIV. EXPLORATION AND DEVELOPMENT 301 THE GENERAL RELATIONS OF THE GEOLOGIST TO EXPLORATION AND DEVELOPMENT 301 PARTLY EXPLORED VERSUS VIRGIN TERRITORIES 303 THE USE OF ALL AVAILABLE INFORMATION 304 COÖPERATION IN EXPLORATION 305 ECONOMIC FACTORS IN EXPLORATION 306 GEOLOGIC FACTORS IN EXPLORATION 307 MINERAL PROVINCES AND EPOCHS 308 CLASSIFICATION OF MINERAL LANDS 309 OUTCROPS OF MINERAL DEPOSITS 311 Some illustrative cases 312 Topography and climate as aids in searching for mineral outcrops 314 Size and depth of ore bodies as determined from outcrops 315 The use of placers in tracing mineral outcrops 316 The use of magnetic surveys in tracing mineral ledges 317 THE USE OF ELECTRICAL CONDUCTIVITY AND OTHER QUALITIES OF ROCKS IN EXPLORATION 319 THE USE OF STRUCTURE AND METAMORPHISM IN EXPLORATION 319 DRILLING IN EXPLORATION 320 QUANTITATIVE ASPECTS OF GEOLOGIC EXPLORATION 321 ORIGIN OF MINERAL DEPOSITS AS A FACTOR IN EXPLORATION 322 LAKE SUPERIOR IRON ORE EXPLORATION AS AN ILLUSTRATION 323 DEVELOPMENT AND EXPLOITATION OF MINERAL DEPOSITS 326 CHAPTER XV. VALUATION AND TAXATION OF MINERAL RESOURCES 328 POPULAR CONCEPTION OF MINERAL VALUATION 328 VALUATION AND TAXATION OF MINES 329 Intrinsic and extrinsic factors in valuation 329 Values of mineral deposits not often established by market transfers 331 The ad valorem method of valuation 331 Other methods of mineral valuation and taxation 335 GENERAL COMMENTS ON TAXATION OF MINERAL RESOURCES 338 CHAPTER XVI. LAWS RELATING TO MINERAL RESOURCES 342 I. LAWS RELATING TO OWNERSHIP AND CONTROL OF MINERAL RESOURCES 342 On alienated lands 343 On the public domain 344 Nationalization of mineral resources 345 Effect of ownership laws on exploration 347 Use of geology in relation to ownership laws 349 II. LAWS RELATING TO EXTRACTION OF MINERAL RESOURCES 355 III. LAWS RELATING TO DISTRIBUTION AND TRANSPORTATION OF MINERAL RESOURCES 355 IV. OTHER RELATIONS OF GEOLOGY TO LAW 356 CHAPTER XVII. CONSERVATION OF MINERAL RESOURCES 359 THE PROBLEM 359 DIFFERENCES BETWEEN PRIVATE AND PUBLIC EFFORTS IN CONSERVATION 363 THE INTEREST RATE AS A GUIDE IN CONSERVATION 364 ANTI-CONSERVATIONAL EFFECTS OF WAR 365 CONSERVATION OF COAL 366 MEASURES INTRODUCED OR PROPOSED TO CONSERVE COAL 367 (A) Mining and preparation of coal 368 Progress in above methods 370 (B) Improvement of labor and living conditions at the mines 372 (C) Introduction or modification of laws to regulate or to remove certain restrictions on the coal industry 373 (D) Distribution and transportation of coal 376 (E) Utilization of coal 377 (F) Substitutes for coal as a source of power 378 DIVISION OF RESPONSIBILITY BETWEEN GOVERNMENT AND PRIVATE INTERESTS IN THE CONSERVATION OF COAL 379 CONSERVATION OF MINERALS OTHER THAN COAL 382 CHAPTER XVIII. INTERNATIONAL ASPECTS OF MINERAL RESOURCES 383 WORLD MOVEMENT OF MINERALS 383 Movement of minerals under pre-war conditions of international trade 385 Changes during the war 385 Post-war condition of the mineral trade 387 TENDENCIES TOWARD INTERNATIONAL COÖPERATION AND POSSIBILITY OF INTERNATIONAL CONTROL OF MINERALS 389 Methods of international coöperation 391 CONSERVATION IN ITS INTERNATIONAL RELATIONS 393 EXPLORATION IN ITS INTERNATIONAL RELATIONS 395 VALUATION IN ITS INTERNATIONAL RELATIONS 396 RELATIVE POSITION OF THE UNITED STATES IN REGARD TO SUPPLIES OF MINERALS 396 THE COAL AND IRON SITUATION OF WESTERN EUROPE UNDER THE TERMS OF THE PEACE 400 CONCLUSION 403 LITERATURE 403 CHAPTER XIX. GEOLOGY AND WAR 405 GEOLOGY BEHIND THE FRONT 405 GEOLOGY AT THE FRONT 408 EFFECT OF THE WAR ON THE SCIENCE OF ECONOMIC GEOLOGY 412 CHAPTER XX. GEOLOGY AND ENGINEERING CONSTRUCTION 413 FOUNDATIONS 413 SURFACE WATERS 414 TUNNELS 414 SLIDES 415 SUBSIDENCE 417 RAILWAY BUILDING 417 ROAD BUILDING 418 GEOLOGY IN ENGINEERING COURSES 419 CHAPTER XXI. THE TRAINING, OPPORTUNITIES AND ETHICS OF THE ECONOMIC GEOLOGIST 420 PURE VERSUS APPLIED SCIENCE 420 COURSE OF STUDY SUGGESTED 422 Field work 425 Specialization in studies 426 A degree of Economic Geology 427 THE OPPORTUNITIES OF THE ECONOMIC GEOLOGIST 428 ETHICS OF THE ECONOMIC GEOLOGIST 430 ILLUSTRATIONS FIGURE PAGE 1. Graphic representation of volume change in weathering of a Georgia granite 21 2. Commercial (financial) control of the mineral resources of the world 64 3. Political (territorial) control of the mineral resources of the world 64 4. The fertilizer situation in the United States 100 5. Diagram showing the chemical composition and heat efficiency of the several ranks of coal 122 6. Origin and development of coal 123 7. Chart showing the present tendency of the United States in respect to its unmined reserve of petroleum 134 8. The annual output of the principal oil fields of the United States for the last twenty years 135 9. Curve showing the usual decline in oil field production after the period of maximum output is reached 136 10. Chart showing the relative values of the principal petroleum products manufactured in the United States from 1899 to 1914 138 11. Alteration of Lake Superior iron formation to iron ore by the leaching of silica 168 12. Representing in terms of weight the mineralogical changes in the katamorphism of serpentine rocks to iron ore, eastern Cuba 172 13. Diagram showing gradation from syenite to bauxite in terms of volume 245 CHAPTER I INTRODUCTION SURVEY OF FIELD In adapting ourselves to physical environment it has been necessary to learn something about the earth. Mainly within the last century has this knowledge been organized into the science of geology, and only within the last few decades have the complex and increasing demands of modern civilization required the applications of geology to practical uses, resulting in the development of the science generally known as _economic geology_. This science is not sharply marked off from the science of geology proper; almost any phase of geology may at some time or some place take on its economic aspect. The usefulness of economic geology was first recognized in relation to mineral resources,--and particularly in relation to metallic resources, their discovery and development,--but the science has been found to have much wider practical application. The practice of the economic geologist in recent years has taken on many new phases. The geologist is called upon to study the geologic features of mineral deposits, their occurrence, structure, and origin. The basic information thus acquired is useful in estimating reserves and life of mineral deposits. This leads naturally to considerations of valuation. Because valuation plays such a large part in any tax program, the geologist is being used by tax boards of the federal and state governments. Both in the formulation of laws relating to mineral resources, and in the litigation growing out of the infraction of these laws, the economic geologist plays a part. One cannot go very far with the study of mineral resources without consideration of the question of conservation. Geologists are called on not only for broad surveys of the mineral reserves, but for the formulation of general principles of conservation and their application to specific mines and minerals. The geologist's familiarity with the distribution and nature of mineral resources has given him a part in coping with broad questions of international use of natural resources. War conditions made it necessary to use new sources of supply, new channels of distribution, and new methods of utilization. The economic geologist came into touch with questions of international trade, tariffs, and shipping. But economic geology is not solely confined to mineral resources. In relation to engineering enterprises of the greatest variety--canals, aqueducts, tunnels, dams, building excavations, foundations, etc.--geology now figures largely, both in war and in peace. The nature, amount, and distribution of underground water supplies are so involved with geologic considerations that a considerable number of geologists give up their time wholly to this phase of the subject. It might seem from this list of activities that geology is spreading too far into the fields of engineering and commerce, but there are equally rapid extensions of other fields of knowledge toward geology. The organization of these intermediate fields is required both in the interest of science and in the interest of better adaptation of the race to its environment. The geologist is required to do his part in these new fields, but not to abandon his traditional field. It is proposed in this volume to discuss the economic aspects of geology without exhaustive discussion of the principles of geology which are involved. Practically the whole range of geologic science has some sort of economic application, and it would be futile to attempt in one volume even a survey of the science of geology as a whole. Our purpose is rather to indicate and illustrate, in some perspective, the general nature of the application of geology to practical affairs. In professional preparation for the practice of economic geology there is no easy short-cut. Students sometimes think that a smattering of geological principles, combined with a little business and economic information, may be sufficient. Analysis of professional successes should make it clear that economic geologists are most effective and in most demand, not primarily because of business aptitude, though this helps, but because of their proficiency in the science of geology itself. In short, to enter successfully the field of economic geology one should first become a scientist, if only in a limited field. The traditional conception of the geologist as a musty and stooped individual, with a bag, hammer, and magnifying glass, collecting specimens to deposit in a dusty museum, will doubtless survive as a caricature, but will hardly serve to identify the economic geologist in his present-day work. In writing this book, it is hoped in some measure to convey an impression of the breadth and variety in this field. Few other sciences offer so wide a range of opportunity, from the purely scientific to the practical and commercial, coupled with travel, exploration, and even adventure. ECONOMIC APPLICATIONS OF THE SEVERAL BRANCHES OF GEOLOGY AND OF OTHER SCIENCES There is no phase of geology which at some time or place does not have its economic application. Many references to these applications are made in other chapters. It is proposed here to indicate briefly some of the phases of geologic science which are most necessary to the practice of economic geology. The student in his preparation cannot afford to eliminate any of them on the ground that they are merely "scientific" or "academic" or "theoretical." MINERALOGY AND PETROLOGY Mineralogy, the study of minerals, and petrology, the study of rocks (aggregations of minerals), are of course elementary requisites in preparation. There must be familiarity with the principal minerals and rocks, and especially with the methods and processes of their identification, with their nature, and with their origin. This involves a study of their crystallography, chemical composition, physical qualities, and optical properties as studied with the microscope. In recent years the microscopical study of polished and etched surfaces of ores has proved a valuable tool. STRATIGRAPHY AND PALEONTOLOGY Stratigraphy and paleontology are concerned with the sedimentary and life history of the earth. The determination of the ages of the earth's strata and of the conditions of their deposition is required in the practice of economic geology. For example, a detailed knowledge of the succession of rocks and their ages, as determined by fossils and other stratigraphic evidence, is vital to the interpretation of conditions in an oil or coal field, and to the successful exploration and development of its deposits. The success of certain paleontologists and stratigraphic specialists in oil exploration is an evidence of this situation. Certain iron ores, phosphates, salts, potash, and other minerals, as well as many of the common rocks used for economic purposes, are found in sedimentary deposits, and require for their successful exploration and development the application of stratigraphic and paleontologic knowledge. Closely related to stratigraphy (as well as to physiography, see pp. 6-10) is the study of sedimentation,--_i. e._, the study of the physical, chemical, climatic, and topographic conditions of the deposition of sediments. This is coming to play an increasingly large part in geologic work, and is essential to the interpretation of many mineral deposits, particularly those in which stratigraphic and physiographic questions are involved. Still another aspect of the problem of stratigraphy and sedimentation is covered by the study of _paleogeography_, or the areal distribution of the faunas and sediments of geologic periods caused by the alternating submergence and emergence of land areas. In the search for the treasures of sedimentary deposits, a knowledge of ancient geographies and of ancient faunas makes it possible to eliminate certain regions from consideration. From a study of the faunas of eastern Kansas and Missouri, and of those along the eastern part of the Rocky Mountains, it has been inferred that a ridge must have extended across eastern Kansas during early Pennsylvanian time,--a conclusion which is of considerable economic importance in relation to oil exploration. STRUCTURAL GEOLOGY Structural geology is the study of the physical forms and relations of rocks which result mainly from deformation by earth forces. If rocks remained in their original forms the structural problem would be a comparatively easy one, but usually they do not. Often they are faulted and folded and mashed to such an extent that it is difficult to go behind the superposed structural features to the original conditions in order to work out the geologic history. Not only is structural study necessary for the interpretation of geologic history, but it is often more directly applicable to economic problems,--as when, for instance, ore deposits have been formed in the cracks and joints of rocks, and the ore deposits themselves have been faulted and folded. Water resources are often located in the cracks and other openings of rocks, and are limited in their distribution and flow because of the complex attitude of deformed rocks. Oil and gas deposits often bear a well-defined relation to structural features, the working out of which is almost essential to their discovery. It is not desirable to stop with the merely descriptive aspects of structural geology, as is so often done; for much light can be thrown on the economic applications of this subject by consideration of the underlying principles of mechanics,--involving the relations of earth stresses to rock structures. The mere field mapping and description of faults and joints is useful, but in some cases it is necessary to go a step further and to ascertain the mechanical conditions of their origin in order to interpret them clearly. If, for illustration, there are successive groups of mineralized veins in a mining camp, the later ones cutting the earlier ones, these might be treated as separate structural units. But if it can be shown that the several sets of veins have formed from a single movement, that there is no sharp genetic separation between the different sets and that they are a part of a single system, this interpretation throws new light on exploration and development, and even on questions of ownership and extralateral rights (Chapter XVI). PHYSIOGRAPHY Physiography is a phase of geology which investigates the surface features of the earth. It has to do not only with the description and classification of surface forms, present and past (physical geography or geomorphology), but with the processes and history of their development. The subject is closely related to geography, climatology, sedimentation, and hydrology. As one of the latest phases of geology to be organized and taught, its economic applications have been comparatively recent and are not yet widely recognized. Because of this fact its economic applications may be summarized at somewhat greater length than those of the other branches of geology above mentioned, which are to be more or less taken for granted. The central feature of physiography is the so-called erosion cycle or topographic cycle. Erosion, acting through the agencies of wind, water, and ice, is constantly at work on the earth's surface; the eroded materials are in large part carried off by streams, ultimately to be deposited in the ocean near the continental margins. The final result is the reduction of the land surface to an approximate plain, called a _peneplain_, somewhere near sea level. Geological history shows that such peneplains are often elevated again with reference to sea level, by earth forces or by subsidence of the sea, when erosion again begins its work,--first cutting narrow, steep gulches and valleys, and leaving broad intervening uplands, in which condition the erosion surface is described as that of _topographic youth_; then forming wider and more extensive valleys, leaving only points and ridges of the original peneplains, in which stage the surface is said to represent _topographic maturity_; then rounding off and reducing the elevations, leaving few or none of the original points on the peneplain, widening the valleys still further and tending to reduce the whole country to a nearly flat surface, resulting in the condition of _topographic old age_. The final stage is again the peneplain. This cycle of events is called the _erosion cycle_ or _topographic cycle_. Uplift may begin again before the surface is reduced to base level; in fact, there is a constant oscillation and contest between erosion and relative uplift of the land surface. The action of the erosion cycle on rocks of differing resistance to erosion and of diverse structure gives rise to the great variety of surface forms. The physiographer sees these forms, not as heterogeneous units, but as parts of a definite system and as stages in an orderly series of events. He is able to see into the topographic conditions beyond the range of immediate and direct observation. He is able to determine what these forms were in the past and to predict their condition in the future. He is able to read from the topography the underground structure which has determined that topography. A given structure may in different stages of topographic development give quite diverse topographic forms. In such a case it is important to realize that the diversity is only superficial. On the other hand, a slight local divergence from the usual topographic forms in a given region may reflect a similar local divergence in the underground structure. Thus it is that an appreciation of the physiographic details may suggest important variations in the underground structure which would otherwise pass undiscovered. Many mineral deposits owe their origin or enrichment to weathering and other related processes which are preliminary to erosion. These processes vary in intensity, distribution, and depth, with the stage of erosion, or in relation to the phase of the erosion cycle. They vary with the climatic conditions which obtain on the erosion surface. Mineral deposits are therefore often closely related to the topographic features, present and past, in kind, shape, and distribution. A few illustrative cases follow. Many of the great copper deposits of the western United States owe their values to a secondary enrichment through the agency of waters working down from the surface. When this fact of secondary enrichment was discovered, it was naturally assumed that the process was related to the present erosion surface and to present climatic and hydrologic conditions. Certain inferences were drawn, therefore, as to depth and distribution of the enriched ores. This conception, however, proved to be too narrow; for evidences were found in many cases that the copper deposits had been concentrated in previous erosion cycles, and therefore in relation to erosion surfaces, now partly buried, different from the present surface. The importance of this knowledge from an exploring and development standpoint is clear. It has made it possible to find and follow rich ores, far from the present erosion surface, which would otherwise have been disclosed solely by chance. Studies of this kind in the copper camps are yet so recent that much remains to be learned. The economic geologist advising exploration and development in copper ores who does not in the future take physiographic factors into account is likely to go wrong in essential ways, as he has done in some cases in the past. Not only is it necessary to relate the secondary enrichment of copper deposits to the erosion surface, present or past, but by a study of the conditions it must be ascertained how closely erosion has followed after the processes of enrichment. In some cases erosion has followed so slowly as to leave large zones of secondary enrichment. In other cases erosion has followed up so closely after the processes of secondary enrichment as to remove from the surface important parts of the secondarily enriched deposits. The iron ores of the Lake Superior region are the result of the action of waters from the surface on so-called iron formations or jaspers. Here again it was at first supposed that the enrichment was related to the present erosion surface; but upon further studies the fact was disclosed that the concentration of the ores took place in the period between the deposition of Keweenawan and Cambrian rocks, and thus a new light was thrown on the possibilities as to depth and distribution of the ores. The old pre-Cambrian surface, with reference to which the concentration took place, can be followed with some precision beneath the present surface. This makes it possible to forecast a quite different depth and distribution of the ores from that which might be inferred from present surface conditions. Present surface conditions, of low relief, considerable humidity, and with the water table usually not more than 100 feet from the surface, do not promise ore deposits at great depth. The erosion which formed the old pre-Cambrian surface, however, started on a country of great relief and semi-arid climate, conditions which favored deep penetration of the surface waters which concentrated the ores. The iron ores of eastern Cuba are formed by the weathering of a serpentine rock on an elevated plateau of low relief, where the sluggish streams are unable rapidly to carry off the products of weathering. Where streams have cut into this plateau and where the plateau breaks down with sharp slopes to the ocean, erosion has removed the products of weathering, and therefore the iron ore. An important element, then, in iron ore exploration in this country is the location of regions of slight erosion in the serpentine area. One of the largest discoveries was made purely on a topographic basis. It was inferred merely from a study of topography that a certain large unexplored area ought to carry iron ore. Subsequent work in the thick and almost impenetrable jungle disclosed it. Bauxite deposits in several parts of the world require somewhat similar conditions of concentration, and a study of the physiographic features is an important factor in their location and interpretation. A physiographic problem of another sort is the determination of the conditions surrounding the origin of sedimentary ores. Certain mineral deposits, like the "Clinton" iron ores, the copper ores in the "Red Beds" of southwestern United States and in the Mansfield slates of Germany, many salt deposits, and almost the entire group of placer deposits of gold, tin, and other metals, are the result of sedimentation, from waters which derived their materials from the erosion of the land surface. It is sometimes possible from the study of these deposits to discover the position and configuration of the shore line, the depth of water, and the probable continuity and extent of the deposits. Similar questions are met in the study of coal and oil. This general problem is one of the phases of geology which is now receiving a large amount of attention, not only from the standpoint of ore deposition, but from a broader geologic standpoint. In spite of the fact that sedimentary processes of great variety can be observed in operation today, it is yet extremely difficult to infer from a given sedimentary deposit the precise conditions which determined its deposition and limited its distribution. For instance, sedimentary iron formations furnish a large part of the world's iron ore. The surface distribution, the structure, the features of secondary enrichment, are all pretty well understood; likewise the general conditions of sedimentation are reasonably clear,--but the close interpretation of these conditions, to enable us to predict the extent of one of these deposits, or to explain its presence in one place and absence in another, is in an early and sketchy stage. An understanding of the principles and methods of physiography is also vital to an intelligent application of geology to water resources, to soils, to dam and reservoir construction, and to a great variety of engineering undertakings, but as these subjects involve the application of many other phases of geology, they are considered in separate chapters. (Chapters V, VI, and XX.) ROCK ALTERATIONS OR METAMORPHISM This is one of the newer special phases of geology which for a long time was regarded as the plaything of the petrographer or student of rocks. With the systematic development of the subject, however, it was found that the extremely numerous and complex alterations of rocks and minerals may be definitely grouped, and that they are controlled by broad principles. It became apparent also that these principles apply both to the economic and non-economic minerals and rocks,--in other words, that the segregation of economic minerals is a mere incident in pervasive cycles of the alterations which affect all rocks. Metamorphic geology, therefore, for some geologists becomes a convenient approach to the subject of economic geology. It has the great advantage that it tends to keep all minerals and all processes of ore deposition in proper perspective with relation to rocks and rock processes in general. It is not argued that this is the only approach or that it is the best for all purposes. A brief account of this phase of geology is given in Chapter II. APPLICATION OF OTHER SCIENCES Geology is sometimes defined as the application of other sciences to the earth. Considered broadly, there is no phase of science which is not involved in economic geology. In other chapters in this book many references are made to applications of engineering, mathematics, physics, chemistry, metallurgy, biology, and economics. At different times and places the requirements for earth materials are quite different. In the Stone Age there was little use for metals; in later ages the use of metals broadened. The multiplicity of demands of modern civilization, the increasing knowledge of processes of metallurgy, chemistry and physics, better transportation, better organization of commercial life, and many other factors, tend to bring new earth materials into use,--and, therefore, into the field of economic geology. A comparatively few years ago alumina, one of the most common and abundant substances of the earth's crust, was in no general demand except for very limited use as an ornament. Little attention was paid to it by economic geologists as a commercial product; now, however, aluminum is in great demand, and the raw materials which produce it have become the subjects of intensive study by economic geologists. In short, economic geology includes the consideration of man in reaction to his physical environment. There are some earth materials and some conditions of the earth environment which do not yet come within the field of economic geology. But so large a proportion of them do, that the "complete economic geologist" should indeed be almost omniscient. When one considers what an insignificantly small portion of this field can be covered by any individual, it is apparent that the title of economic geologist implies no mastery of the entire field. There is yet no crowding. TREATMENT OF THE SUBJECT IN THIS VOLUME In scope and manner of treatment this volume follows somewhat the writer's presentation of the subject in university teaching. The purpose is to explain the nature of the economic demands for the science of geology, and to discuss something of the philosophy of the finding and use of raw materials. Somewhat generalized statistics are used as a means of gaining perspective. No effort has been made for detailed accuracy or for completeness. So far as possible the quantitative features are expressed in general proportions, and where specific figures are given they are meant to indicate only such general proportions. The thought has been not to be so specific that the figures would soon be out of date. All standard statistical sources have been drawn on, but the principal sources have been the results of the various special investigations called out by the war, in which the writer had a part. On the geologic side many sources have been drawn on outside of the writer's own experience. For the most part, no specific references or acknowledgments are made, on the ground that the book aims to present the general features which are now the more or less common knowledge of economic geologists. To make the references really adequate for exhaustive study would not only burden the text, but would require a specificity of treatment which it has been hoped to avoid. The illustrative cases chosen for discussion are often taken from the writer's field of experience. This field has been principally the Lake Superior region, but has included also the principal mineral deposits of North America, Cuba, and limited areas in South America and Europe. Thus the Lake Superior iron and copper region might seem to be brought forward more than is warranted by its scientific or economic importance. For this, the writer offers no apology. An author's perspective is largely determined by his background of training and experience, and a frank recognition of this fact may aid in determining the weight to be given to his conclusions. It might even add to scientific efficiency if each writer were to confine his discussion almost solely to matters within his own range of observation and study. The writer's indebtedness for information derived from the printed page and for personal discussion and advice is of wide range. He would express his warm appreciation of the friendly spirit of coöperation and advice with which this effort has been aided--a spirit which he likes to think is particularly characteristic of the profession of economic geology. In particular he would acknowledge the efficient aid of Mr. Julian D. Conover in preparation and revision of the manuscript. CHAPTER II THE COMMON ELEMENTS, MINERALS, AND ROCKS OF THE EARTH AND THEIR ORIGINS A list of the solid substances of the earth making up the so-called lithosphere (or rock sphere) in order of their abundance, does not at all correspond to a list made in order of commercial importance. Some of the most valuable substances constitute such a small proportion of the total mass of the lithosphere that they hardly figure at all in a table of the common substances. RELATIVE ABUNDANCE OF THE PRINCIPAL ELEMENTS OF THE LITHOSPHERE When reduced to the simplest terms of elements the outer ten miles of the lithosphere consists of:[1] PERCENTAGE OF PRINCIPAL ELEMENTS IN THE LITHOSPHERE Oxygen 47.33 Silicon 27.74 Aluminum 7.85 Iron 4.50 Calcium 3.47 Magnesium 2.24 Sodium 2.46 Potassium 2.46 ----- 98.05 The remainder of the elements exist in quantities of less than 1 per cent. None of these principal elements occur separately in nature and none of them are mined as elements for economic purposes. RELATIVE ABUNDANCE OF THE PRINCIPAL MINERALS OF THE LITHOSPHERE Minerals exceptionally consist of single elements, but ordinarily are combinations of two or more elements; for instance, quartz consists of a chemical combination of silicon and oxygen. The proportions of the common minerals in the outer ten miles of the lithosphere are in round numbers as follows: PERCENTAGE OF COMMON MINERALS IN LITHOSPHERE Feldspar 49 Quartz 21 Augite, hornblende, and olivine 15 Mica 8 Magnetite 3 Titanite and ilmenite 1 Kaolin, limonite, hematite, dolomite, calcite, chlorite, etc. 3 --- 100 In making up this table it is assumed that the rocks to a depth of ten miles are about 95 per cent of igneous type, that is, crystallized from molten magma, and about 5 per cent of sedimentary type, that is, formed from the weathering and erosion of igneous rocks or preëxisting sediments, and deposited in beds or layers, either by water or by air (see pp. 16-17). More reliable figures for the relative abundance of the minerals are available for each of the two classes of rocks, igneous and sedimentary. The igneous rocks contain minerals in about the following proportions: PERCENTAGE OF COMMON MINERALS IN IGNEOUS ROCKS Feldspar 50 Quartz 21 Augite, hornblende, olivine, etc. 17 Mica 8 Magnetite 3 Titanite and ilmenite 1 --- 100 The sedimentary rocks contain minerals in about the following proportions: PERCENTAGE OF COMMON MINERALS IN SEDIMENTARY ROCKS Quartz 35 Feldspar 16 White mica 15 Kaolin (clay) 9 Dolomite 9 Chlorite 5 Calcite 4 Limonite 4 Gypsum, carbon, rutile, apatite, magnetite, etc. 3 --- 100 The sedimentary rocks comprise three main divisions: (1) The muds and clays, with their altered equivalents, shale, slate, etc.; (2) the sands, with their altered equivalents, sandstone, quartzite, quartz-schist, etc.; (3) the marls, limestones, and dolomites, with their altered equivalents, marble, talc-schist, etc. For brevity these groups are referred to respectively as shale, sandstone, and limestone. The proportions of minerals in each of these groups of rocks are as follows: PERCENTAGE OF COMMON MINERALS IN SHALE, SANDSTONE, AND LIMESTONE -----------+----------+-----------+----------- | Average | Average | Average | shale | sandstone | limestone -----------+----------+-----------+----------- Quartz | 31.91 | 69.76 | 3.71 Kaolin | 10.00 | 7.98 | 1.03 White mica | 18.40 | | Chlorite | 6.40 | 1.15 | Limonite | 4.75 | .80 | Dolomite | 7.90 | 3.44 | 36.25[1] Calcite | | 7.21 | 56.56 Gypsum | 1.17 | .12 | .10 Feldspar | 17.60 | 8.41 | 2.20 Magnetite | | .58 | Rutile | .66 | .12 | .06 Ilmenite | | .25 | Apatite | .40 | .18 | .09 Carbon | .81 | | -----------+----------+-----------+----------- Total | 100.00 | 100.00 | 100.00 -----------+----------+-----------+----------- 1: Includes small amount of FeCO_{3}. In comparing the mineral composition of igneous and sedimentary rocks, it will be noted that the most abundant single mineral of the igneous rocks, and the most abundant mineral of the lithosphere as a whole, is _feldspar_; that next in order is _quartz_; and that third comes a group of dark green minerals typified by augite and hornblende, commonly called _ferro-magnesian silicates_ because they consist of iron and magnesia, with other bases, in combination with silica. The sedimentary rocks, which are ultimately derived from the destruction of the igneous rocks, contrast with the igneous rocks mainly in their smaller proportions of feldspars and ferro-magnesian minerals, their higher proportions of quartz and white mica (sericite or muscovite), and their content of kaolin, dolomite, calcite, chlorite, limonite, etc., which are nearly absent from the unaltered igneous rocks. Evidently the development of sediments from igneous rocks has involved the destruction of much of the feldspars and ferro-magnesian silicates, and the building from the elements of these destroyed minerals of more quartz, white mica, clay, dolomite, calcite, chlorite and limonite. The composition of the minerals of the sedimentary rocks is such as to indicate that the constituents of the air and water have been added in important amounts to accomplish this change of mineral character. For instance, carbon dioxide of the atmosphere has been added to lime and magnesia of the igneous rocks to make calcite and dolomite, water has been added to some of the alumina and silica of the igneous rocks to make kaolin or clay, and both oxygen and water have been added to the iron of the igneous rocks to make limonite. RELATIVE ABUNDANCE OF THE PRINCIPAL ROCKS OF THE LITHOSPHERE Just as elements combine chemically to form minerals, so do minerals combine mechanically, either loosely or compactly, to form rocks. For instance, quartz is a mineral. An aggregation of quartz particles forms sand or sandstone or quartzite. Most rocks contain more than one kind of mineral. Sedimentary rocks occupy considerable areas of the earth's surface, but they are relatively superficial. It has been estimated that if spread evenly and continuously over the earth, which they are not, they would constitute a shell scarcely a half mile thick.[2] Igneous rocks are relatively more abundant deep below the surface. If the sediments be assumed to be limited to a volume equivalent to a half-mile shell, and the remainder of the rocks be assumed to be igneous, it is evident that to a depth of ten miles 95 per cent of the rocks are igneous. Our actual observation is confined to a shallow superficial zone in which sediments make up at least half of all the rocks. Igneous rocks can be divided for convenience into two main types: (1) granite and allied rocks, containing a good deal of silica and therefore _acid_ in a chemical sense, and (2) basalt and allied types, containing less silica and more lime, magnesia, iron, soda and potassa, and therefore _basic_ in a chemical sense. The former are light-colored gray and pink rocks while the latter are dark-colored green and gray rocks. Granite and basalt as technically defined are very common igneous rocks,--so common that the names are sometimes used to classify igneous rocks in general into two great groups, the granitic and the basaltic. It has been estimated that about 65 per cent of the igneous rocks are of the granitic group and 35 per cent of the basaltic group. Sedimentary rocks, as already indicated, consist principally of three groups, which for convenience are named shale, sandstone, and limestone. If we approximate the average composition of each group and the average composition of the igneous rocks from which they are ultimately derived, it can be calculated that sedimentary rocks must form in the proportions of 82 per cent shale, 12 per cent sandstone, and 6 per cent limestone. Only this combination of the three sediments will yield an average composition comparable with that of the parent igneous rocks. As actually observed in the field the sandstones and limestones are in relatively higher percentage than is here indicated, suggesting that part of the shales may have been deposited in deep seas where they cannot be observed, and that part may have been so changed or metamorphosed that they are no longer recognized as shales. SOILS AND CLAYS Weathered and disintegrated rocks at the surface form soils and clays. No estimate is made of abundance, but obviously the total volume of these products is small as compared with the major classes of earth materials above noted, and in large part they may be included with these major classes. WATER (HYDROSPHERE) It has been estimated that all the water of the earth, including the ocean, surface waters, and underground waters, constitutes about 7 per cent of the volume of the earth to a depth of 10 miles.[3] COMPARISON OF LISTS OF MOST ABUNDANT ROCKS AND MINERALS WITH COMMERCIAL ROCKS AND MINERALS Of the common rocks and minerals figuring as the more abundant materials of the earth's crust, only a few are prominently represented in the tables of mineral resources. Of these water and soils stand first. Others are the common igneous and sedimentary rocks used for building and road materials. Missing from the lists of the most abundant minerals and rocks, are the greater part of the commercially important mineral resources--including such as coal, oil, gas, iron ore, copper, gold, and silver,--implying that these mineral products, notwithstanding their great absolute bulk and commercial importance, occur in relatively insignificant amounts as compared with the common rock minerals of the earth. THE ORIGIN OF COMMON ROCKS AND MINERALS The common rocks and minerals develop in a general sequence, starting with igneous processes, and passing through stages of weathering, erosion, sedimentary processes, and alterations beneath the surface. The commercial minerals are incidental developments under the same processes. IGNEOUS PROCESSES The earliest known rocks are largely igneous. Sedimentary rocks are formed from the breaking down of igneous rocks, and the origin of rocks therefore starts with the formation of igneous rocks. Igneous rocks are formed by the cooling of molten rock material. The ultimate source of this molten material does not here concern us. It may come from deep within the earth or from comparatively few miles down. It may include preëxisting rock of any kind which has been locally fused within the earth. Wherever and however formed, its tendency is to travel upward toward the surface. It may stop far below the surface and cool slowly, forming coarsely crystallized rocks of the granite and gabbro types. Igneous rocks so formed are called _plutonic_ intrusive rocks. Or the molten mass may come well toward the surface and crystallize more rapidly into rocks of less coarse, and often porphyritic, textures. Such intrusive rocks are porphyries, diabases, etc. Or the molten mass may actually overflow at the surface or be thrown out from volcanoes with explosive force. It then cools quickly and forms finely crystalline rocks of the rhyolite and basalt types. These are called effusives or extrusives, or lavas or volcanics, to distinguish them from intrusives formed below the surface. The intrusive masses may take various forms, called stocks, batholiths, laccoliths, sills, sheets and dikes, definitions and illustrations of which are given in any geological textbook. The effusives or volcanics at the surface take the form of sheets, flows, tuffs, agglomerates, etc. Some of the igneous rocks are themselves "mineral" products, as for instance building stones and road materials. Certain basic intrusive igneous rocks contain titaniferous magnetites or iron ores as original constituents. Others carry diamonds as original constituents. Certain special varieties of igneous rocks, known as pegmatites, carry coarsely crystallized mica and feldspar of commercial value, as well as a considerable variety of precious gems and other commercial minerals. Pegmatites are closely related to igneous after-effects, discussed under the next heading. As a whole, the mineral products formed directly in igneous rocks constitute a much less important class than mineral products formed in other ways, as described below. =Igneous after-effects.= The later stages in the formation of igneous rocks are frequently accompanied by the expulsion of hot waters and gases which carry with them mineral substances. These become deposited in openings in adjacent rocks, or replace them, or are deposited in previously hardened portions of the parent igneous mass itself. They form "contact-metamorphic" and certain vein deposits. Pegmatites, referred to above, are in a broad sense in this class of "igneous after-effects," in that they are late developments in igneous intrusions and often grade into veins clearly formed by aqueous or gaseous solutions. Among the valuable minerals of the igneous after-effect class are ores of gold, silver, copper, iron, antimony, mercury, zinc, lead, and others. While mineral products of much value have this origin, most of them have needed enrichment by weathering to give them the value they now have. WEATHERING OF IGNEOUS ROCKS AND VEINS No sooner do igneous rocks appear at or near the earth's surface, either by extrusion or as a result of removal by erosion of the overlying cover, than they are attacked vigorously by the gases and waters of the atmosphere and hydrosphere as well as by various organisms,--with maximum effect at the surface, but with notable effects extending as far down as these agents penetrate. The effectiveness of these agents is also governed by the climatic and topographic conditions. Under conditions of extreme cold or extreme aridity, weathering takes the form mainly of mechanical disintegration, and chemical change is less conspicuous. Under ordinary conditions, however, processes of chemical decomposition are very apparent. The result is definitely known. The rocks become softened, loose, and incoherent. Voids and openings appear. The volume tends to increase, if all end products are taken into account. The original minerals, largely feldspar, ferro-magnesian minerals, and quartz, become changed to clay, mixed with quartz or sand, calcite or dolomite, and iron oxide, together with residual particles of the original feldspars and ferro-magnesian minerals which have only partly decomposed. In terms of elements or chemical composition, water, oxygen, and carbon dioxide, all common constituents of the atmosphere and hydrosphere, have been added; and certain substances such as soda, potassa, lime, magnesia, and silica have in part been carried away by circulating waters, to be redeposited elsewhere as sediments, vein fillings, and cements. Figure 1 illustrates the actual mineral and volume changes in the weathering of a granite--one of the most common rocks. The minerals anorthite, albite, and orthoclase named in this figure are all feldspars; sylvite and halite are chlorides of potash and soda. The weathering processes tend to destroy the original minerals, textures, and chemical composition. They are collectively known as _katamorphic_ alterations, meaning destructive changes. The zone in which these changes are at a maximum is called the _zone of weathering_. This general zone is principally above the surface or level of the ground-waters, but for some rocks it extends well below this level. In some regions the ground-water level may be nearly at the surface, and in others, especially where arid, it may be two thousand or more feet down. Disintegrated weathered rocks form a blanket of variable thickness, which is sometimes spoken of as the residual mantle, or "mantle rock." [Illustration: FIG. 1. Graphic representation of volume change in weathering of a Georgia granite.] Mineral products formed by weathering from common igneous rocks include soils, clay, bauxite, and certain iron, chromite, and nickel ores. Again the commercial importance of this group is not large, as compared with products formed in other ways described below. The same weathering processes described above for igneous rocks cause considerable changes of economic significance in deposits formed as igneous after-effects. In some cases they result in removing the less valuable minerals, thus concentrating the more valuable ones, as well as in softening the rock and making it easier to work; and in other cases they tend to remove the valuable constituents, which may then be redeposited directly below or may be carried completely out of the vicinity. The _oxide zones_ of many ore bodies are formed by these processes. SEDIMENTARY PROCESSES Sedimentary rocks are formed by the removal and deposition of the weathered products of a land surface. Air, water, and ice, moving under the influence of gravity and other forces, all aid in this transfer. The broken or altered rock materials may be merely moved down slopes a little way and redeposited on the surface, forming one type of _terrestrial_ or _subaërial deposits_, or they may be transferred and sorted by streams. When deposited in streams or near their mouths, they are known as _river_, _alluvial_, or _delta deposits_. When carried to lakes and deposited they form _lake deposits_. Ultimately the greater part of them are likely to be carried to the ocean and deposited as _marine sediments_. Part of the weathered substances are carried mechanically as clay and sand, which go to make up the _shale_ and _sandstone_ sediments. Part are carried in solution, as for example lime carbonate and magnesium carbonate, which go to make up _limestone_ and _dolomite_. Some of the dissolved substances are never redeposited, but remain in solution as salts in the sea, the most abundant of which is sodium chloride. Some of the dissolved substances of weathering, such as calcite, quartz, and iron oxide, are carried down and deposited in openings of the rocks, where they act as cements. The sediments as a whole consist of three main types,--_shales_ (kaolin, quartz, etc.), _sandstones_ (quartz, feldspar, etc.), and _limestones_ or _dolomites_ (carbonates of lime and magnesia). Of these, the shale group is by far the most abundant. There are of course many sediments with composition intermediate between these types. There are also sediments made up of large undecomposed fragments of the original rocks, cemented to form _conglomerates_, or made up of small fragments of the original rocks cemented to form _arkoses_ and _graywackes_. These, however, may be regarded as simply stages in the alteration, which in repeated cycles of weathering must ultimately result in producing the three main groups,--shales, sandstones, and limestones. Mineral products formed by sedimentary processes include sandstones, limestones, and shales, used as building stone and road materials; certain sedimentary deposits of iron, like the Clinton ores of the southeastern United States and the Brazilian ores; important phosphate deposits; most deposits of salt, gypsum, potash, nitrates, etc.; comparatively few and unimportant copper deposits; and important placer deposits of gold, tin, and other metals, and precious stones. With the aid of organic agencies, sedimentary processes also account for the primary deposition of coal and oil. WEATHERING OF SEDIMENTARY ROCKS After sedimentary rocks are formed, and in many cases covered by later sediments, they may be brought again by earth movements and erosion to the surface, where they in turn are weathered. The weathering of sedimentary rocks proceeds along lines already indicated for the igneous rocks. Residual mantles of impure clay and sand are commonly formed. The mineral composition of sedimentary rocks being different from that of igneous rocks to start with, the resulting products are in slightly different proportions; but the changes are the same in kind and tend merely to carry the general process of alteration farther in the same direction,--that is, toward the production of a few substances like clay, quartz, iron oxide, and calcite, which are transported and redeposited to form clay, sand, and limestone. Cycles of this kind may be repeated indefinitely. By weathering of sedimentary rocks are produced some soils, certain commercial clays, iron ores, lead and zinc ores, and other valuable mineral products. CONSOLIDATION, CEMENTATION, AND OTHER SUBSURFACE ALTERATIONS OF ROCKS. =Cementation.= No sooner are residual weathered mantles formed or sedimentary rocks deposited, whether under air or water, than processes of consolidation begin. Settling, infiltration of cementing materials, and new growths, or recrystallization, of the original minerals of the rock all play a part in the process. The mud or clay becomes a shale, the sand becomes sandstone or quartzite, the marl becomes limestone or marble. All the minute openings between the grains, as well as larger openings such as fissures and joints, may thus be filled. At the same time the cementing materials may replace some of the original minerals of the rock, the new minerals either preserving or destroying the original textures. This process is sometimes called _metasomatic replacement_. Igneous rocks as a rule are compact, and hence are not so much subject to the processes of cementation as sedimentary rocks; but certain of the more porous phases of the surface lavas, as well as any joints in igneous rocks, may become cemented. All of these changes may be grouped under the general term _cementation_. A special phase of consolidation and cementation is produced near intrusive igneous rocks through the action of the heat and pressure and the expelled substances of the igneous rock. This is called _contact metamorphism_ or _thermal metamorphism_. The processes are even more effective when acting in connection with the more intense metamorphism described under the next heading. By cementation some of the common rocks, especially the sediments, become sufficiently compact and strong to be useful as commercial products, such as building stones and road materials. More important as mineral products are the cementing materials themselves. These are commonly quartz, calcite, or iron oxide, of no especial value, but locally they include commercially valuable minerals containing gold, copper, silver, lead, zinc, and many other mineral products. It is a matter of simple and direct observation, about which there is no controversy, that many minerals are deposited as cements in the openings in rocks or replacing rocks. As to the source of the solutions bringing in these minerals, on the other hand, there has been much disagreement. In general, the common cementing materials such as quartz and calcite, as well as some of the commercial minerals, are clearly formed as by-products of weathering, and are transported and redeposited by the waters penetrating downward from the surface. The so-called _secondary enrichment_ of many valuable veins is merely one of the special phases of cementation from a superficial source. In other cases it is believed that deep circulation of ordinary ground-waters may pick up dispersed mineral substances through a considerable zone, and redeposit them in concentrated form in veins and other trunk channels. For still other cementing materials, it is suspected that the ultimate source is in igneous intrusions; in fact, deposits of this general character show all gradations from those clearly formed by surface waters, independently of igneous activity, to those of a contact-metamorphic nature and others belonging under the head of "igneous after-effects." Hypothesis and inference play a considerable part in arriving at any conclusion as to the source of cementing materials,--with the result that there is often wide latitude for difference of opinion and of emphasis on the relative importance of the different sources of ore minerals. =Dynamic and contact metamorphism.= Beneath the surface rocks are not only cemented, but may be deformed or mashed by dynamic movements caused by great earth stresses; the rocks may undergo rock flowage. The result is often a remarkable transformation of the character of the rocks, making it difficult to recognize their original nature. Also, igneous intrusions may crowd and mash the adjacent rocks, at the same time changing them by heat and contributions of new materials. This process may be called _contact metamorphism_, but in so far as it results in mashing of the rocks it is closely allied to _dynamic metamorphism_. The former term is also applied to less profound changes in connection with igneous intrusions, which result merely in cementation without mashing. Dynamic and contact metamorphism may in some cases produce rocks identical in appearance with those produced by ordinary processes of cementation and recrystallization without movement. For instance, it is difficult to tell how much movement there has been in the production of a marble, because both kinds of processes seem to produce much the same result. Commonly, however, the effect of dynamic metamorphism is to produce a parallel arrangement of mineral particles and to segregate the mineral particles of like kind into bands, giving a _foliated_ or _schistose_ or _gneissic_ structure, and the rocks then become known as slates, schists, or gneisses. Commonly they possess a capacity to part along parallel surfaces, called _cleavage_. The development of the schistose or gneissic structure is accompanied by the recrystallization of the rock materials, producing new minerals of a platy or columnar type adapted to this parallel arrangement. Even the composition of the rock may be substantially changed, though this is perhaps not the most common case. Whereas by weathering the rock is loosened up and disintegrated, substances like carbon dioxide, oxygen, and water are abundantly added, and light minerals of simple composition tend to develop,--by dynamic metamorphism on the other hand, carbon dioxide, oxygen, and water are usually expelled, the minerals are combined to make heavier and more complex minerals, pore space is eliminated, and altogether the rock becomes much more dense and crystalline. While segregation of materials is characteristic of the surficial products of weathering, the opposite tendency, of mixing and aggregation, is the rule under dynamic metamorphism, notwithstanding the minor segregation above noted. Dynamic metamorphism is for the most part unfavorable to the development of mineral products. Ore bodies brought into a zone where these processes are active may be profoundly modified, but not ordinarily enriched. One of the exceptions to this general rule is the development of the cleavage of a slate, which enables it to be readily split and thereby gives it value. Contact metamorphism, on the other hand, may develop valuable mineral deposits (see pp. 20, 45-46). THE METAMORPHIC CYCLE AS AN AID IN STUDYING MINERAL DEPOSITS All of the chemical, mineralogical, and textural changes in rocks above described may be collectively referred to as _metamorphism_. The phase of metamorphism dealing with surficial weathering, similar changes below the surface, and the formation of sediments, is called _katamorphism_ or destructive change. The phase of metamorphism dealing with the constructive changes in rocks, due to cementation, dynamic movements, and igneous influences, is called _anamorphism_. Some geologists confine the term metamorphism to the changes involved in contact and dynamic metamorphism, and call the resulting products _metamorphic rocks_. The zone in which katamorphism is most active, usually near the surface, is called the _zone of katamorphism_. The deeper zone in which anamorphism is preponderant is called the _zone of anamorphism_. There are no definite limits of depth to these zones. A given rock may be undergoing katamorphism while rocks on either side at the same depth are suffering anamorphism. By katamorphism rocks break down to produce the surficial rocks, and by anamorphism the surficial rocks are again consolidated and altered to produce highly crystalline rocks, which are not dissimilar in many of their characteristics to the igneous rocks from which all rocks trace their ultimate origin. In other words, anamorphism tends toward the reproduction of igneous rocks, though it seldom fully accomplishes this result. These two main groups of changes together constitute the _metamorphic cycle_. Some rocks go through all phases of the cycle, but others may pass directly from one phase to an advanced phase without going through the intermediate stages. For instance, an igneous rock may become a schist without going through the intermediate stage of sedimentation. Rocks are not permanent in their condition, but at practically all times and places are undergoing some kind of metamorphism which tends to adapt them to their environment. The conception of rocks as representing phases or stages in a progressive series of changes called the metamorphic cycle aids greatly in correlating and holding in mind many details of rock nature and origin, and brings into some sort of perspective the conditions which have produced rocks. A schistose sediment comes to be regarded as an end product of a long series of alterations, beginning with igneous rocks and passing through the stages of weathering, sedimentation, cementation, etc., each of which stages has been responsible for certain mineralogical, chemical, and textural features now characterizing the rock. The alternation of constructive and destructive changes of the metamorphic cycle, and the repetitions of the cycle itself, periodically work over the earth materials into new forms. Usually the cycles are not complete, in the sense that they seldom bring the rock back to exactly the same condition from which it started. More sediments are formed than are changed to schists and gneisses, and more schists and gneisses are formed than are changed back to igneous rocks. Salts in the ocean continuously accumulate. The net result of the metamorphic cycle, is, therefore, the accumulation of materials of the same kinds. Incidental to these accumulations is the segregation of commercial mineral products. The metamorphic cycle becomes a logical and convenient geologic basis for correlating, interpreting, and classifying mineral products. Because of the great variety of materials and conditions represented in mineral deposits, prodigious efforts are required to remember them as independent entities; but as incidents or stages in the well-known progress of the metamorphic cycle, their essential characteristics may be easily remembered and kept in some perspective. Ores of certain metals, such as iron, occur in almost every phase of the metamorphic cycle,--as igneous after-effects, as weathered products, as sediments, and as schists. The ores of each of these several phases have group characteristics which serve to distinguish them in important particulars from ores belonging to other phases of the cycle. Having established the position of any particular ore in the metamorphic cycle, a number of safe inferences are possible as to mineralogical composition, shape, extent, and other conditions, knowledge of which is necessary for an estimate of commercial possibilities. FOOTNOTES: [1] Clarke, F. W., Data of geochemistry: _Bull. 695, U. S. Geol. Survey_, 1920, p. 35. [2] Clarke, F. W., Data of geochemistry: _Bull. 695, U. S. Geol. Survey_, 1920, p. 33. [3] Clarke, F. W., Data of geochemistry: _Bull. 695, U. S. Geol. Survey_, 1920, pp. 22-23. CHAPTER III SOME SALIENT FEATURES OF THE GEOLOGY AND CLASSIFICATION OF MINERAL DEPOSITS VARIOUS METHODS OF CLASSIFICATION Mineral products may be classified according to use, commercial importance, geographic distribution, form and structure, mineralogical and chemical composition, or origin. Each of these classifications is useful for some purposes. The geologist usually prefers a classification based on origin or genesis. In the following chapters on mineral resources, however, such a classification is not the primary one, because of the desire to emphasize economic features. The mineral commodities are treated as units and by group uses. Some mineral commodities have so many different kinds of origin in different regions that to distribute them among several genetic groups in description would make it impossible to preserve the unity necessary for consideration of the economic features. While in the descriptive chapters many references are made to origin, it may be difficult for the reader to assemble them in perspective; for this reason we summarize at the outset some of the salient features of origin of mineral deposits and of their geologic classification. To the layman the reason for emphasis on origin is often not clear. The "practical" man frequently regards this phase of the subject as merely incidental to the immediate economic questions--a playground for harmless theorists. The answer of the economic geologist is that in no other way than by a knowledge of origin is it possible to arrive at an understanding of conditions which so well enables one to answer many practical questions. In the exploration for mineral deposits, it is obvious that an understanding of the kinds of geologic conditions and processes under which a given type of deposit is known to develop results in the elimination of much unpromising territory, and the concentration of work on favorable localities. In forming any estimate of mineral deposits beyond the ground immediately opened up,--for instance, in estimating depth, form, change in values, mineralogical character, or interruptions due to faulting,--it is difficult to form any intelligent conception of the probabilities unless the history of the deposit is understood. If, for instance, the ore is known to be formed by hot waters, associated with the cooling of igneous rocks, different conditions are to be expected below the zone of observation than if the ore is formed by surface waters. If the ore body is formed as a single episode under simple geologic conditions, the interpretation of the possibilities in the situation may be quite different from the interpretation applied where the history has been more complex. If the surface conditions suggest possibilities of secondary enrichment of the ores, the interpretation of the conditions underground will be different from those applied where there is no evidence of such enrichment. Where a mineral deposit is completely opened up in three dimensions, it is often possible to work out economic questions of tonnage, grade, shape, and values, without the aid of geology. Also, where conditions are comparatively simple and uniform throughout a district, the local knowledge of other mines may be a sufficient basis for answering these questions for any new property developed. Empirical methods may suffice. However, it is seldom that the conditions are so simple that some geological inference is not necessary. Even where problems are settled without calling in the geologist, geological inferences are required in the interpretation of, and projection from, the known facts. It is often the case that the practical man has in his mind a rather elaborate assortment of geologic hypotheses, based on his individual experience, which make the so-called theories of the geologist seem conservative in comparison. The geologist comes to the particular problem with a background of established geologic principles and observations, and his first thought is to ascertain all the local conditions which will aid in deciphering the complete history of the mineral deposit. There is no fact bearing on the history, however remote from practical questions, which may not be potentially valuable. With this digression to explain the geologist's emphasis on origin of mineral products, we may return to a consideration of a few of the principles of rock and mineral genesis which have been found to be significant in the study of mineral products. In the preceding chapters it has been indicated that mineral deposits are mere incidents in the mass of common rocks; that they are made by the same processes which make common rocks, that none of the processes affecting mineral deposits are unique for these minerals, and that most common rocks are on occasion themselves used as mineral resources. These facts are emphasized in order to make it clear that the study of mineral deposits cannot be dissociated from the study of rocks, and that the study of the latter is essential to bring mineral deposits into their proper perspective. Absorption in the details of a mineral deposit makes it easy for the investigator to forget or minimize these relations. Nevertheless, in the study of mineral deposits, and especially deposits of the metallic minerals, certain geologic features stand out conspicuously against the common background indicated above. Our discussion of these features will follow the order of rock genesis indicated in the description of the metamorphic cycle. NAMES Any classification of mineral deposits on the basis of origin is more or less arbitrary. The sharp lines implied by the use of class names do not exist in nature. Mineral deposits are so complex and so interrelated in origin, that a classification according to genesis indicates only the essential and central class features; it does not sharply define the limits of the classes. It is practically impossible for any geologist to present a classification which will be accepted without qualification by other geologists, although there may be agreement on essential features. Difficulties in reaching agreement are increased by the inheritance from the past of names, definitions, and classifications which do not exactly fit present conceptions based on fuller information,--but which, nevertheless, have become so firmly established in the literature that it is difficult to avoid their use. In the progress of investigation many new names are coined to fit more precisely the particular situation in hand, but only in fortunate cases do these new names stand up against the traditional currency and authority of old names. The geologist is often in despair in his attempt to express his ideas clearly and precisely, and at the same time to use terms which will be understandable by his readers and will not arouse needless controversy. As illustrative of the above remarks reference may be made to a few terms commonly used in economic geology, such as _primary_, _secondary_, _syngenetic_, _epigenetic_, _supergene_, _hypogene_, _protore_, etc. The most commonly used of these terms are _primary_ and _secondary_. It is almost impossible to define them in a way which will cover all the conceptions for which they have been used, and yet in their context they have been very useful in conveying essential ideas. An ore formed by direct processes of sedimentation has sometimes been called primary, whereas an ore formed by later enrichment of these sediments has been called secondary. An ore formed directly by igneous processes has been called primary, while an ore formed by enrichment of such primary ore by later processes has been called secondary. It is clear, however, that these terms are merely relative, with application to a specific sequence, and that they do not fix the absolute position of the ore in a sequence applying to all ores. For instance, ores deposited directly as sediments or placers may be derived from the erosion of preëxisting ore bodies,--in which case it may sometimes be convenient to refer to the sedimentary ores or placers as secondary and the earlier ores as primary. Or a sulphide deposit originating through igneous agencies may undergo two or three successive enrichments, each successive one secondary to the preceding, but primary to the one following. In spite of these obvious difficulties, the terms primary and secondary may be entirely intelligible as indicating relative order of development under a given set of conditions. The term _syngenetic_ has been used for mineral deposits formed by processes similar to those which have formed the enclosing rocks and in general simultaneously with them, and _epigenetic_ for those introduced into preëxisting rocks. In certain cases _syngenetic_ may be roughly synonymous with _primary_, and _epigenetic_ with _secondary_, and yet a primary ore may be epigenetic. For instance, zinc sulphides in the Mississippi valley limestones (pp. 54-55) are epigenetic, and yet are primary with reference to a later enrichment. The two sets of terms are meant to convey somewhat different ideas and are not interchangeable. Ransome[4] has suggested, especially for vein and contact deposits, a series of names which has the considerable advantage of definiteness:--_hypogene ores_, formed in general by ascending non-oxidizing solutions, perhaps hot; _supergene ores_, formed in general by oxidizing and surface solutions, initially cold and downward moving; and _protores_, or metallized rock or vein substances which are too low in tenor to be classed as ores, but which would have been converted into ores had the enriching process been carried far enough. In this connection Ransome defines primary ore as unenriched material that can be profitably mined. In view of the general use of the terms primary and secondary as expressing a sequential relation of ore development, it is doubtful whether this more precise definition will supersede the older usage. Also it may be noted that commercial conditions might require, under these definitions, the designation of an ore as a protore at one time or place and as a primary ore at another. Hypogene ores are dominantly primary, and supergene ores are dominantly secondary, but either may include both primary and secondary ores. The terms of these several classifications overlap, and seek to express different aspects of the same situation. While almost synonymous in certain applications they are not in others. In this text the writer has certainly not escaped the difficulties in regard to names above referred to, nor in fact has he made any exceptional effort to do so. His chief purpose is to convey, in somewhat elementary terms, an understandable idea of the central features of economic geology. In the main, the most widely accepted terms are used. Almost at every turn it would be possible, in the interests of precision, to introduce qualifying discussions of names,--but at the expense of continuity and perspective in the presentation of the principal subject-matter. The writer does not wish to minimize the necessity for careful and precise nomenclature; but he regards it important that the student focus his attention on the central objective facts of the subject, and that he do not become misled by the sometimes over-strenuous advocacy of certain names or classifications in preference to others. If the facts are understood, he will ordinarily have no difficulty in judging the significance of the variety of names proposed to express these facts. If, on the other hand, the student approaches the subject with a ready-made set of names and definitions learned by rote, he is in danger of perceiving his facts from one angle only and through a distorted perspective. MINERAL DEPOSITS AS MAGMATIC SEGREGATIONS IN IGNEOUS ROCKS In this class are included deposits which crystallize within the body of igneous rock, almost, if not quite, simultaneously with the adjacent rock. These deposits form one of the main types of _syngenetic_ deposits. The titaniferous magnetites constitute a widely distributed but at present commercially unavailable class of iron ores. The magnetite crystals of these deposits interpenetrate with the other constituents of an igneous rock, commonly of a gabbro type, and the deposits themselves are essentially igneous rocks. Their shapes are for the most part irregular, their boundaries ill-defined, and their concentration varying. While their magmatic origin is clear, there is little agreement as to the precise conditions which determined their segregation in the molten rock. There is often a tendency for the ores to follow certain primary sheeted structures in the igneous mass, a fact for which the reason is not obvious. The Sudbury nickel ores, of Ontario, Canada, the principal source of the world's nickel, lie mainly within and along the lower margin of a great intrusive igneous mass of a basic type called _norite_, and locally the ores project beyond the margin into adjacent rocks. Their textures and their intercrystallization with the primary minerals of the igneous rock have suggested that they are essentially a part of the norite mass, and that they crystallized during some segregative processes which were effective before the magma had solidified. Near the ores there are likely to be granitic rocks, which, like the ores, seem to be segregations from the norite magma. Locally both the ores and the associated granitic rocks replace the main norite body in such a fashion as to indicate their slightly later crystallization. However, the intimate association of the ores with the primary minerals in the magma, together with their absence from higher parts of the norite and from the extraneous rocks far from the contact, indicate to other investigators that they were not brought in from outside in vagrant solutions which followed the intrusion of the main magma, but that they were segregated within the magma essentially in place. The occurrence of these heavy ores near the base of the norite naturally suggests that they were segregated by sinking to the bottom of the molten magma, but this conclusion implies certain physical conditions of the magma which have not yet been proved. Again the precise nature of the process and the part played in it by aqueous and gaseous solutions are subject to some doubt and controversy. The settlement of this problem awaits the solution of the more general problem of the origin and crystallization of magmas. In this general class of igneous deposits may be mentioned also diamonds, platinum, chromite, corundum, and other mineral products, although for the formation of commercial ores of many of these substances further concentration by weathering and sedimentation has been required. Pegmatites are coarsely crystalline acid dike rocks which often accompany a large igneous intrusion and which have obviously crystallized somewhat later than the main igneous mass. They may constitute either sharply delimited dikes or more irregular bodies which grade into the surrounding igneous mass. They have a composition roughly similar to the associated igneous rock, but usually a different proportion of minerals. They are probably the result of the differentiation of the parent magma. The pegmatites are of especial interest to the economic geologist because of the frequency with which they carry commercial minerals, such as the precious stones, mica, feldspar, cassiterite (tin ore), and others. They show a complete gradation from dikes of definitely igneous characteristics to veins consisting largely of quartz in which evidence of igneous origin is not so clear. The pegmatites thus afford a connecting link between ores of direct igneous sources and ores formed as "igneous after-effects," which are discussed in the next paragraph. Aplites are fine-grained acid igneous rocks of somewhat the same composition as the pegmatites and often show the same general relations to ores. MINERAL DEPOSITS WITHIN AND ADJACENT TO IGNEOUS ROCKS WHICH WERE FORMED IMMEDIATELY AFTER THE COOLING AND CRYSTALLIZATION OF THE MAGMAS THROUGH THE AGENCY OF HOT MAGMATIC SOLUTIONS. These deposits are closely associated in place and age with igneous rocks, either intrusive or extrusive, and are usually considered to have come from approximately the same source; and yet they afford distinct evidence of having been deposited after the adjacent igneous rocks were completely crystallized and fractured. They are thus _epigenetic_ deposits. They are not themselves igneous rocks and they do not constitute pegmatites, but they often grade into pegmatites and belong to the same general stage in the sequence of events. They include deposits formed by contact metamorphism. They are sometimes designated by the general term "igneous after-effects"--a term also applied in some cases to pegmatites. Some geologists discriminate between "deep vein" deposits (p. 43) and "contact-metamorphic" deposits, but the two are so closely related in place and origin that for our purposes they will be considered together. The ores of this class are clearly deposited from vagrant solutions which wander through openings of all kinds in the igneous rock and outward into the adjacent country rocks. They also replace the wall rocks; limestone is especially susceptible. This is a phase of contact metamorphism. Some of the most important metalliferous deposits belong in this class, including most of the gold, silver, copper, iron, lead and zinc ores of the western United States and the copper deposits of Lake Superior. In general, ores of this class are more abundant about intrusive igneous rocks, that is about igneous rocks which have stopped and cooled before reaching the surface,--than in association with extrusive igneous rocks which have poured over the surface as lava flows--but the latter are by no means insignificant, including as they do such deposits as the Lake Superior copper ores, the Kennecott copper ores of Alaska, some of the gold-silver deposits of Goldfield and other Nevada camps, and many others. There is general similarity in the succession of events shown by study of ore bodies related to intrusives. First, the invasion of the magma, resulting in contact metamorphism of the adjacent rocks, sometimes with, and often without conspicuous crowding effects on the invaded rocks; second, the cooling, crystallization, and cracking of both the igneous rock and the adjacent rock; third, the introduction of ore-bearing solutions into these cracks,--sometimes as a single episode, sometimes as a long continued and complex process forming various types of minerals at successive stages. This order may in some cases be repeated in cycles, and overlapping of the successive events is a common feature. One of the interesting facts is the way in which the igneous mass has invaded and extensively altered the country rocks in some mineral districts,--in some cases by crowding and crumpling them, and in others without greatly disturbing their structural attitudes. The latter is illustrated in the Bingham district of Utah and the Philipsburg district of Montana. In such cases there is so little evidence of crowding of the country rocks as to raise the question how such large masses of intrusives could be introduced without greater disturbing structural effect. This leads naturally to consideration of the general problem of the manner of progress of magmas through adjacent rocks,--a subject which is still largely in the realm of speculation, but which is not thereby eliminated from the field of controversy. Facts of this kind seem to favor the position of certain geologists that magmas may assimilate the rocks they invade. EVIDENCE OF IGNEOUS SOURCE No one ever saw one of these deposits in the process of formation; the conclusion, therefore, that they originated from hot solutions, either aqueous or gaseous, or both, which were essentially "after-effects" of igneous activity and came from the same primary source as the associated igneous rocks, is an inference based on circumstantial evidence of the kind below summarized: (1) The close association both in place and age with igneous rocks. This applies not only to individual deposits, but to certain groups of deposits which have common characteristics, and which constitute a metallogenic province; also to groups of the same geologic age, which indicate a metallogenic epoch (pp. 308-309). The association with igneous rocks in one place might be a coincidence but its frequent repetition can hardly be so explained. A zonal arrangement of minerals about intrusives is often noted. Geologic evidence often shows the processes of ore deposition to have been complete before the next succeeding geologic event,--as for instance in the Tonopah district of Nevada (p. 236), where the ores have been formed in relation to certain volcanic flows and have been covered by later flows not carrying ore, without any considerable erosion interval between the two events. (2) The general contrast in mineralogical and chemical composition, texture, and mineral associations, between these ore minerals and the minerals known to be formed by ordinary surficial agencies under ordinary temperatures. The latter carry distinctive evidences of their origin. When, therefore, a mineral group is found which shows contrasting evidences, it is clear that some other agencies have been at work; and the natural assumption is that the solutions were hot rather than cold; that they came from below rather than above. (3) The contrast between the character and composition of these ores (and their associated gangue) and the character and composition of the wall rocks, together with the absence of leaching of the wall rocks, favor the conclusion that the ore minerals are foreign substances introduced from extraneous sources. The source not being apparent above and the processes there observed not being of a kind to produce these results, it is concluded that the depositing solutions were hot and came from below. (4) The fact that many of the ore minerals are never known to develop under ordinary temperatures at the surface. For some of them, experimental work has also indicated high temperature as a requisite to their formation. Quartz, which is a common associate of the ores and often constitutes the principal gangue, serves as a geologic thermometer in that it possesses an inversion point or temperature above which it crystallizes in a certain form, below which in another. In deposits of this class it has often been found to crystallize at the higher temperatures. The quartz sometimes shows bubbles containing liquid, gas, and small heavy crystals, probably of ferric oxide, as in the Clifton-Morenci district of Arizona. It is clear that the ore-bearing solutions in these cavities, before the crystallization of the heavy mineral inclusions, held dissolved not only much larger quantities of mineral substances than can be taken up by water at ordinary temperatures, but also a substance like ferric oxide which is entirely insoluble under ordinary cool conditions. (5) The association of the ores with minerals carrying fluorine and boron, with many silicate minerals, such as garnet, amphiboles, pyroxenes, mica (sericite) and others, and with other minerals which are known to be characteristic developments within or near igneous masses and which are not known to form under weathering agencies at the surface. Various characteristic groupings of these associated minerals are noted. In limestone much of the mass may be replaced by garnet and other silicates in a matrix of quartz. In igneous rock the ore-bearing solutions may have altered the wall rock to a dense mixture of quartz, sericite, and chlorite. Where sericite is dominant, the alteration is called sericitic alteration. Where chlorite is important, it is sometimes called chloritic or "propylitic" alteration. The chloritic phases are usually farther from the ore deposit than the sericitic phases, indicating less intense and probably cooler conditions of deposition. Locally other minerals are associated with the ores, as, for instance, in the Goldfield district of Nevada (p. 230), where alunite replaces the igneous rock. Alunite is a potassium-aluminum sulphate, which differs from sericite in that sulphur takes the place of silicon. In the quartzites of the lead-silver mines of the Coeur d'Alene district of Idaho (p. 212), siderite or iron carbonate is a characteristic gangue material replacing the wall rock. Quartz in some cases, as noted above, gives evidence of high temperature origin and therefore of igneous association. Jasperoid quartz, as well illustrated in the Tintic district of Utah (p. 235), may show texture and crystallization suggestive of deposition from colloidal solution,--a process which can occur under both cold and hot conditions, but which is believed to be accelerated by heat. Certain minerals, such as magnetite, ilmenite, spinel, corundum, etc., are often found as primary segregations within the mass of igneous rock. These and other minerals, including minerals of tin and tungsten, monazite, tourmaline, rutile, and various precious stones, are characteristically developed in pegmatites, which are known to be igneous rocks, crystallized in the later stages of igneous intrusion. When, therefore, such minerals are found in other ore deposits an igneous source is a plausible inference. For instance, in the copper veins of Butte, Montana (p. 201), are found cassiterite (tin oxide) and tungsten minerals. Their presence, therefore, adds another item to the evidence of a hot-water source from below. (6) The occasional existence of hot springs in the vicinity of these ore deposits. Where hot springs are of recent age they may suggest by their heat, steady flow, and mineral content, that they are originating from emanations from the still cooling magmas. In the Tonopah camp (p. 236), cold and hot springs exist side by side, exhibiting such contrasts as to suggest that some are due to ordinary circulation from the surface and that others may have a deep source below in the cooling igneous rocks. This evidence is not conclusive. Hot springs in general fail to show evidence of ore deposition on any scale approximating that which must have been involved in the formation of this class of ore bodies. Much has been made of the slight amounts of metallic minerals found in a few hot springs, but the mineral content is small and the conclusion by no means certain that the waters are primary waters from the cooling of igneous rocks below. In this connection the mercury deposits of California (p. 259), contribute a unique line of evidence. In areas of recent lavas, mercuric sulphide (cinnabar) is actually being deposited from hot springs of supposed magmatic origin, the waters of which carry sodium carbonate, sodium sulphide, and hydrogen sulphide,--a chemical combination known experimentally to dissolve mercury sulphide. The oxidation and neutralization of these hot-spring solutions near the surface throws out the mercury sulphide. At the same time the sulphuric acid thus formed extensively leaches and bleaches the surrounding rocks. Such bleaching is common about mercury deposits. When it is remembered that the mercury deposits contain minor amounts of gold and silver and sulphides of other metals; that they are closely associated with gold and silver deposits; and further that such gold, silver, and other sulphide deposits often contain minor amounts of mercury,--it is easy to assume the possibility that these minerals may likewise have had their origin in hot solutions from below. The presence of mercury in a deposit then becomes suggestive of hot-water conditions. (7) Ores sometimes occur in inverted troughs indicating lodgment by solutions from below, as, for instance, in the saddle-reef gold ores of Nova Scotia and Australia, and in certain copper ores of the Jerome camp of Arizona (p. 204.) This occurrence does not indicate whether the solutions were hot or cold, magmatic or meteoric, but in connection with other evidences has sometimes been regarded as significant of a magmatic source beneath. Perhaps no one of these lines of evidence is conclusive; but together they make a strong case for the conclusion that the solutions which deposited the ores of this class were hot, came from deep sources, and were probably primary solutions given off by cooling magmas. The conclusion that some ores are derived from igneous sources, based on evidence of the kind above outlined, does not mean necessarily that the ore is derived from the immediately adjacent part of the cooling magma. In fact the evidence is decisive, in perhaps the majority of cases, that the source of the mineral solutions was somewhat below; that these solutions may have originated in the same melting-pot with the magma, but that they came up independently and a little later,--perhaps along the same channels, perhaps along others. POSSIBLE INFLUENCE OF METEORIC WATERS IN DEPOSITION OF ORES OF THIS CLASS It is hardly safe, with existing knowledge, to apply the above conclusion to all ore deposits with igneous associations, or in any case to eliminate entirely another agency,--namely, ground-waters of surface or meteoric origin, which are now present and may be presumed often to have been present in the rocks into which the ores were introduced. Such waters may have been heated and started in vigorous circulation by the introduction of igneous masses, and thereby may have been enabled to effectively search out and segregate minutely disseminated ore particles from wide areas. This has been suggested as a probability for the Kennecott copper ores of Alaska (p. 200) and for the copper ores of Ely, Nevada. In the Goldfield camp (p. 230) the ores are closely associated with alunite in such a manner as to suggest a common origin. It has been found difficult to explain the presence of the alunite except through the agency of surface oxidizing waters acting on hydrogen sulphide coming from below. In the early days of economic geology there was relatively more emphasis on the possible effectiveness of ground-waters in concentrating ores of this type. With the recognition of evidence of a deeper source related to magmas, the emphasis has swung rapidly to the other extreme. While the evidence is sound that the magmatic process has been an important one, it is difficult to see how and to just what extent this process may have been related to the action of ground-waters,--which were probably present in a heated condition near the contact. It may never be possible to discriminate closely between these two agencies. It seems likely that at some stages the two were so intimately associated that the net result of deposition cannot be specifically assigned either to one or to the other. ZONAL ARRANGEMENT OF MINERALS RELATED TO IGNEOUS ROCKS Evidence is accumulating in many mining districts that ore deposits of these igneous associations were deposited with a rough zonal arrangement about the igneous rock. At Bingham, Tintic, and Butte (pp. 204, 208, 235), copper ores are on the whole closest to the igneous rock, and the lead, zinc, and silver ores are farther away. Furthermore, the quartz gangue near the igneous rock is likely to contain minerals characteristic of hot solutions, while farther away such minerals as dolomite and calcite appear in the gangue, suggestive of cooler conditions. In Cornwall (p. 262), tin ores occur close to the intrusives, and lead-silver ores farther away. The gradations are by no means uniform; shoots of one class of ore may locally cut abruptly across or through those of another class. The existence of zones horizontally or areally arranged about intrusives suggests also the possibility of a vertical zonal arrangement with reference to the deep sources of the solutions. Of course when secondary concentration from the surface, described later, is taken into account, there may be a marked zonal distribution in a vertical direction, but this is not primary zoning. A few veins and districts show evidence of vertical zoning apparently related to primary deposition; for the most part, however, in any one mine or camp there is yet little evidence of primary vertical zoning. On the other hand, certain groups of minerals are characteristic of intense conditions of heat and pressure, as indicated by the coarse recrystallization and high degree of metamorphism of the rocks with which they are associated; and other groups have such associations as to indicate much less intense conditions of temperature and pressure. Depth is only one factor determining intensity of conditions, but it affords a convenient way to indicate them; so mineral deposits associated with igneous rocks are sometimes classified by economic geologists on the basis of deep, intermediate, and shallow depths of formation. There are a considerable number of minerals which are formed in all three of these zones, although in differing proportions. There are comparatively few which are uniformly characteristic of a single zone. On the whole, it is possible to contrast satisfactorily mineral deposits representing very intense metamorphic conditions, usually associated with formation at great depth, with those formed at or near the surface; but there are many deposits with intermediate characteristics which it is difficult to place satisfactorily. The accessible deposits of the deep zone are associated with plutonic igneous rocks which have been deeply eroded, and not with surface lavas. They are characterized by minerals of gold, tin, iron, titanium, zinc, and copper, and sometimes of tungsten and molybdenum, in a gangue of quartz, which contains also minerals such as garnet, corundum, amphibole, pyroxene, tourmaline, spinel, and mica. The deep-zone minerals are not unlike the pegmatite minerals in their grouping and associations. Deposits formed at shallow depths are related to extrusive rocks and to intrusives near the surface. Erosion has not been deep. Mercury, silver and gold (tellurides, native metals, and silver sulphides), antimony, lead, and zinc minerals are characteristic, together with alunite, adularia, and barite. Metallic copper also is not infrequent. Very often the gangue material is more largely calcite than quartz, whereas calcite is not present in the deep zone.[5] The trend of evidence in recent years has favored the conclusion that the principal ores associated with igneous rocks have not developed at very great depths. Even within our narrow range of observation there is a difference in favor of the shallower depths, and the greatest depths we can observe are after all but trivial on the scale of the earth. A survey of the ore deposits of Utah has suggested the generalization that ores are more commonly related to intrusive stocks than to the forms known as laccoliths, and that within and about intrusive stocks the ores are much more abundant near the top or apex of a stock than lower down.[6] In parts of the region where erosion has removed all but the deeper portions of the stocks, ore bodies are less abundant. It will be of interest to follow the testing of this generalization in other parts of the world. The scientist is constantly groping for underlying simple truth. Such glimpses of order and symmetry in the distribution of ore around igneous rocks as are afforded by the facts above stated, tempt the imagination to a conception of a simple type or pattern of ore distribution around intrusions. For this reason we should not lose sight of the fact that, in the present state of knowledge, the common and obvious case is one of irregular and heterogeneous distribution, and that there are many variations and contradictions even to the simplest generalization that can be made. The observer is repeatedly struck by the freakish distribution of ores about igneous masses, as compared with their regularity of arrangement under sedimentary processes to be discussed later. It is yet unexplained how an intrusive like the Butte granite can produce so many different types of ores at different places along its periphery or within its mass, and yet all apparently under much the same general conditions and range of time. It is difficult also to discern the laws under which successive migrations of magma, from what seems to be a single deep-seated source or melting-pot, may carry widely contrasting mineral solutions. Far below the surface, beyond our range of observation, it is clear that there is a wonderful laboratory for the compounding and refinement of ores, but as to its precise location and the nature of its processes we can only guess. Other features of distribution of minerals associated with igneous rocks are indicated by their grouping in metallogenic provinces and epochs (see pp. 308-309). THE RELATION OF CONTACT METAMORPHISM TO ORE BODIES OF THE FOREGOING CLASS. The deposition of ores of igneous source in the country rock into which the igneous rocks are intruded is a phase of contact metamorphism. Ordinarily where this deposition occurs there are further extensive replacements and alterations of the country rock, resulting in the development of great masses of quartz, garnet, pyroxene, amphibole, and other silicates, and in some cases of calcite, dolomite, siderite, barite, alunite, and other minerals. Looked at broadly, the deposition of ores at igneous contacts under contact metamorphism is a mere incident in the much more widespread and extensive alterations of this kind. Hence it is that the subject of contact metamorphism is of interest to economic geologists. The minerals here formed which do not constitute ores throw much light on the nature of the ore-bearing solutions, the conditions of temperature and pressure, and the processes which locally and incidentally develop the ore bodies. The subject, however, is a complex one, the full discussion of which belongs in treatises on metamorphism.[7] We may note only a few salient features. For many hundreds of yards the rocks adjacent to the intrusions may be metamorphosed almost beyond recognition. This is especially true of the limestone, which may be changed completely to solid masses of quartz and silicates. The shales and sandstones are ordinarily less vitally affected. The shales become dense, highly crystalline rocks of a "hornstone" type, with porphyritic developments of silicate minerals. The sands and sandstones become highly crystalline quartzites, spotted with porphyritic developments of silicates. Occasionally even these rocks may be extensively replaced by other minerals, as in the Coeur d'Alene district, where quartzites adjacent to the ore veins may be completely replaced by iron carbonate. A question of special interest to economic geologists is the source of the materials for the new minerals in these extensively altered zones. In some cases the minerals are known to be the result of recrystallization of materials already in the rock, after the elimination of certain substances such as carbon dioxide and lime, under the pressures and temperatures of the contact conditions. In such cases there has obviously been large reduction in volume to close the voids created by the elimination of substances. In the majority of cases, the new substances or minerals are clearly introduced from the igneous source, replacing the wall rock volume for volume so precisely that such original textures and structures as bedding are not destroyed. In many cases the result is clearly due to a combination of recrystallization of materials already present and introduction of minerals by magmatic solutions from without. So obvious is the evidence of the introduction of materials from without, that there has been a tendency in some quarters to overlook the extensive recrystallization of substances already present; and the varying emphasis placed on these two processes by different observers has led to some controversy. SECONDARY CONCENTRATION IN PLACE OF THE FOREGOING CLASSES OF MINERAL DEPOSITS THROUGH THE AGENCY OF SURFACE SOLUTIONS Mineral deposits of direct magmatic segregation are seldom much affected by surficial alteration, perhaps because of their coarse crystallization and their intermingling with resistant crystalline rocks. Mineral deposits of the "igneous after-effect" type may be profoundly altered through surficial agencies. The more soluble constituents are taken away, leaving the less soluble. The parts that remain are likely to be converted into oxides, carbonates, and hydrates, through reaction with oxygen, carbon dioxide, and water, which are always present at the surface and at shallow depths. These processes are most effective at the surface and down to the level of permanent ground-water, though locally they may extend deeper. This altered upper part of the ore bodies is usually called the _oxide zone_. It may represent either an enrichment or a depletion of ore values, depending on whether the ore minerals are taken into solution less rapidly or more rapidly than the associated minerals and rocks; all are removed to some extent. In certain deposits, there is evidence that both zinc and copper have been taken out of the upper zone in great quantity; but they happen to be associated with limestone, which has dissolved still more rapidly, with the result that there is a residual accumulation of copper and zinc values. Manganese, iron, and quartz are usually more resistant than the other minerals and tend to remain concentrated above. The same is true to some extent of gold and silver. The abundance of iron oxide thus left explains the name "iron cap" or "gossan" so often applied to the upper part of the oxide zone. Not infrequently, and especially in copper ores, the upper part of the oxide zone is nearly or entirely barren of values and is called the _capping_. The depth or thickness of the oxide zone depends on topography, depth of water table, climatic conditions, and speed of erosion. A fortunate combination of conditions may result in a deep oxide zone with important accumulations of values. In other cases erosion may follow oxidation so rapidly as to prevent the growth of a thick oxide zone. It is clear from the study of many ore deposits that the process of oxidation has not proceeded uniformly to the present, but has depended upon a fortunate combination of factors which has not been often repeated during geologic time. As illustrative of this, the principal oxidation of the Bisbee copper ores of Arizona (p. 204) occurred before Tertiary time, with reference to a place that has since been covered by later sediments. The conditions in the Ray, Miami, and Jerome copper camps of Arizona (pp. 203-205) likewise indicate maximum oxidation at an early period. The Lake Superior iron ore deposits (pp. 167-170) were mainly concentrated before Cambrian time, during the base-leveling of a mountainous country in an arid or semi-arid climate. The oxide zone of these deposits has no close relation to the present topography or to the present ground-water level. In the Kennecott (Alaska) copper deposits all oxidation has been stopped since glacial time by the freezing of the aqueous solutions. At Butte and at Bingham the main concentration of the ores is believed to have occurred in an earlier physiographic cycle than the present one. The _cyclic_ nature of the formation of oxide zones is of comparatively recent recognition, and much more will doubtless be found out about it in the comparatively near future. Its practical bearing on exploration is obvious (see p. 325). It should be clearly recognized that oxidizing processes are not limited to the zone above the ground-water level. Locally oxidizing solutions may penetrate and do effective work to much greater depths, especially where the rocks traversed at higher elevations are of such composition or in such a stage of alteration as not to extract most of the oxygen. Consequently the presence of oxide ores below the water table is not necessarily proof that the water table has risen since their formation. On the other hand, the facts of observation do indicate generally a marked difference, in circulation and chemical effect, between waters above and below this horizon, and show that oxidation is dominantly accomplished above rather than below this datum surface. During the formation of the oxide zone, erosion removes some of the ore materials entirely from the area, both mechanically and in solution. Part of the material in solution, however, is known to penetrate downward and to be redeposited in parts of the ore body below the oxide zone,--that is, usually below the water table. Evidence of this process is decisive in regard to several minerals. Copper is known to be taken into solution as copper sulphate at the surface, and to be redeposited as chalcocite where these sulphate solutions come in contact with chalcopyrite or pyrite below. Not only has the process been duplicated in the laboratory, but the common coating of chalcocite around grains of pyrite and chalcopyrite below the water level indicates that this process has been really effective. Sulphides of zinc, lead, silver, and other metals are similarly concentrated, in varying degrees. The zone of deposition of secondary sulphides thus formed is called the zone of _secondary sulphide enrichment_. Ores consisting mainly of secondary sulphides are also called _supergene_ ores (p. 33). In some deposits, as in the copper deposits of Ray and Miami, there is found, below the secondary sulphide zone, a lean sulphide zone which is evidently of primary nature. The mineralized material of this zone, where too lean to mine, has been called a _protore_. With the discovery of undoubted evidence of secondary sulphide enrichment, there was a natural tendency to magnify its importance as a cause of values. Continued study of sulphide deposits, while not disproving its existence and local importance, has in some districts shown clearly that the process has its limitations as a factor in ore concentration, and that it is not safe to assume its effectiveness in all camps or under all conditions. At Butte for instance, secondary chalcocite is clearly to be recognized. The natural inference was that as the veins were followed deeper the proportion of chalcocite would rapidly diminish, and that a leaner primary zone of chalcopyrite, enargite and other primary minerals would be met. However, the great abundance of chalcocite in solid masses which have now been proved to a depth of 3500 feet, far below the probable range of waters from the surface in any geologic period, seems to indicate that much of the chalcocite is primary. The present tendency at Butte is to consider as secondary chalcocite only certain sooty phases to be found in upper levels. The solid masses of chalcocite in the Kennecott copper mines seem hardly explainable as the result of secondary sulphide enrichment. No traces of other primary minerals are present and the chalcocite here is regarded as probably primary. The possible magnification of the process of secondary enrichment above referred to has had for its logical consequence a tendency to over-emphasize the persistence of primary ores in depth. The very use of the terms "secondary" and "primary" has suggested antithesis between surficial and deep ores. Progress in investigation, as indicated on previous pages, seems to indicate that the primary ores are not uniformly deep and that in many cases they are distinctly limited to a given set of formations or conditions comparatively near the surface. In general the processes of oxidation and secondary sulphide enrichment have been studied mainly by qualitative methods with the aid of the microscope and by considerations of possible chemical processes. These methods have disclosed the nature but not the quantitative range and relations of the different processes. Much remains to be done in the way of large scale quantitative analysis of ores at different depths, as a check to inferences drawn by other methods. One may know, for instance, that a mineral is soluble and is actually removed from the oxide zone and redeposited below. The natural inference, therefore, is that the mineral will be found to be depleted above and enriched below. In many cases its actual distribution is the reverse,--indicating that this process has been only one of the factors in the net result, the more rapid solution and deposition of other materials being another factor. If one were to approach the study of the concentration of iron ores with the fixed idea of insolubility of quartz from a chemical standpoint, and were to draw conclusions accordingly, he would fail to present a true picture of the situation. While quartz is insoluble as compared with most minerals, it is nevertheless more soluble than iron oxide, and therefore the net result of concentration at the surface is to accumulate the iron rather than the silica. Descriptions of enrichment processes as published in many reports are often misleading in this regard. They may be correct in indicating the actual existence of a process, but may lead the reader to assumptions as to net results which are incorrect. RESIDUAL MINERAL DEPOSITS FORMED BY THE WEATHERING OF IGNEOUS ROCKS IN PLACE Igneous rocks not containing mineral deposits may on weathering change to mineral deposits. The lateritic iron ores such as those of Cuba (p. 172), many bauxite deposits, many residual clays, and certain chromite and nickel deposits are conspicuous representatives of this class. The chemical and mineralogical changes involved in the formation of these deposits are pretty well understood. Certain constituents of the original rock are leached out and carried away, leaving other constituents, as oxides and hydrates, in sufficiently large percentage in the mass to be commercially available. The accumulation of large deposits depends on the existence of climatic and erosional conditions which determine that the residual deposit shall remain in place rather than be carried off by erosion as fast as made. In the glaciated parts of the world, deposits of this nature have usually been removed and dispersed in the glacial drift. When the minerals of these deposits are eroded, transported, and redeposited in concentrated form, they come under the class of placer or sedimentary deposits described under the following heading. There are of course many intermediate stages, where the residual deposit is only locally moved and where the distinction between this class of deposits and that next described is an arbitrary one. MINERAL DEPOSITS FORMED DIRECTLY AS PLACERS AND SEDIMENTS Mineral deposits of this class are of large value, including as they do salt, gypsum, potash, sulphur, phosphates, nitrates, and important fractions of the ores of iron, manganese, gold, tin, tungsten, platinum, and precious stones; also many common rocks of commercial importance. The minerals of these deposits are derived from the weathering and erosion of land surfaces, either igneous or sedimentary. They are deposited both under air and under water, both mechanically and chemically (in part by the aid of organisms). These deposits form the principal type of _syngenetic_ deposits (p. 32); the term _sedigenetic_ deposits has also been applied to them. MECHANICALLY DEPOSITED MINERALS Mechanical erosion of preëxisting mineral deposits or rocks and their transportation, sorting, and deposition are responsible for the placers of gold, tin, tungsten, platinum, and various precious stones, and for certain iron sands and conglomerates. Sands, sandstones, shales, and certain clays and bauxites also belong in this group. These deposits may be formed under air or under water, and under various climatic and topographic conditions. During the process of formation the minerals of differing density are more or less sorted out and tend to become segregated in layers. The process is not unlike the artificial process of mechanical concentration where ores are crushed, shaken up, and treated with running water. The process is most effective for minerals which are resistant to abrasion and to solution, and of such density as to differentiate them from the other minerals of the parent rock. The origin of deposits of this kind is fairly obvious where they are of recent age and have not been subsequently altered or buried. A considerable amount of experimental work has brought out clearly the main elements of the processes. Physiographic and climatic conditions play an important part, and cannot be safely overlooked by anyone studying such deposits. Extensive copper deposits exist as sediments (pp. 205-206). It is not clear to what extent they are mechanically or to what extent chemically deposited. For the most part the concentration of copper in this manner has not been sufficient to yield deposits of large commercial value; the mineral is too much dispersed. Relatively small amounts are mined in the Mansfield shales of Germany and the Nonesuch shales and sandstones of the Lake Superior country. The Clinton and similar iron ores of the United States and Newfoundland, the pre-Cambrian iron ores of Brazil, and the Jurassic iron ores of England and western Europe (pp. 166-167) are now commonly agreed to be direct sedimentary deposits in which mechanical agencies of sorting and deposition played a considerable part. How far chemical and bacterial agencies have also been effective is not clear. The climatic, topographic, and other physiographic and sedimentary conditions which cause the deposition of this great group of ores present one of the great unsolved problems of economic geology. The study of present-day conditions of deposition affords little clue as to the peculiar combination of conditions which was necessary to accomplish such remarkable results in the past. On the whole, minerals of this mechanically deposited group are not greatly affected by later surficial alteration and concentration, because, having already been subjected to weathering, they are in a condition to resist such influences. CHEMICALLY AND ORGANICALLY DEPOSITED MINERALS The products of surface weathering and erosion are in part carried away in chemical solution and redeposited as sediments. Sediments thus formed include limestone and dolomite, siderite, salt, gypsum, potash, sulphur, phosphates, nitrates, and other minerals. Precipitation may be caused by chemical reactions, by organic secretion, or by evaporation of the solutions. The processes are qualitatively understood and it is usually possible to ascertain with reasonable accuracy the conditions of depth of water, relation to shore line, climate, nature of erosion, and other similar factors; yet the vast scale of some of these deposits, and their erratic areal and stratigraphic distribution, present unsolved problems as to the precise combinations of factors which have made such results possible. Chemically and organically deposited minerals of this class are usually susceptible to further alteration by surface weathering, and some of them, for instance the phosphates and siderites, are thus secondarily concentrated. These processes are discussed under the next heading. In general the great unsolved problem of the origin of the entire group of mineral deposits in placers and sediments relates to the scale of the results. Observation of present-day processes and conditions of deposition of these minerals affords satisfactory evidence of their nature, but fails to give us a clear idea of the precise combinations of agencies and conditions necessary to produce such vast results as are represented by the mineral deposits. For example, solution of iron on a land surface and redeposition in bogs and lagoons (as actually observed to be taking place today) show how some iron-ore sediments may be formed; but these processes are entirely inadequate to explain the deposition of iron ores in thick masses over broad areas without intermingling of other sediments--as represented by the Clinton iron ores of North America, the Jurassic ores of Europe and England, and the ancient iron ores of Brazil. The Paleozoic seas in northern and eastern United States encroached over land areas to the north and east and deposited ordinary sediments such as sandstone, shale, and limestone. Suddenly, without, so far as known, tapping any new sources of supply on the ancient land areas, and without any yet ascertainable change in topographic or climatic conditions, they deposited enormous masses of iron ore. There is clearly some cyclic factor in the situation which we do not yet understand. The various deposits of salt, gypsum, potash, sulphur, and other minerals are known to be the result of evaporation, and the deposition of each of these minerals is known to be related to the degree of evaporation as well as to temperature, pressure, and factors such as mass action and crystallization of double salts. The nature of the processes is fairly well understood; but again, observation of the present-day operation of these processes fails to give us much clue to the enormous accumulations at certain times and places in the past. It is difficult to say just what conditions of climate, in combination with particular physiographic factors, could have preserved uniformity of conditions for the long periods necessary to account for some of the enormously thick salt deposits. Again some cyclic factor in the situation remains to be worked out. SEDIMENTARY MINERAL DEPOSITS WHICH HAVE REQUIRED FURTHER CONCENTRATION TO MAKE THEM COMMERCIALLY AVAILABLE The conditions for the direct deposition of sedimentary mineral deposits of the foregoing class are also responsible for the deposition of minerals in more dispersed or disseminated form, requiring further concentration through surface agencies to render them commercially available. Some of these deposits are discussed below. The lead and zinc ores of the Mississippi Valley, Virginia, Tennessee, Silesia, Belgium, and Germany (pp. 211-212, 216-219) are in sedimentary rocks far removed from igneous sources. Lead and zinc were deposited in more or less dispersed form with the enclosing sediments. It is supposed that deposition was originally chemical and was favored by the presence of organic material, which is a rather common accompaniment of the sediments. It is supposed further that these organic participants were originally localized during sedimentation in so-called estuarine channels and shore-line embayments. When subsequently exposed to weathering, the lead and zinc minerals were dissolved and redeposited in more concentrated form in fissures and as replacements of limestone. Agreement as to origin of these deposits, so far as it exists, does not go beyond these broad generalizations. There is controversy as to whether the original sources of the ore minerals were the sediments directly above, from which the mineral solutions have been transferred downward during weathering and erosion, or whether the original minerals were below and have been transferred upward by artesian circulation, or whether they were situated laterally and have been brought to their present position by movement along the beds, or whether there has been some combination of these processes. It is the writer's view that the evidence thus far gathered favors on the whole the conclusion of direct downward concentration from overlying sources which have been removed by erosion, although this conclusion fails to explain why certain sulphide deposits give so little evidence of important downward transfer from their present position. This matter is further discussed on pages 216-219. The choice of the various alternatives has some practical bearing on exploration. Since these ores were brought into approximately their present position, they have undergone considerable oxidation near the surface and secondary sulphide enrichment below. The chemical and mineralogical changes are pretty well understood, but the quantitative range of these changes and their relative importance in determining the net result are far from known. Undoubted evidence of secondary sulphide enrichment has led in some quarters to an assumption of effectiveness in producing values which is apparently not borne out by quantitative tests. A group of mineral deposits in sandstones in Utah is regarded as due to chemical concentration of material originally disseminated in the rock. They include silver, copper, manganese, uranium, and radium deposits. The Silver Reef deposits, including silver, copper, uranium, and vanadium, are commercially the most important of this type.[8] The ore minerals are commonly associated with carbonized material representing plant remains, and have replaced the calcareous and cementing material of the rock, and also some of the quartz grains. The deposits are regarded as having been formed by circulating waters which collected the minerals disseminated through the sedimentary rocks, and deposited them on contact with carbonaceous matter, earlier sulphides, or other precipitating agents. The circulation in some places is believed to have been of artesian character and to have been controlled to a large extent by structural features. The Silver Reef deposits are near the crest of a prominent anticline. Most of the minerals have been later altered by surface solutions. Another great group of ores to be considered under this head are the iron ores of Lake Superior,--which were originally deposited as sediments, called jaspers or iron formations, with too low a percentage of iron to be of use, and which have required a secondary concentration by surficial agencies to render them valuable. The process of concentration has been a simple one. The iron minerals have been oxidized in place and the non-ferrous minerals have been leached out, leaving iron ores. This process contrasts with the concentration described above, in that there is little evidence of collection of iron minerals from disseminated sources. The Lake Superior iron ores are essentially residual concentrations in place. The outstanding problems of secondary concentration relate to the structural features which determined the channels through which the oxidizing and leaching waters worked, and to the topographic and climatic conditions which existed at the time the work was done. As with many other classes of ores, it was first assumed that these processes were related to the present erosion surface; but it is now known that concentration happened long ago under conditions far different from those now existing. These deposits contribute to the rapidly accumulating evidence of the _cyclic_ nature of ore concentration. Our least satisfactory knowledge of the Lake Superior ores relates to the peculiar conditions which determined the initial stage of sedimentation of the so-called iron formation. As in the case of the Clinton iron ores, no present-day sedimentation gives an adequate clue. Students of the problem have fallen back on the association of the iron formation with contemporaneous volcanic rocks, as affording a possible explanation of the wide departure from ordinary conditions of sedimentation evidenced by these formations.[9] Coal deposits are direct results of sedimentation of organic material. They are mainly accumulations of vegetable matter in place. To make them available for use, however, they undergo a long period of condensation and distillation. Conditions of primary deposition may be inferred from modern swamps and bogs; but, as in the case of sediments described under the preceding heading, we are sometimes at a loss to explain the magnitude of the process, and especially to explain the maintenance of proper surface conditions of plant growth and accumulation for the long periods during which subsidence of land areas and encroachment of seas are believed to have been taking place. The processes of secondary concentration are also understood qualitatively, but much remains to be learned about the influences of pressure and heat, the effect of impervious capping rocks, and other factors. Various oil shales and asphaltic deposits are essentially original sediments which have subsequently undergone more or less decay and distillation. The migration of the distillates to suitable underground reservoirs is responsible for the accumulation of oil and gas pools. Oil and gas are distillates from these oil shales and asphaltic deposits, and also from other organic sediments such as carbonaceous limestones. The distillates have migrated to their present positions under pressure of ground-waters. The stratigraphic horizons favorable to their accumulation are generally recognized. The geologist is concerned in identifying these horizons and in ascertaining where they exist underground. He is further concerned in analysis of the various structural conditions which will give a clue to the existence of local reservoirs in which the oil or gas may have been accumulated. So capricious are the oil migrations that the most intensive study of these conditions still leaves vast undiscovered possibilities. ANAMORPHISM OF MINERAL DEPOSITS Mineral deposits formed in any one of the ways indicated above may undergo repeated vicissitudes, both at the surface and deep below the surface, with consequent modifications of character. They may be cemented or replaced by introduction of mineral solutions from without. They may be deformed by great earth pressures, undergoing what is called dynamic metamorphism (pp. 25-27), which tends to distort them and give them schistose and crystalline characters. They may be intruded by igneous rocks, causing considerable chemical, mineralogical, and structural changes. All these changes may take place near the surface, but on the whole they are more abundant and have more marked effects deep below the surface. In general all these changes of the deeper zone tend to make the rocks more crystalline and dense and to make the minerals more complex. Cavities are closed. The process is in the main an integrating and constructive one which has been called _anamorphism_, to contrast it with the disintegrating and destructive processes near the surface, which have been called _katamorphism_ (see also pp. 27-28). There is little in the process of anamorphism in the way of sorting and segregation which tends to enrich and concentrate the metallic ore bodies. On the contrary the process tends to lock up the valuable minerals in resistant combinations with other substances, making them more difficult to recover in mining. Later igneous intrusions or the ordinary ground-waters may bring in minerals which locally enrich ores under anamorphic conditions, but these are relatively minor effects. An illustration of the general effect is afforded by a comparison of the Cuban iron ores, which are soft and can be easily taken out, with the Cle Elum iron ores of Washington, which seem to be of much the same origin, but which have subsequently been buried by other rocks and rendered hard and crystalline. In the first case the ores can be mined easily and cheaply with steam shovels at the surface. In the second, underground methods of mining are required, which cost too much for the grade of ore recovered. On the other hand, the same general kind of anamorphic processes, when applied to coal, result in concentration and improvement of grade. The same is true up to a certain point in the concentration of oil; but where the process goes too far, the oil may be lost (pp. 140-141). CONCLUSION Mineral deposits are formed and modified by practically all known geologic processes, but looked at broadly the main values are produced in three principal ways: (1) As after effects of igneous intrusion, through the agency of aqueous and gaseous solutions given off from the cooling magma. (2) Through the sorting processes of sedimentation,--the same processes which form sandstone, shale, and limestone. Organic agencies are important factors in these processes. (3) Through weathering of the rock surface in place, which may develop values either by dissolving out the valuable minerals and redepositing them in concentrated form, or by dissolving out the non-valuable minerals and leaving the valuable minerals concentrated in place. The latter process is by far the more important. The overwhelming preponderance of values of mineral deposits as a whole is found in the second of the classes named. Under all these conditions it appears that the maximum results are obtained at and near the surface. On the scale of the earth even the so-called deep veins may be regarded as deposits from solutions reaching the more open and cooler outer portions of the earth. However, valuable mineral deposits are found in the deepest rocks which have been exposed by erosion, and the question of what would be found at still greater depths, closer to the center of the earth, is a matter of pure speculation. Ultimately all minerals are derived from igneous sources within the earth. The direct contributions from these sources are only in small part of sufficient concentration to be of value; for the most part they need sorting and segregation under surface conditions. We can only speculate as to causes of the occurrence of valuable minerals in certain igneous rocks and not in others. Many granites are intruded into the outer shell of the earth, but only a few carry "minerals"; also, of a series of intrusions in the same locality, only one may carry valuable minerals. It is clear that in some fashion these minerals are primarily segregated within the earth. Causes of this segregation are so involved with the problem of the origin of the earth as a whole that no adequate explanation can yet be offered. Our inductive reasoning from known facts is as yet limited to the segregation within a given mass of magma, and even here the conditions are only dimly perceived. A discussion of these ultimate problems is beyond the scope of this book. FOOTNOTES: [4] Ransome, Frederick Leslie, Copper deposits near Superior, Arizona: _Bull. 540, U. S. Geol. Survey_, 1914, pp. 152-153; The copper deposits of Ray and Miami, Arizona: _Prof. Paper 115, U. S. Geol. Survey_, 1919, p. 156; Discussion: _Econ. Geol._, vol. 8, 1913, p. 721. [5] For more specific definitions of vertical zones of ore deposition in association with igneous rocks see Spurr, J. E., Theory of ore deposition: _Econ. Geol._, vol. 7, 1912, pp. 489-490; Lindgren, W., _Mineral deposits_, McGraw-Hill Book Co., 2d ed., 1919, Chapters XXIV-XXVI; and Emmons, W. H., _The principles of economic geology_, McGraw-Hill Book Co., 1918, Chapters VI-VIII. An excellent discussion of a case of vertical and areal zoning of minerals is contained in _Ore deposits of the Boulder batholith of Montana_, by Paul Billingsley and J. A. Grimes, Bull. Am. Inst. Min. Engrs., vol. 58, 1918, pp. 284-368. [6] Butler, B. S., Loughlin, G. F., Heikes, V. C., and others, The ore deposits of Utah: _Prof. Paper 111, U. S. Geol. Survey_, 1920, p. 201. [7] Leith, C. K., and Mead, W. J., _Metamorphic Geology_, Pt. 2, Henry Holt and Company, New York, 1915. [8] Butler, B. S., Loughlin, G. F., Heikes, V. C., and others, The ore deposits of Utah: _Prof. Paper 111, U. S. Geol. Survey_, 1920, pp. 152-158. [9] Van Hise, C. R., and Leith, C. K., Geology of the Lake Superior region. _Mon. 52, U. S. Geol. Survey_, 1911, pp. 506-518; and references there given. CHAPTER IV MINERAL RESOURCES--SOME GENERAL QUANTITATIVE CONSIDERATIONS Of the 1,500 known mineral species, perhaps 200 figure in commerce as mineral resources. For the mineral substances used commercially, the term "mineral" is used in this chapter with a broad significance to cover any or all of the materials from which the needed elements are extracted,--whether these materials be single minerals or groups of minerals; whether they be rocks or ores; whether they be liquid or solid. The following figures are generalizations based on the miscellaneous information available. The purpose is to indicate the general perspective rather than the detail which would be necessary for precise statement. WORLD ANNUAL PRODUCTION OF MINERALS IN SHORT TONS Exclusive of water, but inclusive of petroleum, the world's annual output of mineral resources amounts to two billions of tons. This figure refers to the crude mineral as it comes from the ground and not to the mineral in its concentrated form. Of this total extraction, coal amounts to nearly 70 per cent, stone and clay 10 per cent, iron ore about 9 per cent, petroleum 4 per cent, copper ore 3 per cent, and all the remaining minerals constitute less than 6 per cent. If spread out on the surface in a uniform mass with an estimated average density based on relative proportions of the crude minerals, this annual production would cover a square mile to a depth of 2,300 feet. Of the total annual production 85 per cent comes from countries bordering the North Atlantic basin; 75 per cent is accounted for by the United States, England, and Germany; the United States has 39 per cent of the total, England 18 per cent, and Germany 18 per cent. By continents, Europe accounts for nearly 51 per cent, North America for nearly 42 per cent, Asia for nearly 4 per cent, and the remaining continents for nearly 4 per cent. The United States mineral production in recent years has been about 900,000,000 tons. According to the United States census of 1920, nearly half of all the establishments or businesses engaged in quarrying or mining operations in this country are operating in oil and gas. Of the crude materials extracted from the ground perhaps 10 per cent, including gold, silver, copper, lead, zinc, nickel, and other ores, are concentrated mainly at the mine, with the result that this fraction of the tonnage in large part does not travel beyond the mine. About 90 per cent of the total production, therefore, figures largely in the transportation of mineral resources. It is estimated that roughly two-thirds of the annual world production is used or smelted within the countries of origin, the remaining one-third being exported. Of the minerals moving internationally, coal and iron constitute 90 per cent of the tonnage. The metal smelting capacity of the world in terms of yearly production of crude metal is estimated at nearly 100,000,000 short tons. Of this amount about 80 per cent is located in the United States, England, and Germany. The United States alone has over half of the total. Of the oil-refining capacity the United States controls nearly 70 per cent. One of the significant features of the situation above summarized is the concentration of production and smelting in a comparatively few places in the world. This statement applies with even more force to the individual mineral commodities. Water may be regarded as a mineral resource in so far as it is utilized as a commodity for drinking, washing, power, irrigation, and other industrial uses. For purposes of navigation and drainage, or as a deterrent in excavation, it would probably not be so classed. While it is not easy to define the limits of water's use as a mineral resource, it is clear that even with a narrow interpretation the total tonnage extracted from the earth as a mineral resource exceeds in amount all other mineral resources combined. WORLD ANNUAL PRODUCTION OF MINERALS IN TERMS OF VALUE In terms of value, mineral resources appear in different perspective. The annual world value of mineral production, exclusive of water, is approximately $9,000,000,000. This figure is obtained by dividing the annual value of the United States output of each of the principal minerals by the percentage which the United States output constitutes in the world output, and adding the figures thus obtained. The values here used are mainly selling prices at the mines. It is impossible to reduce the figures absolutely to the value of the mineral as it comes from the ground; there are always some items of transportation included. This method of figuring is of course only the roughest approximation; the values as obtained in the United States cannot be accurately exterpolated for the rest of the world because of locally varying conditions. However, the figures will serve for rough comparative purposes. Of this total value coal represents roughly 61 per cent, petroleum 12 per cent, iron 6 per cent, copper 5 per cent, and gold 3 per cent. In terms of value, about 25 per cent of the world's mineral production is available for export beyond the countries of origin. Of this exportable surplus the United States has about 40 per cent, consisting principally of coal, copper, and formerly petroleum. The value of the United States annual mineral production in recent years has been from about $3,500,000,000 to $5,500,000,000. Annual imports of mineral products into the United States have averaged recently in the general vicinity of $450,000,000, the larger items being copper, tin, fertilizers, petroleum, gems and precious stones, manganese, nickel, and tungsten. Again the perspective is changed when the value of water resources is considered. As a physiologically indispensable resource, the value of water in one sense is infinite. There is no way of putting an accurate value on the total annual output used for drinking and domestic purposes,--although even here some notion of the magnitude of the figures involved may be obtained by considering the average per capita cost of water in cities where figures are kept, and multiplying this into the world population. This calculation would not imply that any such amount is actually paid for water, because the local use of springs, wells, and streams can hardly be figured on a cash basis; but, if human effort the world over in securing the necessary water is about as efficient as in the average American city, the figures would indicate the total money equivalent of this effort. SIGNIFICANCE OF GEOGRAPHIC DISTRIBUTION OF MINERAL PRODUCTION The remarkable concentration of the world's mining and smelting around the North Atlantic basin, indicated by the foregoing figures, does not mean that nature has concentrated the mineral deposits here to this extent. It is an expression rather of the localized application of energy to mineral resources by the people of this part of the world. The application of the same amount of energy in other parts of the world would essentially change the distribution of current mineral production. The controlling factor is not the amount of minerals present in the ground; this is known to be large in other parts of the world and more will be found when necessary. Controlling factors must be looked for in historical, ethnological, and environmental conditions. This subject is further discussed in the chapters on the several resources, and particularly in relation to iron and steel. THE INCREASING RATE OF PRODUCTION The extraction of mineral resources on the huge scale above indicated is of comparatively recent date. From 1880 to the end of 1918 the value of the annual mineral production of the United States has increased from $367,000,000 to more than $5,500,000,000, or nearly fifteen times; measured in another way, it has increased from a little over $7 per capita to more than $52.[10] More coal has been mined in the United States since 1905 than in all the preceding history of the country. More iron ore has been mined since 1906 than in all the preceding history. The gold production of the United States practically started with the California gold rush in 1849. The great South African gold production began in 1888. Production of diamonds in South Africa began about 1869. The large use of all fertilizer minerals is of comparatively recent date. The world's oil production is greater now each year than it was for any ten years preceding 1891, and more oil has come out of the ground since 1908 than in all the preceding history of the world. The use of bauxite on a large scale as aluminum ore dated practically from the introduction of patented electrolytic methods of reduction in 1889. In one sense the world has just entered on a gigantic experiment in the use of earth materials. The most striking feature of this experiment relates to the vast acquisition of power indicated by the accelerating rate of production and consumption of the energy resources--coal, oil, and gas (and water power). Since 1890 the per capita consumption of coal in the United States has trebled and the per capita consumption of oil has become five times as great as it was. If the power from these sources used annually in recent years be translated roughly into man power, it appears that every man, woman, and child in the United States has potential control of the equivalent of thirty laborers,--as against seven in 1890. Energy is being released on a scale never before approximated, with consequences which we can yet hardly ascertain and appraise. This consideration cannot but raise the question as to the ability of modern civilization to control and coödinate the dynamic factors in the situation. CAPITAL VALUE OF WORLD MINERAL RESERVES It is impossible to deduce accurately the capital value of mineral resources from values of annual output, but again some approximation may be made. The profit on the extraction of mineral resources on the whole, considering the cost of exploration, is probably no greater than in other industries (p. 330). If we assume a 6 per cent return, which perhaps is somewhere near the world-wide standard of interest rate for money, and capitalize the value of the world's annual output at this rate, we obtain a world capital value for mineral resources, exclusive of water, of 150 billions of dollars. This assumes an indefinitely long life for reserves. This assumption may need some qualifications, but it is the writer's view (Chapter XVII) that it is justified for a sufficiently long period to substantiate the above method of calculation. [Illustration: FIG. 2. Commercial (financial) control of the mineral resources of the world.] [Illustration: FIG. 3. Political (territorial) control of the mineral resources of the world.] POLITICAL AND COMMERCIAL CONTROL OF MINERAL RESOURCES The occurrence of a mineral resource within a country does not necessarily mean control by that particular political unit. A citizen of the United States may own a mineral resource in South America. Commercial control of this sort was demonstrated during the war to be of more far-reaching significance than had been supposed, and it became necessary to ascertain, not only the output of the different countries, but the commercial control of this output. Investigation of this subject for twenty-three leading commodities shows that the political and commercial control are by no means the same. These are partly summarized in the accompanying graphs from Spurr.[11] It is to be noted that the graphs show the control of many commodities as it existed in 1913, the last normal year before the war. Changes during and since the war have of course largely altered the situation for certain commodities, notably for iron, coal, and potash. These developments are summarized in the discussion of the individual resources. It is also to be noted that the commercial or financial control of the world's minerals, under the influence of the fostering and protective policies of certain governments discussed in Chapter XVIII, is at present in a state of flux. Considerable changes are taking place today and are to be looked for in the future. RESERVES OF MINERAL RESOURCES Annual production figures are only to a very partial extent an indication of the distribution of the great reserves of mineral resources. For instance, there are enormous reserves of coal in China which are not yet utilized to any large extent. The minerals of South America and Africa are in a very early stage of development. The total world reserves will of course not be known until exploration and development of the world's resources are complete--a time which will probably never come. Figures of reserves represent only our present partial state of knowledge and are likely to be considerably modified in the future. Furthermore, the quantitative accuracy of knowledge of reserves is so variable in different parts of the world that it is almost impossible to make up world figures which have any great validity. There are, however, certain broad facts ascertainable. Every country in the globe is deficient in supplies of some minerals. The United States is better off than any other country, but still lacks many mineral commodities (see pp. 396-399.) No single continent has sufficient reserves of all mineral commodities. For the world, however, it may be stated with reasonable certainty that the reserves of the principal minerals are now known to be ample with the exception of those of oil, tin, and perhaps gold and silver. By _ample_ we mean sufficient to give no cause for worry for the next few decades. For many mineral commodities the amounts now actually in sight will not last long, but the possibilities of extension and discovery are so great that a long future availability of these commodities can be counted upon with reasonable safety. The present shortages in oil, tin, and other minerals mentioned may be only temporary. There is a large part of the world still to be explored, and the present reserves merely mark a stage in this exploration. Nevertheless, the ratio of reserves and discovery on the one hand to accelerated use on the other gives cause for much concern. Looking forward to the future, the problem of mineral reserves in general is not one of the possible ultimate amount which the earth may contain--presumably in no case is this deficient--but of the success with which the resource may be found and developed to keep up with the rapid acceleration of demand. In the chapter on conservation the suggestion is made that future difficulties are more likely to arise from failure to coödinate the dynamic factors of supply and demand, than from absolute shortage of material in the earth. FOOTNOTES: [10] Bastin, Edson S., and McCaskey, H. D., The work on mineral resources done by the U. S. Geological Survey: _Min. Res. of the United States for 1918, U. S. Geol. Survey_, pt. 1, 1920, p. 3a. [11] Spurr, J. E., Who owns the earth?: _Eng. and Min. Jour._, vol. 109, 1920, pp. 389-390. CHAPTER V WATER AS A MINERAL RESOURCE GENERAL GEOLOGIC RELATIONS With the solid earth as the special care of geology, it may seem presumptuous for the geologist to claim the waters thereof, but he does not disclaim this inheritance. Water is so all-pervasive that it is more or less taken for granted; and so many and so intricate are its relations that it is not easy to make an objective survey of the water problem in its relation to geology. The original source of water, as well as of air, is in molten magmas coming from below. These carry water and gases,--some of which are released and some of which are locked up in the rocks on cooling, to be later released during the alterations of the rocks. It is supposed, whatever theory of the origin of the earth we favor, that in its early stages the earth lacked both hydrosphere and atmosphere, and that during the growth of the earth these gradually accumulated on and near the surface in the manner stated. During alterations at the surface water is added to the mineral constitution of the rocks, and by alterations deep below the surface it may be subtracted. Water is the agent through which most mineral and chemical changes of rocks are accomplished. It is the agent also which is mainly responsible for the segregation of mineral deposits. Water, both as running water and in the solid form of ice, plays an important part in determining the configuration of the earth's surface. Water is the medium in which most sedimentary rocks are formed. It is an important agent in the development of soil and in organic growth. These various influences of water on geological processes touch the economic field at many points, especially in relation to the concentration of ores and to the development of soils and surface forms. Water comes even more directly into the field of economic geology as a mineral resource. Water supplies, for the greatest variety of purposes, involve geologic considerations at almost every turn. Finally, water may be an aid or a hindrance to excavation and to a great variety of structural operations, both in war and in peace; and in this relation it again affords geologic problems. The part played by water in geologic processes, such as that of mineral segregation, is more or less incidentally discussed in other chapters. We may consider more fully in this chapter the application of geology to the general subject of water supplies. From the geological point of view, water is a mineral,--one of the most important of minerals,--as well as a constituent of other minerals. It becomes a mineral resource when directly used by man. It is ordinarily listed as a mineral resource when shipped and sold as "mineral water," but there is obviously no satisfactory line between waters so named and water supplies in general, for most of them are used for the same purposes and none of them are free from mineral matter. Water which is pumped and piped for municipal water supply is as much a mineral resource as water which is bottled and sold under a trade name. Likewise water which is used for irrigation, water power, and a wide variety of other purposes may logically be considered a mineral resource. Notwithstanding the immense economic importance of water as a mineral resource its value is more or less taken for granted, and considerations of valuation and taxation are much less in evidence than in the case of other mineral resources. Water must be had, regardless of value, and market considerations are to a much less extent a limiting factor. Economic applications of geology to this resource are rather more confined to matters of exploration, development, total supply, and conservation, than to attempts to fix money value. DISTRIBUTION OF UNDERGROUND WATER Free water exists in the openings in rocks where it is sometimes called _hygroscopic_ water. There is also a large amount of water combined molecularly with many of the minerals of rocks, in which form it is called _water of constitution_. This water is fixed in the rock so that it is not available for use, though some of the processes of rock alteration liberate it and contribute it to the free water. The immediate source of underground water, both free and combined, is mainly the surface or rain waters. A subordinate amount may come directly from igneous emanations or from destruction of certain hydrous minerals. Ultimately, as already indicated, even the surface water originates from such sources. The openings in rocks consist of joints and many other fractures, small spaces between the grains of rocks (pore space), and amygdaloidal and other openings characteristic of surface volcanic rocks. Many of these openings are capillary and sub-capillary in size. Most rocks, even dense igneous rocks, are porous in some degree, and certain rocks are porous in a very high degree. The voids in some surface materials may amount to 84 per cent of the total volume. In general the largest and most continuous openings are near the surface,--where rocks on the whole are more largely of the sedimentary type and are more fractured, disintegrated, and decomposed, than they are deep within the earth. The largest supplies of water are in the unconsolidated sediments. The water in igneous and other dense rocks is ordinarily in more limited quantity. APPROXIMATE QUANTITY OF WATER WHICH WILL BE ABSORBED BY SOILS AND ROCKS[1] ------------------------------------------------------ _Volume of water asborbed _Material_ per 100 of material_ ------------------------------------------------------ Sandy soil[2] 45.4 Chalk soil[2] 49.5 Clay[2] 50-52.7 Loam[2] 45.1-60.1 Garden earth [2] 69.0 Coarse sand [2] 39.4 Peat subsoil[2] 84.0 Sand 30-40 Sandstone 5-20 Limestone and dolomite 1-8 Chalk 6-27 Granite 03.-.8 ------------------------------------------------------ 1: Mead, Daniel W., _Hydrology_: McGraw-Hill Book Co., New York, 1919, p. 393. 2: Woodward, H. B., _Geology of soils and substrata_: Edward Arnold, London, 1912. Immediately at the surface, the openings of rocks may not be filled with water; but below the surface, at distances varying with climatic and topographic conditions, the water saturates the openings of the rocks and forms what is sometimes called the _zone of saturation_ or the _sea of underground water_. The top surface of this zone is called the _water table_, or the _ground-water level_. The space between the water table and the earth's surface is sometimes referred to as the _vadose zone_ or the _zone of weathering_, since it is the belt in which weathering processes are most active. The zone of weathering is not necessarily dry. Water from the surface enters and sinks through it and water also rises through it from below; it may contain suspended pockets of water surrounded by dry rocks; it is not continuously and fully saturated. The water table or ground-water level may be near or at the surface in low and humid areas, and it may be two thousand feet or more below the surface in arid regions of high topographic relief. Because of the influence of capillarity, the water table is not a horizontal surface. It shows irregularities more or less following the surface contours, though not nearly so sharply accentuated. The lower limit of the ground-water is more irregular than the upper surface and is less definitely known. In general, openings in rocks tend to diminish with depth, due to cementation and to closing of cavities by pressures which are too great for the rock to withstand. But rocks differ so widely in their original character, and in their response to physical and chemical environment, that it is not unusual to find dense and impervious rocks above, and open and porous rocks below. The lower limit of the zone of abundant underground water varies accordingly. A well may encounter nearly dry rock at a comparatively shallow depth, or it may reach a porous water-bearing stratum at considerable depth. At the greater depths pockets of water are sometimes found which have a composition different from that of the surface water, and which evidently are isolated from the surface water by zones of non-pervious rock. Attempts have been made to calculate the total volume of underground water by measuring the openings of rocks and making assumptions as to the depth to which such openings may extend. In this manner it has been estimated that, if all the ground-water were assembled in a single body, it would make a shell between eighty and two hundred feet thick (depending on the assumptions) over all the continental areas. MOVEMENT OF UNDERGROUND WATER Availability of water supplies is determined by the movement or flow of water as well as by its distribution and amount. The natural flow of water underground is caused by gravity in the larger openings, but in the smaller openings adhesion and capillarity are also important forces. Of all the water falling on the surface, some may not go below the surface at all but may immediately evaporate or join the runoff--that is, the surface streams. Another part may penetrate a little distance into the zone of weathering and then join the runoff. Of the water which reaches the zone of saturation, a part may soon come to the surface in low areas and join the runoff, and a part may penetrate deeply. Above the zone of saturation gravity carries the water downward in devious courses until it reaches the water table. Thereafter its course is determined largely by the lowest point of escape from the water table. In other words, the water table is an irregular surface; and under the influence of gravity the water tends to move from the high to the low points of this surface. Between the point of entrance and the point of escape from the water table, the water follows various courses, depending upon the porosity and the openings in the rocks. In general it fills all of the available openings, and uses the entire available cross section in making its progress from one point to another. The difference in height or the "head" between the point of entrance and the point of escape, together with the porosity of the rock and other factors, determine the general speed of its movement (see p. 73). With equal porosity the flow is at a maximum along a line directly connecting the two points, and on more devious courses the flow is less. The surface water first enters the ground through innumerable small openings. Soon, however, it tends to be concentrated into channels of easiest flow, with the result that in the later part of its underground course it may be much concentrated in large trunk channels. These channels may consist of joints, or frequently of very coarse and pervious beds. The sedimentary rocks as a whole contain the most voids, and therefore the largest flow and largest supply of water is often localized in them. Of the sedimentary rocks, sandstones and limestones usually contain the largest and most continuous openings, and thus afford the freest circulation for water. The voids in fine-grained shales may equal in volume those in sandstones and limestones, but the openings are so small and discontinuous that the water does not flow freely. Regardless of total amount of water, unless there are continuous openings of some size the flow may be small. The relations of more porous rocks to containing impervious strata also profoundly affect the flow of underground water. Between impervious strata the circulation may be concentrated and vigorous within the porous bed. Where the porous bed is not so contained, the movement may be more dispersed and less vigorous locally. The inclination of the beds, of course, also affects the direction and amount of the flow. The influence of gravity upon underground water may locally tend toward a state of equilibrium in which there is little movement. In such a case the water is substantially ponded, and moves only when tapped by artificial openings. WELLS AND SPRINGS Underground water becomes available for use by means of springs and through wells or bore holes. Water rises to the surface in natural springs at points where the pressure or _head_, due to its entrance into the ground at a higher level, is sufficient to force it to the surface after a longer or shorter underground course. The movement may be all downward and lateral to the point of escape, or it may be downward, lateral, and upward. Ordinarily, the course of spring waters does not carry them far below the surface. Heat and gases may be added beneath the surface by contact with or contributions from cooling igneous rocks. These may accelerate the upward movement of spring waters, and yield thermal and gas-charged waters, as in the springs and geysers of Yellowstone Park. When a well is sunk to tap the underground water supply, the water may not rise in the artificial opening but may have to be lifted to the surface. If, however, the water is confined beneath an impervious stratum and is under pressure from the water of higher areas, a well opening may simply allow it to move upward under its own pressure or head. This pressure may carry it upward only a few feet or quite to the surface or beyond, in which latter case the well is called an _artesian_ well. The essential condition for an artesian circulation is a porous zone, inclining downward from the surface beneath an impervious stratum which tends to confine and pond the water. The water at any point in the water-bearing rock is under pressure which is more or less equivalent to the weight of the column of water determined by the difference in height between this point and the point of entrance or feeding area of the water. If the feeding area is higher than the collar of the well, the water will rise quite to the surface; if not, it will rise only part way. Capillary resistance, however, may and usually does lessen the theoretical pressure so figured. The flow in deep artesian circulations is ordinarily a slow one. For the artesian wells of southern Wisconsin, it has been calculated that waters entering the outcrop of the southward dipping sandstone and limestone layers in the northern part of the state have required two or three hundred years to reach a point in the southern part of the state where they are tapped. Because of this slow movement, a large number of wells in any one spot may exhaust the local supply faster than it is replenished from the remainder of the formation. The drilling of additional wells near at hand in such cases does not increase the total yield, but merely divides it among a larger number of wells. The porosity of the rocks, and therefore the flow of an artesian circulation, may in some cases be artificially increased by blasting and shattering. COMPOSITION OF UNDERGROUND WATERS Underground waters are never entirely free from dissolved mineral substances, and seldom are they free from suspended particles. Some waters are desired because they contain very small quantities of dissolved mineral matter. Others are prized because they have an unusually high content of certain mineral substances. In determining the deleterious or beneficial effect of dissolved substances, much depends on the purpose for which the water is to be used,--whether for drinking, washing, steam boilers, or irrigation. Near the surface underground waters may carry bacteria, as well as animal and vegetable refuse, which from a sanitary standpoint are usually objectionable. Deeper waters are more likely to lack this contamination because of filtration through rocks and soils. The dissolved mineral substances of underground water are derived for the most part from the solution of rocks with which the waters come in contact, particularly at or near the surface. Through the agency of underground water most of the mineral and chemical changes of rocks are produced. The dissolved substances in solution at any time and place may therefore be regarded as by-products of rock alterations. Locally they may to some extent be derived from direct emanations from cooling igneous masses. The most common mineral substances contained in waters are lime and magnesia. Less common, but abundant locally, are soda, potash, iron, and silica. Waters contain also certain acid and gaseous substances, the most common of which is carbon dioxide; and less widespread, but locally abundant, are chlorine and sulphur dioxide. Where lime and magnesia are abundant the water is ordinarily classed as a hard water. Where absent, or subordinate to soda and potash, the water is ordinarily classed as a soft water. Large amounts of the acid substances like chlorine and sulphur are detrimental for most purposes. Where there are unusual amounts of carbon dioxide or other gases present, they may by expansion cause the water to bubble. If we were to attempt to describe and define the characteristics, with reference to dissolved mineral content and temperature, which make a given water more desirable than another, we should enter a field of the most amazing complexity and one with many surprising contradictions. For the most widespread use, the most desirable water is a cold water as free from mineral content as possible, and especially one lacking an excess of lime and magnesia which make it hard; also lacking an excess of acid constituents like sulphur dioxide, carbon dioxide, or chlorine, which give the water a taste, or which make impossible its use in boilers. Locally and for special reasons, waters of other qualities are in demand. Waters so excessively carbonated as to bubble, sulphureted waters, chlorine waters, waters high in iron, high in silica, high in potash, high in soda, or high in magnesia, or waters of high temperature, may come to be regarded as desirable. It is an interesting fact that any water with unusual taste, or unusual mineral content, or unusual temperature, is likely to be regarded as having medicinal value. Sometimes this view is based on scientific knowledge; sometimes it is an empirical conclusion based on experience; and again it may be merely superstition. In one case the desirable feature may be the presence of a large amount of carbon dioxide; in another case it may be its absence. In one case the desirable feature may be high temperature; in another case low temperature. The same combination of qualities which in a certain locality may be regarded as highly desirable, may be regarded as highly detrimental somewhere else where certain other types of waters are in vogue. Proprietary rights and advertising have brought certain waters into use for drinking purposes which are not essentially different from more widely available waters which are not regarded as having special value. Two springs located side by side, or a spring and a deep well, whose waters have exactly the same chemical characteristics, may be used and valued on entirely different scales. Any attempt to classify mineral waters sold to the public in any scientific way discloses a most intricate and confused situation. One can only conclude that the popularity of certain waters is not based alone on objective qualities of composition, but rather on causes which lie in the fields of psychology and commerce. The part played by sentiment in putting value on water is well illustrated by the general preference for spring waters as compared with well waters. In the public mind, "spring water" denotes water of unusual purity and of more desirable mineral content than well water. Illustrations could be cited of districts in which the surface or spring waters have a composition not different from that of the deeper well waters, and are much more likely to be contaminated because of proximity to the surface; and yet people will pay considerable sums for the spring water in preference to the cheaply available well water. RELATION OF GEOLOGY TO UNDERGROUND WATER SUPPLY It is obvious that a knowledge of geology is helpful in locating an underground water supply. Locally the facts may become so well known empirically that the well driller is able to get satisfactory results without using anything but the crudest geologic knowledge; but in general, attention to geologic considerations tends to eliminate failures in well drilling and to insure a more certain and satisfactory water supply. In drilling for water, it is essential to know the nature, succession, and structure of the rocks beneath the surface in order to be able to identify and correlate them from drill samples. The mere identification of samples is often sufficient to determine whether a well has been drilled far enough or too far to secure the maximum results. In order to arrive at any advance approximation of results for a given locality, a knowledge of the general geology of the entire region may be necessary. Especially for expensive deep artesian wells it is necessary to work out the geologic possibilities well in advance. It is useless, for instance, to look for artesian water in a granite; but in an area of gently inclined strata, with alternations of porous and impervious layers, the expert may often figure with a considerable degree of certainty the depth at which a given porous stratum will be found, and the pressure under which the water will be in this particular stratum at a given point. Even the mineral content of the water may in some cases be predicted from geologic study. One of the most obvious and immediately useful services of the geologist in most localities is the collection and preservation of well samples for purposes of identification and correlation of rock formations, and as a guide to further drilling. Failure to preserve samples has often led to useless and expensive duplication of work. The problem of water supply in some localities is comparatively simple and easy. In other areas there is an infinite variety of geologic conditions which affect the problem, and the geologist finds it necessary to bring to bear all the scientific knowledge of any sort which can be used,--particularly knowledge in relation to the type of rock, the stratigraphy and the structure. SURFACE WATER SUPPLIES Where underground water is not abundant or not cheaply available, or where larger amounts of water are needed, as in large cities or for irrigation purposes, surface water is used. In general, surface waters are more likely to be contaminated by vegetable and animal matter and to require purification for drinking purposes. Surface waters are also used for irrigation, water power, drainage, the carrying of sewage, etc. This great variety of uses brings the consideration of surface waters into many fields other than geology, but an understanding and interpretation of the geological conditions is none the less fundamental. This is evidenced by the inclusion of geologic discussions in most textbooks of hydrology, and in the reports of the Hydrographic Branch of the U. S. Geological Survey. The very fact that this important branch of governmental investigation is in a charge of the U. S. Geological Survey indicates its close relation to geology. The principles of geology used in the study of surface waters relate chiefly to physiography (see Chapter I). It is usually necessary to know the total quantity of flow, its annual and seasonal variation, and the possible methods of equalization or concentration; the maximum quantity of flow, the variation during periods of flood, and the possibilities of reduction or control; the minimum flow and its possible modification by storage or an auxiliary supply. These questions are obviously related to the size and shape of the catchment area, the topography, the rock structure, the relation between underground flow or absorption and the runoff, and other physiographic factors. Quoting from D. W. Mead:[12] Geological conditions are frequently of great importance in their influence on the quantity and regularity of runoff. If the geological deposits of the drainage area are highly impervious, the surface flow will receive and transmit the water into the mass only through the cracks and fissures in the rock. Pervious materials, such as sandstones, sands, gravels, and cracked or fissured rocks, induce seepage, retard runoff, and, if such deposits are underlaid with an impervious bed, provide underground storage which impounds water away from the conditions which permit evaporation, and hence tends to increase runoff and equalize flow. On the other hand, if such pervious deposits possess other outlets outside of the stream channel and drainage area, they may result in the withdrawal of more or less of the seepage waters entirely from the ultimate flow of the stream. Coarse sands and gravels will rapidly imbibe the rainfall into their structure. Fine and loose beds of sand also rapidly receive and transmit the rainfall unless the precipitation is exceedingly heavy under which conditions some of it may flow away on the surface. Many of the highly pervious indurated formations receive water slowly and require a considerable time of contact in order to receive and remove the maximum amount. In flat, pervious areas, rainfalls of a certain intensity are frequently essential to the production of any resulting stream flow. In a certain Colorado drainage area, the drainage channel is normally dry except after a rainfall of one-half inch or more. A less rainfall, except under the condition of a previously saturated area, evaporates and sinks through the soil and into the deep lying pervious sand rock under the surface which transmits it beyond the drainage area. Such results are frequently greatly obscured by the interference of other factors, such as temperature, vegetation, etc. * * * * * The natural storage of any drainage area and the possibilities of artificial storage depend principally upon its topography and geology. Storage equalizes flow, although the withdrawal of precipitation by snow or ice storage in northern areas often reduces winter flow to the minimum for the year. Both surface and sub-surface storage sometimes hold the water from the streams at times when it might be advantageously used. Storage, while essential to regulation, is not always an advantage to immediate flow conditions. UNDERGROUND AND SURFACE WATERS IN RELATION TO EXCAVATION AND CONSTRUCTION Scarcely more than a mention of this subject is necessary. In mining, the pumping charge is one of the great factors of cost. A forecast of the amount and flow of water to be encountered in mining is based on the geologic conditions. The same is true in excavating tunnels, canals, and deep foundations. Detailed study of the amount and nature of water in the rock and soil of the Panama Canal has been vital to a knowledge of the cause and possibilities of prevention of slides. Rock slides in general are closely related to the amount and distribution of the water content. The importance of ground-water as a detriment in military operations was shown during the recent war in trenching and other field works. At the outset, with the possible exception of the German army, a lack of scientific study of ground-water conditions led to much unnecessary difficulty. It soon became necessary to study and map the water conditions in great detail in advance of operations. Much of this work was done by geologists (see Chapter XIX). Geological considerations are involved in a great variety of engineering undertakings related to river and harbor improvements, dam sites, etc., mentioned in Chapter XX. FOOTNOTES: [12] Mead, Daniel W., _Hydrology_: McGraw-Hill Book Co., New York, 1919, pp. 447-448, 456. CHAPTER VI THE COMMON ROCKS AND SOILS AS MINERAL RESOURCES ECONOMIC FEATURES OF THE COMMON ROCKS Under the general heading of common rocks are included the ordinary igneous, sedimentary, and "metamorphic" rocks, and the unconsolidated clays, sands, and gravels characteristic of surface conditions, which are mined and quarried for commercial use. Soils are closely related to this group; but since they present special problems of their own, they are discussed under a separate heading at the end of the chapter. Names of the common rocks will be used with the general commercial significance given them by the United States Geological Survey in its mineral resource reports. Because of their inexhaustible quantity and ready availability, the value of the common rock products is not large per unit of weight; but in the aggregate it ranks high among mineral products. In respect to tonnage, common rocks constitute perhaps 10 per cent of the world annual output of all mineral commodities (exclusive of water). The greater tonnage of the common rocks is used commercially in crushed or comminuted forms for road material, for railroad ballast, and for cement, brick, concrete, and flux. In blocks and structural shapes, of less aggregate tonnage, they are used as building stone, monumental stone, paving blocks, curbing, flagging, roofing, refractory stone, and for many other building and manufacturing purposes. The common rocks are commodities in which most countries of the globe are self-sufficing. International trade in these commodities is insignificant, being confined to small quantities of materials for special purposes, or to local movements of short distances, allowed by good transportation facilities. The common rocks are so abundant and widespread that the conservation of raw materials is not ordinarily a vital problem. Conservational principles do apply, however, to the human energy factor required for their efficient use. In the valuation of common rocks, also, the more important factors are not the intrinsic qualities of the stones, but rather the conditions of their availability for use. Because of bulk and comparatively low intrinsic value, the principal commercial factors in the availability of the common rocks are transportation and ease of quarrying, but these are by no means the only factors determining availability. Their mineral and chemical composition, their texture and structure, their durability, their behavior under pressure and temperature changes, and other factors enter in to important degrees. The weighting and integration of these factors, for the purpose of reaching conclusions as to the availability of particular rock materials, depend also on the purposes for which these materials are to be used. The problem is anything but simple. The search for a particular rock to meet a certain demand within certain limits of cost is often a long and arduous one. On account of the abundance and widespread distribution of common rocks and their variety of uses, there is a good deal of popular misapprehension as to their availability. Many building and manufacturing enterprises have met disastrous checks, because of a tendency to assume availability of stone without making the fullest technical investigation. Many quarrying ventures have come to grief for the same reason. It is easy to assume that, because a granite in a certain locality is profitably quarried and used, some other granite in the same locality has equal chances. However, minor differences in structure, texture, and composition, or in costs of quarrying and transportation, may make all the difference between profit and loss. Even though all these conditions are satisfactorily met, builders and users are often so conservative that a new product finds difficulty in breaking into the market. A well-established building or ornamental stone, or a limestone used for flux, may hold the market for years in the face of competition from equally good and cheaper supplies. The very size of a quarry undertaking may determine its success or failure. GRANITE The term granite, as used commercially, includes true granite and such allied rocks as syenite and gneiss. In fact even quartzite is sometimes called granite in commerce, as in the case of the Baraboo quartzites of Wisconsin, but this is going too far. For statistical purposes, the United States Geological Survey has also included small quantities of diorite and gabbro. The principal uses of granite are, roughly in order of importance, for monumental stone, building stone, crushed stone, paving, curbing, riprap and rubble. Thirty states in the United States produce granite, the leaders being Vermont, Massachusetts, North Carolina, Maine, Wisconsin, Minnesota, and California. BASALT AND RELATED TYPES Basalt and related rocks are sometimes included under the name "trap rock," which comprises,--besides typical basalt and diabase,--fine-grained diorite, gabbro, and other basic rocks, which are less common in occurrence and are similar in chemical and physical properties. The principal use of these rocks is as crushed stone for road and ballast purposes and for concrete. They are produced in some fifteen states, the leaders being New Jersey, Pennsylvania, California, and Connecticut. LIMESTONE, MARL, CHALK In the United States limestone is used principally as crushed stone for road material, railroad ballast, concrete, and cement, as fluxing stone for metallurgical purposes, and in the manufacture of lime. Minor uses are as building stones, paving blocks, curbing, flagging, rubble, and riprap; in alkali works, sugar factories, paper mills, and glass works; and for agricultural purposes. For the making of cement, in metallurgical fluxes, and in most of the manufacturing and agricultural uses, both limestone and lime (limestone with the CO_2 driven out by heating) are used. Lime is also extensively used in the making of mortar for building operations, in tanning leather, and in a great variety of chemical industries. The total quantity of limestone used for all purposes in the United States nearly equals that of iron ore. Nearly every state in the union produces limestone, but the more important producers are Pennsylvania (where a large amount is used for fluxing), Ohio, Indiana, New York, Michigan, and Illinois. Closely associated with limestone in commercial uses, as well as in chemical composition, is calcareous marl, which is used extensively in the manufacture of Portland cement. Chalk is a soft amorphous substance of the same composition as limestone. The main uses of chalk are as a filler in rubber, and as a component of paint and putty. It is also used for polishing. The principal producers of this commodity are England, Denmark, and France, and the chief consumer is the United States. The United States depends upon imports for its supply of chalk for the manufacture of whiting. Before the war two-thirds came from England and a third from France. During the war importation was confined to England, with a small tonnage from Denmark. No deposits of domestic chalk have been exploited commercially. A somewhat inferior whiting, but one capable of being substituted for chalk in most cases, is manufactured from the waste fine material of limestone and marble quarries. MARBLE Marble is limestone which has been coarsely recrystallized by metamorphism. The marble of commerce includes a small quantity of serpentine as quarried and sold in Massachusetts, California, Maryland, Pennsylvania, and Vermont, and also a small amount of so-called onyx marble or travertine obtained from caves and other deposits in Kentucky and other states. The principal uses of marble are for building and monumental stones. Of the twenty-two states producing marble, the leaders are Vermont, Georgia, and Tennessee. A small amount of marble of special beauty, adapted to ornamental purposes, is imported from European countries, especially from Italy. Marble imports from Italy constitute about two-thirds, both in tonnage and value, of all stone imported into the United States. SAND, SANDSTONE, QUARTZITE (AND QUARTZ) Sand is composed mainly of particles of quartz or silica, though sometimes feldspar and other minerals are present. Sandstones are partially cemented sands. Quartzites are completely cemented sands. To some extent these substances are used interchangeably for the same purposes. The principal uses of sand in order of commercial totals are for building purposes--for mortar, concrete, sand-lime brick, etc.,--as molding sand in foundries, as a constituent of glass, in grinding and polishing, in paving, as engine sand, as fire or furnace sand, in the manufacture of ferrosilicon (a steel alloy), and in filters. Reference is made to sand as an abrasive and in the manufacture of steel in Chapters XIII and IX. Almost every state produces some sand, but for some of the more specialized uses, such as glass sand, molding sand, and fire or furnace sand, the distribution is more or less limited. The United States Geological Survey has collected information concerning the distribution of various kinds of sand and gravel, and serves a very useful function in furnishing data as to supplies of material for particular purposes. Fine molding sands have been imported from France, but during the war domestic sources in New York and Ohio were developed sufficiently to meet any requirements. The sandstone of commerce includes the quartzites of Minnesota, South Dakota, and Wisconsin, and the fine-grained sandstones of New York, Pennsylvania, and elsewhere, known to the trade as "bluestone." In Kentucky most of the sandstone quarried is known locally as "freestone." The principal uses of sandstone are for building stone, crushed stone, and ganister (for silica brick and furnace-linings). Other uses are for paving blocks, curbing, flagging, riprap, rubble, grindstones, whetstones, and pulpstones (see also Chapter XIII). Sandstone is sometimes crushed into sand and is used in the manufacture of glass and as molding-sand. Most of the states of the union produce sandstone, the principal producers being Pennsylvania, Ohio, and New York. "SAND AND GRAVEL" Where sand is coarse and impure and mixed with pebbles, it is Ordinarily referred to as "sand and gravel." For sand and gravel the principal uses are for railroad ballast, for road building, and for concrete. Sand and gravel are produced in almost every state in the union, the largest producers being Pennsylvania, Ohio, Illinois, New Jersey, and North Carolina. CLAY, SHALE, SLATE Shale is consolidated clay, usually with a fine lamination due to bedding. Slate is a more dense and crystalline rock, produced usually by the anamorphism of clay or shale under pressure, and characterized by a fine cleavage which is usually inclined to the sedimentary bedding. Clays are used principally for building and paving brick and tile, sewer-pipe, railroad ballast, road material, puddle, Portland cement, and pottery. Clay is mined in almost every state. Ohio, Pennsylvania, New Jersey, and Illinois have the largest production. There has been a considerable importation of high-grade clays, principally from England, for special purposes--such as the filling and coating of paper; the manufacture of china, of porcelain for electrical purposes, and of crucibles; and for use in ultramarine pigments, in sanitary ware, in oilcloth, and as fillers in cotton bleacheries. War experience showed the possibility of substitution of domestic clays for most of these uses; but results were not in all cases satisfactory, and the United States will doubtless continue to use imported clays for some of these special purposes. Shales, because of their thinly bedded character and softness, are of no value as building stones, but are used in the manufacture of brick, tile, pottery, and Portland cement. Slates owe their commercial value primarily to their cleavage, which gives well-defined planes of splitting. The principal uses are for roofing and, in the form of so-called mill stock for sanitary, structural, and electrical purposes. Small amounts are used for tombstones, roads, slate granules for patent roofing, school slates, blackboard material, billiard table material, etc. The color, fineness of the cleavage, and size of the flakes are the principal features determining the use of any particular slate. Ten states produce slate, the principal production coming from Pennsylvania and Vermont. THE FELDSPARS Feldspars are minerals, not rocks, but mention of them is made here because, with quartz, they make up such an overwhelming percentage of earth materials. It is estimated that the feldspars make up 50 per cent of all the igneous rocks and 16 per cent of the sedimentary rocks. As the igneous rocks are so much more abundant than the sedimentary rocks, the percentage of feldspars in the earth approaches the former rather than the latter figure. In most rocks feldspar is in too small grains and is too intimately associated with other minerals to be of commercial importance; in only one type of rock, pegmatite, which is an igneous rock of extremely coarse and irregular texture, are the feldspar crystals sufficiently large and concentrated to be commercially available. Feldspar is used principally in the manufacture of pottery, china ware, porcelain, enamel ware, and enamel brick and tile. In the body of these products it is used to lower the fusing point of the other ingredients and to form a firm bond between their particles. Its use in forming the glaze of ceramic products is also due to its low melting point. A less widespread use of feldspar is as an abrasive (Chapter XIII). One of the varieties of feldspar carries about 15 per cent of potash, and because of the abundance of the mineral there has been much experimental work to ascertain the possibility of separating potash for fertilizer purposes; but, because of cost, this source of potash is not likely for a long time to compete with the potash salts already concentrated by nature. Feldspar is mined in eleven states, but the important production comes from North Carolina and Maine. The United States also imports some feldspar from Canada. HYDRAULIC CEMENT (including Portland, natural, and Puzzolan cements) Cement is a manufactured product made from limestone (or marl) and clay (or shale). Sometimes these two kinds of substances are so combined in nature (as in certain clayey limestones) that they are available for cement manufacture without artificial mixing. It is not our purpose in this volume to discuss manufactured products; but the cement industry involves such a simple transformation of raw materials, and is so closely localized by the distribution of the raw materials, that a mention of some of its outstanding features seems desirable. Hydraulic cement is used almost exclusively as a structural material. It is an essential ingredient of concrete. Originally used chiefly for the bonding of brick and stone masonry and for foundation work, its uses have grown rapidly, especially with the introduction of reinforced concrete. It is being used in the construction of roads, and its latest use is in ship construction. With the exception of satisfactory fuels, the raw materials required for the manufacture of cement are found quite generally throughout the world. While practically all countries produce some cement, much of it of natural grade, only the largest producers make enough for their own requirements and as a result there is a large world movement of this commodity. The world trade is chiefly in Portland cement. Next to the United States, the producing countries having the largest exportable surplus of cement in normal times are Germany and Great Britain. France and Belgium were both large producers and exporters before the war, but the war greatly reduced their capacity to produce for the time being. Sweden, Denmark, Austria, Japan, and Switzerland all produce less extensively but have considerable surplus available for export. Italy and Spain have large productions, which are about sufficient for their own requirements. Holland and Russia import large amounts from the other European countries. The far eastern trade absorbs the excess production of Japan. In South Africa and Australasia, production nearly equals demand. In Canada, although the industry has been growing very rapidly, the demand still exceeds production. In South and Central America, Mexico and the West Indies, the demand is considerable and will probably increase; production has thus far been insufficient. Several modern mills are either recently completed or under construction in these countries, and concessions have been granted for several others. These new mills are largely financed by American capital. The United States is the largest single producer of cement in the world, its annual production being about 45 per cent of the world's total. Domestic consumption has always been nearly as great as the production, and exports have usually not exceeded 4 per cent of the total shipments from the mills. South and Central America offer fields for exportation of cement from the United States. GEOLOGIC FEATURES OF THE COMMON ROCKS To describe the geologic features of the common rocks used in commerce would require a full treatise on the subject of geology. These are the bulk materials of the earth and in them we read the geologic history of the earth. In preceding chapters a brief outline has been given of the relative abundance of the common earth materials and of the processes producing them. In comparison, the metalliferous deposits are the merest incidents in the development of this great group of mineral resources. In this section reference will be made only to a few of the rock qualities and other geologic features which require first attention in determining the availability of a common rock for commercial use. The list is very fragmentary, for the reason that the uses are so many and so varied that to describe all the geologic features which are important from the standpoint of all uses would very soon bring the discussion far beyond the confines of a book of this scope.[13] BUILDING STONE For building stones, the principal geologic features requiring attention are structure, durability, beauty, and coloring. The structures of a rock include jointing, sedimentary stratification, and secondary cleavage. Nearly all rocks are jointed. The joints may be open and conspicuous, or closed and almost imperceptible. The closed joints or incipient joints cause planes of weakness, known variously as rift, grain, etc., which largely determine the shapes of the blocks which may be extracted from a quarry. Where properly distributed, they may facilitate the quarrying of the stone. In other cases they may be injurious, in that they limit the size of the blocks which can be extracted and afford channels for weathering agents. Some rocks of otherwise good qualities are so cut by joints that they are useless for anything but crushed stone. The bedding planes or stratification of sedimentary rocks exercise influences similar to joints, and like joints may be useful or disadvantageous, depending on their spacing. The secondary cleavage of some rocks, notably slates, enables them to be split into flat slabs and thus makes them useful for certain purposes. Proper methods of extraction and use of a rock may minimize the disadvantageous effects of its structural features. The use of channelling machines instead of explosives means less shattering of the rock. By proper dressing of the surface the opening of small crevices may be avoided. Stratified rocks set on bed, so that the bedding planes are horizontal, last longer than if set on edge. The durability of a rock may depend on its perviousness to water which may enter along planes of bedding or incipient fracture planes, or along the minute pore spaces between the mineral particles. The water may cause disastrous chemical changes in the minerals and by its freezing and thawing may cause splitting. For this reason, the less pervious rocks have in general greater durability than the more pervious. Highly pervious rocks used in a dry position or in a dry climate will last longer than elsewhere. Durability is determined also by the different coefficients of expansion of the constituent minerals of the rock. Where the minerals are heterogeneous in this regard, differential stresses are more likely to be set up than where the minerals are homogeneous. Likewise a coarse-textured rock is in general less durable than a fine-textured one. Expansion and contraction of a stone under ordinary temperature changes, and also under fire and freezing, must necessarily be known for many kinds of construction. Minerals resist weathering to different degrees, therefore the mineral composition of a rock is another considerable factor in determining its durability. Where pyrite is present in abundance it easily weathers out, leaving iron-stained pits and releasing sulphuric acid which decomposes the rock. Abundance of mica, especially where segregated along the stratification planes, permits easy splitting of the rock under weathering. Likewise the mica often weathers more quickly than the surrounding minerals, giving a pitted appearance; in marbles and limestones its irregular occurrence may spoil the appearance. Flint or chert in abundance is deleterious to limestones and marbles, because, being more resistant, it stands out in relief on the weathered surface, interferes with smooth cutting and polishing, and often causes the rock to split along the lines of the flint concretions. Abundance of tremolite may also be disadvantageous to limestones and marbles, because it weathers to a greenish-yellow clay and leaves a pitted surface. The crushing strength of a rock has an obvious relation to its structural uses. The rock must be strong enough for the specified load. Most hard rocks ordinarily considered for building purposes are strong enough for the loads to which subjected, and this factor is perhaps ordinarily less important than the structural and mineral features already mentioned. It is often necessary to know the modulus of elasticity and other mechanical constants of a rock, as in cases where it is to be combined with metal or other masonry or to be subjected to exceptional shock. The beauty and coloring of a rock are its esthetic rather than its utilitarian features. They are particularly important in the construction of buildings and monuments for public or ornamental purposes. CRUSHED STONE The largest use of rock or stone is in the crushed form for road building, railway embankments, and concrete, and the prospect is for largely increased demands for such uses in the future. For the purpose of road building, it is necessary to consider a stone's resistance to abrasion, hardness, toughness, cementing value, absorption, and specific gravity. Limestone cements well, but in other qualities it is not desirable for heavy traffic. Shales are soft and clayey, and grind down to a mass which is dry and powdery, and muddy in wet weather. Basalt and related rocks resist abrasion, and cement well. Granites and other coarse-grained igneous rocks do not cement well and are not resistant to abrasion. Many sandstones are very hard and brittle and resist abrasion, but do not cement. The application of geology on a large scale to the study of sources and qualities of crushed stone is now being required in connection with the great state and national projects of highway building. This work is by no means confined to a mere testing of the physical qualities of road-building materials found along the proposed route, but includes a careful study of their geologic occurrence, distribution, and probable amounts. In certain of the northern states specialists in glacial geology are preferred for this purpose. STONE FOR METALLURGICAL PURPOSES The use of limestone and other rock for metallurgical fluxes is dependent very largely on chemical composition. Comparatively few limestones are sufficiently pure for this purpose. For furnace linings, the quartzite or ganister must be exceptionally pure. The field search for rocks of the necessary composition has required geologic service. CLAY For a variety of uses to which clay is put, it is necessary to know its degree of plasticity, tensile strength, shrinkage (both under air and fire), fusibility, color, specific gravity, and chemical properties. The testing of clay for its various possible uses is a highly specialized job, usually beyond the range of a geologist, although certain geologists have been leaders in this type of investigation. More commonly within the range of a geologist are questions concerning origin, field classification, distribution, quantities, and other geologic conditions affecting quality and production. Clay originates from the weathering of common rocks containing silicates, by pretty well understood weathering processes (see Chapter II). It may remain in place above the parent rock, or may be transported and redeposited, either on land or under water, by the agencies of air, water, and ice. The kind of parent rock, the climatic conditions and nature of the weathering, and the degree of sorting during transportation, all determine the composition and texture of the resulting clay,--with the result that a classification on the basis of origin may indicate the broad group characteristics which it is desirable to know for commercial purposes. For instance, residual clays from the weathering of granite may be broadly contrasted with residual clays formed by the weathering of limestone, and both differ in group characteristics from clays in glacial deposits. Classification according to origin also may be useful in indicating general features of depth, quantity, and distribution. However, a genetic classification of clays is often not sufficient to indicate the precise characteristics which it is necessary to know in determining their availability for narrow and special technical requirements. Furthermore, clays suitable for certain commercial requirements may be formed in several different ways, and classification based on specific qualities may therefore not correspond at all to geologic classification based on origin. Geologists have been especially interested in the causes of plasticity of clay and in its manner of hardening when dried. In general these phenomena have been found to be due to content of colloidal substances of a clayey nature, which serve not only to hold the substance together during plastic flow but to bind it during drying. The part played by colloids in the formation of clays, as well as of many other mineral products, is now a question which is receiving intensive study. The same processes which produce clay also produce, under special conditions, iron ores, bauxites, the oxide zones of many sulphide ore bodies, and soils, all of which are referred to on other pages. LIMITATIONS OF GEOLOGIC FIELD IN COMMERCIAL INVESTIGATION OF COMMON ROCKS In general the qualities of the earth materials which determine their availability for use are only to a minor extent the qualities which the geologist ordinarily considers for mapping and descriptive purposes. The usual geological map and report on a district indicate the distribution and general nature of the common rocks, and also the extent to which they are being used as mineral resources. Seldom, however, is there added a sufficiently precise description, for instance of a clay, to enable the reader to determine which, if any, of the many different uses the material might be put to. The variety of uses is so great, and the technical requirements for different purposes are so varied and so variable, that it is almost impossible to make a description which is sufficiently comprehensive, and at the same time sufficiently exact, to give all the information desired for economic purposes. If the geologist is interested in disclosing the commercial possibilities in the raw materials of an area, he may select some of the more promising features and subject them to the technical analysis necessary to determine their availability for special uses. In this phase of his work he may find it necessary to enlist the coöperation of skilled technicians and laboratories in the various special fields. The problem is simplified if the geologist is hunting for a particular material for a specific purpose, for then he fortifies himself with a knowledge of the particular qualities needed and directs his field and laboratory study accordingly. Too often the geologist fails to recognize the complexity and definiteness of the qualities required, and makes statements and recommendations on the use of raw materials based on somewhat general geologic observations. On the other hand, the engineer, or the manufacturer, or the builder often goes wrong and spends money needlessly, by failing to take into consideration general geologic features which may be very helpful in determining the distribution, amount, and general characters of the raw materials needed. It is difficult to draw the line between the proper fields of the geologist and those of the engineer, the metallurgist, and other technicians. It is highly desirable that the specialist in any one of these fields know at least of the existence of the other fields and something of their general nature. Too often his actions indicate he is not acutely conscious even of the existence of these related branches of knowledge. The extent and detail to which the geologist will familiarize himself with these other fields will of course vary with his training and the circumstances of his work. Whatever his limit is, it should be definitely recognized; his work should be thorough up to this limit and his efforts should not be wasted in fields which he is not best qualified to investigate. These remarks apply rather generally to mineral resources, but they are particularly pertinent in relation to the common rock materials which the geologist is daily handling,--for he is likely to assume that he knows all about them and that he is qualified to give professional advice to industries using them. In connection with metallic resources, the metallurgical and other technical requirements are likely to be more definitely recognized and the lines more sharply drawn, with the result that the geologist is perhaps not so likely to venture into problems which he is not qualified to handle. The limits to geologic work here discussed are not necessarily limits separating scientific from non-scientific work. The study and determination of the qualities of rocks necessary for commercial purposes is fully as scientific as a study of the qualities commonly considered in purely geologic work, and the results of technical commercial investigations may be highly illuminating from a purely geological standpoint. When a field of scientific endeavor has been established by custom, any excursion beyond traditional limits is almost sure to be regarded by conservatives in the field as non-scientific, and to be lightly regarded. The writer is fully conscious of the existence of limits and the necessity for their recognition; but he would explain his caution in exceeding these limits on the ground of training and effectiveness, rather than on fear that he is becoming tainted with non-scientific matters the moment he steps beyond the boundaries of his traditional field. SOILS AS A MINERAL RESOURCE Soils are not ordinarily listed as mineral resources; but as weathered and altered rock of great economic value, they belong nearly at the head of the list of mineral products. ORIGIN OF SOILS Soil originate from rocks, igneous, sedimentary, and "metamorphic" by processes of weathering, and by the mixing of the altered mineral products with decayed plant remains or _humus_. The humus averages perhaps 3 or 4 per cent of the soil mass and sometimes constitutes as much as 75 per cent. Not all weathered rock is soil in the agricultural sense. For this purpose the term is mainly restricted to the upper few inches or feet penetrated by plant roots. The general process of soil formation constitutes one of the most important phases of katamorphism--the destructive side of the metamorphic cycle, described in Chapter II. Processes of katamorphism or weathering, usually accompanied by the formation of soils, affect the surface rocks over practically all the continental areas. The weathering of a highly acid igneous rock with much quartz produces a residual soil with much quartz. The weathering of a basic igneous rock without quartz produces a clay soil without quartz, which may be high in iron. Where disintegration has been important the soil contains an abundance of the original silicates of the rock, and less of the altered minerals. The production of soil from sedimentary rocks involves the same processes as alter igneous rocks; but, starting from rocks of different composition, the result is of course different in some respects. Sandstones by weathering yield only a sandy soil. Limestones lose their calcium carbonate by solution, leaving only clay with fragments of quartz or chert as impurities. A foot of soil may represent the weathering of a hundred feet of limestone. Shales may weather into products more nearly like those of the weathering of igneous rocks. Silicates in the shales are broken down to form clay, which is mixed with the iron oxide and quartz. In some localities the soil may accumulate to a considerable depth, allowing the processes of weathering to go to an extreme; in others the processes may be interrupted by erosion, which sweeps off the weathered products at intermediate stages of decomposition and may leave a very thin and little decomposed soil. Soils formed by weathering may remain in place as residual soils, or they may be transported, sorted, and redeposited, either on land or under water. It is estimated by the United States Bureau of Soils[14] that upward of 90 per cent of the soils of the United States which have been thus far mapped owe their occurrence and distribution to transportation by moving water, air, and ice (glaciers), and that less than 10 per cent have remained in place above their parent rock. Glaciers may move the weathered rock products, or they may grind the fresh rocks into a powder called _rock flour_, and thus form soils having more nearly the chemical composition of the unaltered rocks. Glacial soils are ordinarily rather poorly sorted, while wind and water-borne soils are more likely to show a high degree of sorting. The character of a transported soil is less closely related to the parent rock than is that of a residual soil, because the processes of sorting and mixture of materials from different sources intervene to develop deposits of a nature quite different from residual soils; but even transported soil may sometimes be traced to a known rock parentage. Where deposited under water, soil materials may be brought above the water by physiographic changes, and exposed at the surface in condition for immediate use. Or, they may become buried by other sediments and not be exposed again until after they have been pretty well hardened and cemented,--in which case they must again undergo the softening processes of weathering before they become available for use. Where soils become buried under other rocks and become hardened, they are classed as sedimentary rocks and form a part of the geologic record. Many residual and transported soils are to be recognized in the geologic column; in fact a large number of the sedimentary rocks ordinarily dealt with in stratigraphic geology are really transported soils. The development of soils by weathering should not be regarded as a special process of rock alteration, unrelated to processes producing other mineral products. Exactly the same processes that produce soils may yield important deposits of iron ore, bauxite, and clay, and they cause also secondary enrichment of many metallic mineral deposits. For instance the weathering of a syenite rock containing no quartz, under certain conditions, as in Arkansas, results in great bauxite deposits which are truly soils and are useful as such,--but which happen to be more valuable because of their content of bauxite. The weathering of a basic igneous rock, as in Cuba, may produce important residual iron ore deposits, which are also used as soils. Weathering of ferruginous limestone may produce residual iron and manganese ores in clay soils. COMPOSITION OF SOILS AND PLANT GROWTH The mineral ingredients in soils which are essential for plant growth include water, potash, lime, magnesia, nitrates, sulphur, and phosphoric acid--all of which are subordinate in amount to the common products of weathering (pp. 20-22, 23-24). Of these constituents magnesia is almost invariably present in sufficient quantity; while potash, nitrates, lime, sulphur, and phosphoric acid, although often sufficiently abundant in virgin soil, when extracted from the soils by plant growth are liable to exhaustion under ordinary methods of cultivation, and may need to be replenished by fertilizers (Chapter VII). Some soils may be so excessively high in silica, iron, or other constituents, that the remaining constituents are in too small amounts for successful plant growth. Even where soils originally have enough of all the necessary chemical elements, one soil may support plant growth and another may not, for the reason that the necessary constituents are soluble and hence available to the plant roots in one case and are not soluble in the other. Plainly the mineral combinations in which the various elements occur are important factors in making them available for plant use. Similarly a soil of a certain chemical and mineralogical composition may be fruitful under one set of climatic conditions and a soil of like composition may be barren at another locality--indicating that availability of constituents is also determined by climatic and other conditions of weathering. Even with the same chemical composition and the same climatic conditions, there may be such differences in texture between various soils as to make them widely different in yield. The unit of soil classification is the _soil type_, which is a soil having agricultural unity, as determined by texture, chemical character, topography, and climate. The types commonly named are clay, clay loam, silt loam, loam, fine sandy loam, sandy loam, fine sand, and sand. In general the soil materials are so heterogeneous and so remote from specific rock origin, that in such classification the geologic factor of origin is not taken into account. More broadly, soils may be classified into provinces on the basis of geography, similar physiographic conditions, and similarity of parent rocks; for instance, the soils of the Piedmont plateau province, of the arid southwest region, of the glacial and loessal province, etc. In such classification the geologic factors are more important. Soils within a province may be subdivided into "soil series" on the basis of common types of sub-soils, relief, drainage, and origin. USE OF GEOLOGY IN SOIL STUDY While the desirability of particular soils is related in a broad way to the character of the parent rocks, and while by geologic knowledge certain territories can be predicated in advance as being more favorable than others to the development of good soils, so many other factors enter into the question that the geologic factor may be a subordinate one. A soil expert finds a knowledge of geology useful as a basis for a broad study of his subject; but in following up its intricacies he gives attention mainly to other factors, such as the availability of common constituents for plant use, the existence and availability of minute quantities of materials not ordinarily regarded as important by the geologist, the climatic conditions, and the texture. As the geologic factors are many of them comparatively simple, much of the expert work on soils requires only elementary and empirical knowledge of geology. The geologist, although he may understand fully the origin of soils and may indicate certain broad features, must acquire a vast technique not closely related to geology before he becomes effective in soil survey work and diagnosis. For these reasons the mapping and classification of soils, while often started by geologists of state or federal surveys, have in their technical development and application now passed largely into the hands of soil experts in the special soil surveys affiliated with the U. S. Department of Agriculture and with agricultural colleges. FOOTNOTES: [13] A good summary of this subject may be found in _Engineering Geology_, by H. Ries and T. L. Watson, Wiley and Sons, 2d ed., 1915. [14] Marbut, Curtis F., Soils of the United States: _Bull. 96, Bureau of Soils_, 1913, p. 10. CHAPTER VII THE FERTILIZER GROUP OF MINERALS GENERAL COMMENTS Soils are weathered rock more or less mixed with organic material. The weathering processes forming soils are in the field of geologic investigation, but the study of soils in relation to agriculture requires attention to texture and to several of their very minor constituents which have little geologic significance. Soil study has therefore become a highly specialized and technicalized subject,--for which a geological background is essential, but which is usually beyond the range of the geologist. To supply substances which are deficient in soils, however, requires the mining, quarrying, or extraction of important mineral resources, and in this part of the soil problem the geologist is especially interested. Soils may be originally deficient in nitrates, phosphates, or potash; or the continued cropping of soils may take out these materials faster than the natural processes of nature supply them. In some soils there are sufficient phosphates and potash to supply all plant needs indefinitely; but the weathering and alteration processes, through which these materials are rendered soluble and available for plant life, in most cases are unable to keep up with the depletion caused by cropping. A ton of wheat takes out of the soil on an average 47 pounds of nitrogen, 18 pounds of phosphoric acid, 12 pounds of potash. On older soils in Europe it has been found necessary to use on an average 200 pounds of mixed mineral fertilizers annually per acre. On the newer soils of the United States the average thus far used has been less than one-seventh of this amount. The United States has thus far been using up the original materials stored in the soil by nature, but these have not been sufficient to yield anything like the crop output per acre of the more highly fertilized soils of Europe. In addition to the nitrates, phosphates, and potassium salts, important amounts of lime and sulphuric acid, and some gypsum, are used in connection with soils. Lime is derived from crushed limestone (pp. 82-83), and is used primarily to counteract acidity or sourness of the soil; it is, therefore, only indirectly related to fertilizers. Sulphuric acid is used to treat rock phosphates to make them more soluble and available to plant life. It requires the mining of pyrite and sulphur. Gypsum, under the name of "land-plaster," is applied to soils which are deficient in the sulphur required for plant life; increase in its use in the future seems probable. There are also considerable amounts of inert mineral substances which are used as fillers in fertilizers to give bulk to the product, but which have no agricultural value. The proportions of the fertilizer substances used in the United States are roughly summarized in Figure 4. The United States possesses abundant supplies of two of the chief mineral substances entering into commercial fertilizers,--phosphate rock and the sulphur-bearing materials necessary to treat it. For potash the United States is dependent on Europe, unless the domestic industry is very greatly fostered under protective tariff. For the mineral nitrates the United States has been dependent on Chile, and because of the cheapness of the supply will doubtless continue to draw heavily from this source. However, because of the domestic development of plants for the fixation of nitrogen from the air, the recovery of nitrogen from coal in the by-product processes, and the use of nitrogenous plants, the United States is likely to require progressively less of the mineral nitrates from Chile. The fertilizer industry of the United States is yet in its infancy and is likely to have a large growth. Furthermore much remains to be learned about the mixing of fertilizers and the amounts and kinds of materials to be used. The importance of sulphur as a plant food has been realized comparatively recently. The use of fertilizers in the United States has come partly through education and the activity of agricultural schools and partly through advertising by fertilizer companies. The increased use of potash has been due largely to the propaganda of the German sales agents. An examination of a map showing distribution of the use of fertilizers over the country indicates very clearly the erratic distribution of the effects of these various activities. One locality may use large amounts, while adjacent territory of similar physical conditions uses little. The sudden withdrawal of fertilizers for a period of three or four years during the war had very deleterious effects in some localities, but was not so disastrous as expected in others,--emphasizing the fact that the use of fertilizers has been partly fortuitous and not nicely adjusted to specific needs. [Illustration: FIG. 4. FERTILIZER SITUATION IN THE UNITED STATES. SMITHSONIAN INSTITUTION--UNITED STATES NATIONAL MUSEUM] NITRATES ECONOMIC FEATURES There are several sources of nitrogen for fertilizer purposes: mineral nitrates, nitrogen taken from the air by certain plants with the aid of bacteria and plowed into the soil, nitrogen taken directly from the air by combining nitrogen and oxygen atoms in an electric arc, or by combining nitrogen and hydrogen to form ammonia, nitrogen taken from the air to make a compound of calcium, carbon, and nitrogen (cyanamid), nitrogen saved from coal in the form of ammonia as a by-product of coke-manufacture, and nitrogen from various organic wastes. Nitrogen in the form of ammonia is also one of the potential products of oil-shales (p. 150). While the principal use of nitrogenous materials is as fertilizers, additional important quantities are used in ammonia for refrigerating plants, and in the form of nitric acid in a large number of chemical industries. During the war the use of nitrates was largely diverted to explosives manufacture. The geologist is interested principally in the mineral nitrates as a mineral resource, but the other sources of nitrogen, particularly its recovery from coal, also touch his field. Almost the single source of mineral nitrates for the world at present is Chile, where there are deposits of sodium nitrate or Chile saltpeter, containing minor amounts of potassium nitrate. About two-thirds of the Chilean material normally goes to Europe and about one-fourth to the United States. The supply has been commercially controlled chiefly by Great Britain and by Chilean companies backed by British and German capital. The dependence of the world on Chile became painfully apparent during the war. Germany was the only nation which had developed other sources of nitrogenous material to any great extent. The other nations were dependent in a very large degree on the mineral nitrates, both for fertilizer and munition purposes. Total demands far exceeded the total output from Chile, requiring international agreement as to the division of the output among the nations. The stream of several hundred ships carrying nitrates from Chile was one of the vital war arteries. This situation led to strenuous efforts in the belligerent countries toward the development of other sources of nitrogen. The United States, under governmental appropriation, began the building of extensive plants for the fixation of nitrogen from the air, and the building of by-product coke ovens in the place of the old wasteful beehive ovens was accelerated. Germany before the war had already gone far in both of these directions, not only within her own boundaries, but in the building of fixation plants in Scandinavia and Switzerland. War conditions required further development of these processes in Germany, with the result that this country was soon entirely self-supporting in this regard. One of the effects was the almost complete elimination in Germany of anything but the by-product process of coking coal. War-time development of the nitrogen industry in the United States for munition purposes brought the domestic production almost up to the pre-war requirements for fertilizers alone. With the increasing demand for fertilizers and with the cheapness of the Chilean supply of natural nitrates, it is likely that the United States will continue for a good many years to import considerable amounts of Chilean nitrates. It may be noted that, although this country normally consumes about one-fourth of the Chilean product, American interests commercially control less than one-twentieth of the output. Presumably, if for no other purpose than future protection, effort will be made to develop the domestic industry to a point where in a crisis the United States could be independent of Chile. Particularly may an increase in the output of by-product ammonia from coke manufacture be looked for (see also pp. 118-119), since nitrogenous material thus produced need bear no fixed part of the cost of production, and requires no protective tariff. The reserves of Chilean nitrate are known to be sufficient for world requirements for an indefinitely long future. GEOLOGIC FEATURES Mineral nitrates in general, and particularly those of soda and potash, are readily soluble at ordinary temperatures. Mineral nitrate deposits are therefore very rare, and are found only in arid regions or other places where they are protected from rain and ground-water. The only large deposits known are those of northern Chile and some extensions in adjacent parts of Peru and Bolivia. These are located on high desert plateaus, where there is almost a total absence of rain, and form blankets of one to six feet in thickness near the surface. The most important mineral, the sodium nitrate or Chile saltpeter, is mingled with various other soluble salts, including common salt, borax minerals, and potassium nitrate, and with loose clay, sand, and gravel. The nitrate deposits occur largely around and just above slight basin-like depressions in the desert which contain an abundance of common salt. The highest grade material contains 40 to 50 per cent of sodium nitrate, and material to be of shipping grade must run at least 12 to 15 per cent. The origin of the nitrate beds is commonly believed to be similar to that of beds of rock salt (pp. 295-298), borax, and other saline residues. The source of the nitrogen was probably organic matter in the soil, such as former deposits of bird guano, bones (which are actually found in the same desert basin), and ancient vegetable matter. By the action of nitrifying bacteria on this organic matter, nitrate salts are believed to have formed which were leached out by surface and ground waters, and probably carried in solution to enclosed bodies of water. Here they became mingled with various other salts, and all were precipitated out as the waters of the basins evaporated. Deliquescence and later migration of the more soluble nitrates resulted in their accumulation around the edges of the basins. The nitrate beds are thus essentially a product of desiccation. While the origin just set forth is rather generally accepted, several other theories have been advanced. It has been suggested that the deposits were not formed in water basins, but that ground water carrying nitrates in solution has been and is rising to the surface,--where, under the extremely arid conditions, it evaporates rapidly, leaving the nitrates mixed with the surface clays. One group of writers accounts for the deposits by the fixation of atmospheric nitrogen through electrical phenomena. Still others note the frequent presence of nitrogen in volcanic exhalations and the association of the Chilean nitrate beds with surface volcanic rocks; they suggest that these rocks were the source of the nitrogen, which under unusual climatic conditions was leached out and then deposited by evaporation. PHOSPHATES ECONOMIC FEATURES The principal use of natural phosphates is in the manufacture of fertilizers. They are also used in the manufacture of phosphorus, phosphoric acid, and other phosphorus compounds, for matches, for certain metallurgical operations, and for gases used in military operations. The material mined is mainly a phosphate of lime (tricalcium phosphate). To make it available for plant use, it is treated with sulphuric acid to form a soluble superphosphate; hence the importance of sulphuric acid, and its mineral sources pyrite and sulphur, in the fertilizer industry. A small percentage of the phosphate is also ground up and applied directly to the soil in the raw form. Other phosphatic materials are the basic slag from phosphatic iron ores made into Thomas-process steel, guano from the Pacific islands, and bone and refuse (tankage) from the cattle raising and packing countries. These materials are used for the same purposes as the natural phosphates. The United States is the largest factor in the world's phosphate industry, with reference both to production and reserves. The largest and most available of the European sources are in Tunis and Algeria, under French control, and in Egypt, under English control. Belgium and northern France have been considerable producers of phosphates, but, with the development of higher grade deposits in other countries, their production has fallen to a very small fraction of the world's total. There also has been very small and insignificant production in Spain and Great Britain. Russia has large reserves which are practically unmined. While there is comparatively little phosphate rock in western Europe, a considerable amount of the phosphate supply is obtained as a by-product from Thomas slag, derived from phosphatic iron ores. These ores are chiefly from Lorraine and Sweden, but English and Russian ores can be similarly used. Outside of Europe and the United States, there are smaller phosphate supplies in Canada, the Dutch West Indies, Venezuela, Chile, South Australia, New Zealand, and several islands of the Indian and South Pacific Oceans. None of these has yet contributed largely to world production, and their distance from the principal consuming countries bordering the North Atlantic basin is so great that there is not likely to be any great movement to this part of the world. On the other hand, some of the South Sea islands have large reserves of exceptionally high grade guano and bone phosphates, which will doubtless be used in increasing amounts for export to Japan, New Zealand, and other nearby countries. The most important of these islands are now controlled by Great Britain, Japan, and France. A striking feature of the situation is that the central European countries, which have been large consumers of phosphate material, have lost not only the Pacific island phosphates but the Lorraine phosphatic iron ores, and are now almost completely dependent on British, French, and United States phosphate. In the United States, reserves of phosphate are very large. They are mined principally in Florida, Tennessee, and South Carolina; but great reserves, though of lower grade, are known in Arkansas, Montana, Idaho, Wyoming, and Utah. There are possibilities for the development of local phosphate industries in the west, in connection with the manufacture of sulphuric acid from waste smelting gases at nearby mining centers. The Anaconda Copper Mining Company has taken up the manufacture of superphosphate as a means of using sulphuric acid made in relation to its smelting operations. The United States is independent in phosphate supplies and has a surplus for export. This country, England, and France exercise control of the greater part of the world's supply of phosphatic material. In competition for world trade, the Florida and Carolina phosphates are favorably situated for export, but there is strong competition in Europe from the immense fields in French North Africa, which are about equally well situated. GEOLOGIC FEATURES Small amounts of phosphorus are common in igneous rocks, in the form of the mineral apatite (calcium phosphate with calcium chloride or fluoride). Apatite is especially abundant in some pegmatites. In a few places, as in the Adirondacks where magnetic concentration of iron ores leaves a residue containing much apatite, and in Canada and Spain where veins of apatite have been mined, this material is used as a source of phosphate fertilizer. The great bulk of the world's phosphate, however, is obtained from other sources--sedimentary and residual beds described below. Phosphorus in the rocks is dissolved in one form or another by the ground-waters; a part of it is taken up by land plants and animals for the building of their tissues, and another part goes in solution to the sea to be taken up by sea plants and animals. In places where the bones and excrements of land animals or the shells and droppings of sea animals accumulate, deposits of phosphatic material may be built up. In certain places where great numbers of sea birds congregate, as on desert coasts and oceanic islands, guano deposits have been formed. Some of them, like the worked-out deposits of Peru and Chile, are in arid climates and have been well preserved. Others, like those of the West Indies and Oceania, are subjected to the action of occasional rains; and to a large extent the phosphates have been leached out, carried down, and reprecipitated, permeating and partially replacing the underlying limestones. In this way deposits have been formed containing as high as 85 per cent calcium phosphate. Even more important bodies of phosphates have been produced by the accumulation of marine animal remains, probably with the aid of joint chemical, bacterial, and mechanical precipitation. These processes have formed the chief productive deposits of the world, including those of the United States, northern Africa, and Russia, and also the phosphatic iron ores of England and central Europe. The sedimentary features of many phosphate rocks, particularly their oölitic textures, show a marked similarity to the features of the Clinton type of iron ores (pp. 166-167). The marine phosphate beds originally consist principally of calcium phosphate and calcium carbonate in varying proportions. Depending on the amount of secondary enrichment, they form two main types of deposits. The extensive beds of the western United States (in the upper Carboniferous) are hard, and very little enrichment by weathering has taken place; they carry in their richer portions 70 to 80 per cent calcium phosphate, and large sections range only from about 30 to 50 per cent. In the southeastern deposits (Silurian and Devonian in Tennessee and Tertiary in the Carolinas and Florida), there has been considerable enrichment, the rock is softer, and the general grade ranges from 65 to 80 per cent. Both calcium carbonate and calcium phosphate are soluble in ordinary ground waters, but the carbonate is the more soluble of the two. Thus the carbonate has been dissolved out more rapidly, and in addition descending waters carrying the phosphate have frequently deposited it to pick up the carbonate. These enriching processes, sometimes aided by mechanical concentration, have formed high-grade deposits both in the originally phosphatic beds and in various underlying strata. Concretionary and nodular textures are common. The "pebble" deposits of Florida consist of the phosphatic materials broken up and worked over by river waters and advancing shallow seas. PYRITE ECONOMIC FEATURES The principal use of pyrite is in the manufacture of sulphuric acid. Large quantities of acid are used in the manufacture of fertilizers from phosphate rock, and during war times in the manufacture of munitions. Sulphuric acid converts the phosphate rock into superphosphate, which is soluble and available for plant use. Other uses of the acid are referred to in connection with sulphur. Pyrite is also used in Europe for the manufacture of paper from wood-pulp, but in the United States native sulphur has thus far been exclusively used for this purpose. The residue from the roasting of pyrite is a high-grade iron ore material frequently very low in phosphorus, which is desirable in making up mixtures for iron blast furnaces. Most of the countries of Europe are producers of pyrite, and important amounts are also produced in the United States and Canada. The European production is marketed mainly on that continent, but considerable amounts come to the United States from Spain. Before the war domestic sources supplied a fourth to a third of the domestic demand for pyrite. Imports came mainly from Spain and Portugal to consuming centers on the Atlantic seaboard. The curtailment of overseas imports of pyrite during the war increased domestic production by about a third and resulted also in drawing more heavily on Canadian supplies, but the total was not sufficient to meet the demand. The demand was met by the increased use of sulphur from domestic deposits (p. 109). At the close of the war supplies of pyrite had been accumulated to such an extent that, with the prospect of reopening of Spanish importation, pyrite production in the United States practically ceased. War experience has demonstrated the possibility of substitution of sulphur, which the United States has in large and cheaply mined quantities. The future of the pyrite industry in the United States therefore looks cloudy, except for supplies used locally, as in the territory tributary to the Great Lakes, and except for small amounts locally recovered as by-products in the mining of coal or from ores of zinc, lead, and copper. Pyrite production in the past has been chiefly in the Appalachian region, particularly in Virginia and New York, and in California. GEOLOGIC FEATURES Pyrite, the yellow iron sulphide, is the commonest and most abundant of the metallic sulphides. It is formed under a large variety of conditions and associations. Marcasite and pyrrhotite, other iron sulphide minerals, are frequently found with pyrite and are used for the same purposes. The great deposits of Rio Tinto, Spain, which produce about half of the world's pyrite, were formed by replacement of slates by heated solutions from nearby igneous rocks. The ores are in lenticular bodies, and consist of almost massive pyrite with a small amount of quartz and scattered grains and threads of chalcopyrite (copper-iron sulphide). They carry about 50 per cent of sulphur, and the larger part carries about 2 per cent of copper which is also recovered. Similar occurrences of pyrite on a smaller scale are known in many places. Pyrite is very commonly found in vein and replacement deposits of gold, silver, copper, lead, and zinc. In the Mississippi valley it is extracted as a by-product from the lead and zinc ores, and in the Cordilleran region large quantities of by-product pyrite could easily be produced if there were a local demand. The pyrite deposits of the Appalachian region are chiefly lenses in schists; they are of uncertain origin though some are believed to have been formed by replacement of metamorphosed limestones and schists. Under weathering conditions pyrite oxidizes, the sulphur forming sulphuric acid,--an important agent in the secondary enrichment of copper and other sulphides,--and the iron forming the minerals hematite and limonite in the shape of a "gossan" or "iron-cap." Pyrite is likewise frequently found in sediments, apparently being formed mainly by the reducing action of organic matter on iron salts in solution. In Illinois and adjacent states it is obtained as a by-product of coal mining. SULPHUR ECONOMIC FEATURES Sulphur is used for many of the same purposes as pyrite. Under pre-war conditions, the largest use in the United States was in the manufacture of paper pulp by the sulphite process. Minor uses were in agriculture as a fungicide and insecticide, in vulcanizing rubber, and in the manufacture of gunpowder. About 5 per cent of the sulphur of the United States was used in the manufacture of sulphuric acid. During the war this use was greatly increased because of the shortage of pyrite and the large quantities of sulphuric acid necessary for the manufacture of explosives. The replacement of pyrite by sulphur in the manufacture of sulphuric acid has continued since the war, and in the future is likely to continue to play an important part. Sulphuric acid is an essential material for a great range of manufacturing processes. Some of its more important applications are: in the manufacture of superphosphate fertilizer from phosphate rock; in the refining of petroleum products; in the iron, steel, and coke industries; in the manufacture of nitroglycerin and other explosives; and in general metallurgical and chemical practice. The United States is the world's largest sulphur producer. The principal foreign countries producing important amounts of sulphur are Italy, Japan, Spain, and Chile. Europe is the chief market for the Italian sulphur. In spite of increased demands in Europe the Italian production has decreased as the result of unfavorable labor, mining, and transportation conditions, and the deficit has had to be met from the United States. Japan's sulphur production has been increasing. Normally about half of the material exported comes to the United States to supply the needs of the paper industry in the Pacific states, and half goes to Australia and other British colonies. Spain's production is relatively small and has been increasing slowly; most of it is consumed locally. Chile's small production is mainly consumed at home and large additional amounts are imported. The sulphur output of the United States, which in 1913-14 was second to Italy, now amounts to three-fourths of the entire output of the world, and the United States has become a large exporter of sulphur. Supplies are ample and production increasing, with the result that the United States can not only meet its own demands, but can use this commodity extensively in world trade. Small amounts of sulphur are mined in some of the western states, but over 98 per cent of the production comes from Louisiana and Texas. GEOLOGIC FEATURES Native sulphur is found principally in sedimentary beds, where it is associated with gypsum and usually with organic matter. Deposits of this type are known in many places, the most important being those of Sicily and of the Gulf Coast in the United States. In the latter region beds of limestone carry lenses of sulphur and gypsum which are apparently localized in dome-like upbowings of the strata. The deposits are overlain by several hundred feet of loose, water-bearing sands, through which it is difficult to sink a shaft. An ingenious and efficient process of mining is used whereby superheated water is pumped down to melt the sulphur, which is then forced to the surface by compressed air and allowed to consolidate in large bins. The Sicilian deposits are similar lenses in clayey limestones containing 20 to 25 per cent of sulphur, associated with gypsum and bituminous marl; they are mined by shafts. Concerning the origin of these deposits several theories have been advanced. It has been thought that the materials for the deposits were precipitated at the same time as the enclosing sediments; and that the sulphur may have been formed by the oxidation of hydrogen sulphide in the precipitating waters through the agency of air or of sulphur-secreting bacteria, or that it may have been produced by the reduction of gypsum by organic matter or bacteria. Others have suggested that hot waters rising from igneous rocks may have brought in both the sulphur and the gypsum, which in crystallizing caused the upbowing of the strata which is seen in the Gulf fields (see also p. 298). Native sulphur is also found in mineral springs from which hydrogen sulphide issues, where it is produced by the oxidation of the hydrogen sulphide. It likewise occurs in fissures of lava and around volcanic vents, where it has probably been formed by reactions between the volcanic gases and the air. The Japanese and Chilean deposits are of the volcanic type. POTASH ECONOMIC FEATURES Potash is used principally as a component of fertilizers in agriculture. It is also used in the manufacture of soap, certain kinds of glass, matches, certain explosives, and chemical reagents. For a long time potash production was essentially a German monopoly. The principal deposits are in the vicinity of Stassfurt in north central Germany (about the Harz Mountains). Stassfurt salts are undoubtedly ample to supply the world's needs of potash for an indefinite future. However, other deposits, discovered in the Rhine Valley in Alsace in 1904, have been proved to be of great extent; and though the production has hitherto been limited by restrictions imposed by the German Government, it has nevertheless become considerable.[15] The grade (18 per cent K_{2}O) is superior to the general run of material taken from the main German deposits, and the deposits have a regularity of structure and uniformity of material favorable to cheaper mining and refining than obtains in the Stassfurt deposits. Other countries have also developed supplies of potash, some of which will probably continue to produce even in competition with the deposits of recognized importance referred to above. Noteworthy among the newer developments are those in Spain.[16] These have not yet produced on any large scale, but their future production may be considerable. Less important deposits are known in Galicia, Tunis, Russia, and eastern Abyssinia, and the nitrate deposits of Chile contain a small percentage of potash which is being recovered in some of the operations. Prior to the war the United States obtained its potash from Germany. The German potash industry was well organized and protected by the German Government, which made every effort to maintain a world monopoly. During the war the potash exports from Germany were cut off, excepting exports to the neutrals immediately adjoining German territory. The result in the United States was that the price of potash rose so far as to greatly diminish its use as fertilizer. The consequent efforts to increase potash production in the United States met with considerable success, but the maximum production attained was only about one-fourth of the ordinary pre-war requirements. The principal American sources are alkaline beds and brines in Nebraska, Utah, and California, and especially at Searles Lake, California. These furnished 75 per cent of the total output. Minor amounts have been extracted in Utah from the mineral alunite (a sulphate of potassium and aluminum), in Wyoming from leucite (a potassium-aluminum silicate), in California from kelp or seaweed, and in various localities from cement-mill and blast-furnace dusts, from wood ashes, from wool washings, from the waste residues of distilleries and beet-sugar refineries, and from miscellaneous industrial wastes. At the close of the war, sufficient progress had been made in the potash industry to indicate that the United States might become self-supporting in the future, though at high cost. The renewal of importation of cheap potash from Germany, with probable further offerings from Alsace and Spain, makes it impossible for the United States potash production to continue; except, perhaps, for the recovery of by-products which will go on in connection with other industries. Demand for a protective tariff has been the inevitable result (see Chapters XVII and XVIII). GEOLOGIC FEATURES Potassium is one of the eight most abundant elements in the earth. It occurs as a primary constituent of most igneous rocks, some of which carry percentages as high as those in commercial potash salts used for fertilizers. It is present in some sediments and likewise occurs in many schists and gneisses. Various potassium silicates--leucite, feldspar, sericite, and glauconite--and the potassium sulphate, alunite, have received attention and certain of them have been utilized to a small extent, but none of them are normally able to compete on the market. Potential supplies are thus practically unlimited in amount and distribution. Deposits from which the potash can be extracted at a reasonable cost, however, are known in only a few places, where they have been formed as saline sediments. In the decomposition of rocks the potash, like the soda, is readily soluble, but in large part it is absorbed and held by clayey materials and is not carried off. Potash is therefore more sparingly present in river and ocean waters than is soda, and deposits of potash salts are much rarer than those of rock salt and other sodium compounds. The large deposits in the Permian beds of Stassfurt, as well as those in the Tertiary of Alsace and Spain, have been formed by the evaporation of very large quantities of salt water, presumably sea water. They consist of potassium salts, principally the chloride, mixed and intercrystallized with chlorides and sulphates of magnesium, sodium, and calcium. In the Stassfurt deposits the potassium-magnesium salts occupy a relatively thin horizon at the top of about 500 feet of rock salt beds, the whole underlying an area about 200 miles long and 140 miles wide. The principal minerals in the potash horizon are carnallite (hydrous potassium-magnesium chloride), kieserite (hydrous magnesium sulphate), sylvite (potassium chloride), kainite (a hydrous double salt of potassium chloride and magnesium sulphate), and common salt (sodium chloride). The potash beds represent the last stage in the evaporation of the waters of a great closed basin, and the peculiar climatic and topographic conditions which caused their formation have been the subject of much speculation. This subject is further treated in the discussion of common salt beds (pp. 295-298). In the United States the deposits at Searles Lake, California, have been produced by the same processes on a smaller scale. In this case evaporation has not been carried to completion, but the crystallization and separation out of other salts has concentrated the potassium (with the magnesium) in the residual brine or "mother liquor." The deposits of this lake or marsh also contain borax (see p. 276), and differ in proportions of salts from the Stassfurt deposits. This is due to the fact that they were probably derived, not from ocean waters, but from the leaching of materials from the rocks of surrounding uplands, transportation of these materials in solution by rivers and ground waters, and concentration in the desert basin by evaporation. The alkali lakes of Nebraska are believed to be of very recent geologic origin. They lie in depressions in a former sand dune area, and contain large quantities of potash supposedly accumulated by leaching of the ashes resulting from repeated burnings of the grass in the adjacent country. Of other natural mineral sources, alunite is the most important. The principal deposits worked are at Marysville, Utah, but the mineral is a rather common one in the western part of the United States, associated with gold deposits, as at Goldfield, Nevada. Alunite occurs as veins and replacement deposits, often in igneous associations, and is supposed to be of igneous source. Its origin is referred to in connection with the Goldfield ores (p. 230). FOOTNOTES: [15] Gale, Hoyt S., The potash deposits of Alsace: _Bull. 715-B, U. S. Geol. Survey_, 1920, pp. 17-55. [16] Gale, Hoyt S., Potash deposits in Spain: _Bull. 715-A, U. S. Geol. Survey_, 1920, pp. 1-16. CHAPTER VIII THE ENERGY RESOURCES--COAL, OIL, GAS (AND ASPHALT) COAL ECONOMIC FEATURES Coal overshadows all other mineral resources, except water, in production, value, and demand. It is the greatest of the energy sources--coal, petroleum, gas, and water power. Roughly two-thirds of the world's coal is used for power, one-sixth for smelting and metallurgical industries, and one-sixth for heating purposes. Coal constitutes over one-third of the railroad tonnage of the United States and is the largest single tonnage factor in international trade; 70 per cent of the pre-war tonnage of outgoing cargoes from England was coal. =World production and trade.= The great coal-producing countries of the world border the North Atlantic basin. The United States produces about 40 per cent of the world's total, Great Britain about 20 per cent, and Germany about 20 per cent. Other countries producing coal stand in about the following order: Austria-Hungary, France, Russia, Belgium, Japan, China, India, Canada, and New South Wales. There is similarity in the major features of the distribution of coal production and of iron ore production. The great centers of coal production--the Pennsylvania and Illinois fields of the United States, the Midlands district of England, and the lower Rhine or Westphalian fields of Germany--are also the great centers of the iron and steel industries of these countries. As in the case of iron ore, there is rather a striking absence of important coal production in the southern hemisphere and in Asia. A significant item in the world's distribution of coal supplies is England's world-wide system of coaling stations for shipping. The principal coal-producing countries all have large reserves of coal. Outside of these countries the world's most important reserves are in China, which may be looked to for great future development. For the most part, except for the probable Chinese development, it is likely that countries now producing most of the coal will continue to do so in the future, and that outlying parts of the world will continue to be supplied mainly from these countries. The quantity and distribution of the coal reserves of the world have been estimated with perhaps a greater degree of accuracy than those of any other mineral resource. From these estimates it appears that the North American continent contains about half of the world reserves (principally in the United States, with lesser amounts in Canada) and Asia about one-fourth (principally in China, with some in India). Europe contains only one-sixth of the world total, chiefly in the area of the former German Empire and in Great Britain, with smaller quantities in Russia, Austria-Hungary, France, and Belgium. Australasia (New South Wales), Africa (British South Africa), and South America (Chile, Brazil, Peru, and Colombia), together contain less than a tenth of the total reserves. Coal being one of the great bases for modern industrialism, the large reserves of high grade-coals in China have led to the belief that China may some day develop into a great manufacturing nation. Similarly, the deficiency in coal of most of the South American and African countries seems to preclude their developing any very large manufacturing industries, except where water power is available. Coal reserves and the conservation of coal are further discussed in Chapter XVII. The war resulted in considerable disturbances in coal production and distribution. There has not yet been a return to normal conditions, and some of the changes are probably permanent. The great overseas movement of coal from Germany was stopped and that from England curtailed. To some extent the deficiency was supplied by coal exports from the United States, particularly to South America. The shutting off of the normal German export to France and Mediterranean countries, the occupation of the French and Belgian coal fields by the Germans, and the partial restriction of German exports to Scandinavian countries, resulted in Europe's absorbing most of the British coal available for export, and in addition requiring coal from the United States. The stress in the world's coal industry to meet the energy requirements of war is too recent and vivid to require more than mention. The world was made to realize almost for the first time the utterly vital and essential nature of this industry. Since the war, there has been a gradual resumption of England's export of coal along old lines of international trade. The German overseas export trade has not been reëstablished, and cannot be for a long time to come if Germany fulfills the terms of the Peace Treaty. Indeed, because of slow recovery in output of German coal, there is yet considerable lag in the supply available for European countries. The terms of the Peace Treaty lessened the territory of German coal reserves and required considerable additional contributions of coal to be delivered to France, Belgium, Luxemburg, and Italy. The increased export of coal from the United States during the war is likely to be in part continued in the future, although the great bulk of the United States production will in the future, as in the past, be absorbed locally. Most of the coal in the United States available for export is higher in volatile matter than the British and German export coal. This quality will in some degree be a limiting factor in exportation. On the other hand, it may result in wider introduction of briquetting, coking, and other processes, which will tend to improve the local industry and be conservational in their effect. Japan will doubtless hold some of the Asiatic coal market gained during the war. International coal relations are further discussed in Chapter XVIII.[17] =Production in the United States.= The main features of the distribution of coal supplies in the United States are: (1) Localization of the anthracite production and reserves in a limited area in the Lawton region of Pennsylvania. Low-grade anthracite coal also occurs in Rhode Island, North Carolina, Colorado, and Idaho. (2) Localization of the bituminous production in the eastern and interior states of Pennsylvania, West Virginia, Ohio, Indiana, Illinois, and Kentucky. The principal reserves of bituminous coal occur in the same provinces, but important additional reserves are known in Texas, in North and South Carolina, and in the Rocky Mountain and Pacific Coast provinces. (3) The existence of large tonnages of subbituminous coal in the west, which have not been mined to any extent. (4) The existence of large fields of lignite in the Gulf Coast region, and in the Northern Plains region, which have not been mined. =Coke.= About one-sixth of the bituminous coal mined in the United States is made into _coke_, that is, it is subjected to heat in ovens from which oxygen is excluded in order to drive off the volatile gases (chiefly hydrocarbons and water) which constitute about 40 per cent of the weight of the coal. The residual product, the coke, is a light, porous mass with a considerably higher percentage of fixed carbon than bituminous coal. In regard to composition, coking accomplishes artificially somewhat the same result reached by nature in its slow development of high-grade coals, but the texture of coke is far different from that of coal. Not all bituminous coals are suitable for coke manufacture; and such coals are frequently divided into two classes, known as _coking_ and _non-coking_ coals. Coke is used principally for smelting purposes. Because of its spongy, porous texture, it burns more rapidly and intensely than coal. The gases eliminated in coking are wasted in the old-fashioned "beehive" ovens, but in modern "by-product" coke ovens these gases by proper treatment yield valuable coal tar products and ammonia. It is estimated that the sum of the value of the products thus recovered from a ton of coal multiplies the value of the ton of coal at the mine by at least thirteen times. The importance of this fact from the conservational standpoint cannot be too much emphasized. At present over half of the total coke produced in the United States comes from by-product ovens, and this proportion will doubtless increase in the future. BALANCE SHEET SHOWING CONTRAST BETWEEN VALUE OF 1 TON OF BITUMINOUS COAL AT MINE AND VALUE OF PRODUCTS WHICH IT CONTAINS, BASED ON CONDITIONS PREVAILING IN 1915.[1] _Value at _Value at point of mine 1915_ _Quantity_ production, 1915_ ------------------------------------------------------------------------- 1 ton (2,000 pounds) |(1,500 pounds smokeless fuel $5.00[2] bituminous coal |(10,000 cubic feet gas, at contains $1.13 | 90c. per 1,000 9.00[3] |(22 pounds ammonium sulphate at 2.8c. .61 |(2-1/2 gallons benzol, at 30c. .75[4] |(9 gallons tar, at 2.6c. .23[4] Total $1.13[5]| $15.59 ------------------------------------------------------------------------- 1: Gilbert, Chester G., and Pogue, Joseph E., The energy resources of the United States--A field for reconstruction: _Bull. 102, U. S. National Museum_, vol. 1, 1919, p. 11. 2: Figure based upon approximate selling price of anthracite. 3: Figure based upon average price of city gas. 4: These figures would be much higher if an adequate coal products industry were in existence. 5: This figure shows clearly that lowering the cost of production cannot be expected to lower the price of coal. Even if the cost of production were eliminated, the price of coal would merely be a dollar less. =Classification of coals.= The accurate naming and classification of different varieties of coal is not an easy matter. The three main classes,--anthracite, bituminous, and lignite,--have group characteristics determined by their composition, color, texture, origin, and uses, and for general purposes these names have reasonably definite significance. However, there is complete gradation in coal materials from peat through lignite to bituminous and anthracite coals; many varieties fall near the border lines of the main groups, and their specific naming then becomes difficult. In addition, coal is made up of several substances which vary unequally in their proportions. It is difficult to arrange all of these variables in a graded series in such a fashion as to permit of precise naming of the coal. Furthermore, the scientific naming of a coal may not serve the purpose of discriminating coals used for different commercial purposes. Even the commercial names vary among themselves, depending on the use for which the coal is being considered. Thus it is that the naming and classification of coals is a perennial source of difficulty and controversy. The earliest and most widely used classification is based on the ratio between fixed (or non-volatile) carbon and volatile constituents, called the "fuel ratio." For this purpose "proximate" analyses of coal are made, in terms of fixed carbon, volatile matter, moisture, ash, and sulphur. Anthracite has a higher fuel ratio than bituminous coal; that is, it has more fixed carbon in relation to volatile matter. Similarly bituminous coal has a higher fuel ratio than lignite. The fuel ratio measures roughly the heat or calorific power of the coal, in other words, its fuel value. However, some bituminous coals have a higher calorific power than some anthracites, because a large part of their volatile matter is combustible and yields more heat than the corresponding weight of fixed carbon in the anthracite. The fuel ratio pretty well discriminates coals of the higher ranks, and gives a classification corresponding roughly with their commercial uses. For the lower ranks of coal it is not so satisfactory, because the volatile constituents of such coals contain large and varying percentages of non-combustible hydrogen, oxygen, and nitrogen. Also such coals contain larger and more variable amounts of moisture, which is inert to combustion and requires heat for its evaporation. Two coals of the lower ranks with the same fuel ratio may have very different fuel qualities and different commercial uses, because of their different amounts of inert volatile matter and of water. For these coals it is sometimes desirable to supplement the chemical classification by physical criteria. For instance, subbituminous coal may be distinguished from lignite, not by its fuel ratio alone, but by its shiny, black appearance as contrasted with the dull, woody appearance of lignite. Bituminous may be distinguished from subbituminous by the manner of weathering. Other classifications have attempted to recognize these difficulties and still maintain a purely chemical basis by considering separately the combustible and non-combustible volatile constituents. For this purpose, it is necessary to have not merely approximate analyses, but the ultimate analyses in terms of elements. Definitions of the principal kinds of coal by Campbell,[18] of the United States Geological Survey, are as follows: _Anthracite._ Anthracite is generally well known and may be defined as a hard coal having a fuel ratio (fixed carbon divided by the volatile matter) of not more than 50 or 60 and not less than 10. _Semianthracite._ Semianthracite is also a hard coal, but it is not so hard as true anthracite. It is high in fixed carbon, but not so high as anthracite. It may be defined as a hard coal having a fuel ratio ranging from 6 to 10. The lower limit is uncertain, as it is difficult to say where the line should be drawn to separate "hard" from "soft" coal and at the same time to divide the two ranks according to their fuel ratio. _Semibituminous._ The name "semibituminous" is exceedingly unfortunate, as literally it implies that this coal is half the rank of bituminous, whereas it is applied to a kind of coal that is of higher rank than bituminous--really superbituminous. Semibituminous coal may be defined as coal having a fuel ratio ranging from 3 to 7. Its relatively high percentage of fixed carbon makes it nearly smokeless when it is burned properly, and consequently most of these coals go into the market as "smokeless coals." _Bituminous._ The term "bituminous," as generally understood, is applied to a group of coals having a maximum fuel ratio of about 3, and hence it is a kind of coal in which the volatile matter and the fixed carbon are nearly equal; but this criterion cannot be used without qualification, for the same statement might be made of subbituminous coal and lignite. As noted before, the distinguishing feature which serves to separate bituminous coal from coals of lower rank is the manner in which it is affected by weathering. _Subbituminous._ The term "subbituminous" is adopted by the Geological Survey for what has generally been called "black lignite," a term that is objectionable because the coal is not lignitic in the sense of being distinctly woody, and because the use of the term seems to imply that this coal is little better than the brown, woody lignite of North Dakota, whereas many coals of this rank approach in excellence the lowest grade of bituminous coal. Subbituminous coal is generally distinguishable from lignite by its black color and its apparent freedom from distinctly woody texture and structure, and from bituminous coal by its loss of moisture and the consequent breaking down of "slacking" that it undergoes when subjected to alternate wetting and drying. _Lignite._ The term "lignite," as used by the Geological Survey, is restricted to those coals which are distinctly brown and either markedly woody or claylike in their appearance. They are intermediate in quality and in development between peat and subbituminous coal. [Illustration: FIG. 5. Diagrams showing the chemical composition and heat efficiency of the several ranks of coal. Upper diagram: Comparative heat value of the samples of coal represented in the lower diagram, computed on the ash-free basis. Lower diagram: Variation in the fixed carbon, volatile matter, and moisture of coals of different ranks, from lignite to anthracite, computed on samples as received, on the ash-free basis. After Campbell.] GEOLOGIC FEATURES Geologic features of coal may be conveniently described in terms of origin or genesis. Coal has essential features in common with asphalt, oil, and gas. They are all composed of carbon, hydrogen, and oxygen, with minor quantities of other materials, combined in various proportions. They are all "organic" products which owe their origin to the decay of the tissues of plants and perhaps animals. They have all been buried with other rocks beneath the surface. The common geologic processes affecting all rocks have in the main determined the evolution of these organic products and the forms in which we now find them. Originating at the surface, they have participated in the constructive or anamorphic changes of the metamorphic cycle, which occur beneath the surface, and under these influences have undergone various stages of condensation, refinement, distillation, and hardening. All stages in the development of coal have been traced. In brief, the story is this: [Illustration: FIG. 6. Origin and development of coal. After Gilbert.] This exhibit shows the successive chemical stages in the evolution of coal. The striking qualities of the original are lost in the reproduction through the use of designs in the place of realistic coloring, but the effect is retained sufficiently to indicate the nature of the sequence and the directness with which it leads back to an origin in vegetal accumulations. The evolutionary process is seen to take the form of increasing density through the progressive expulsion of volatilizable matters in the course of geologic time. This inference is substantiated beyond reasonable question by the actual presence of organic remains in coal beds. Grasses, trees, and other plants growing in swamps and bogs decay and form a vegetable mold in the nature of _peat_. A peat bog from the top downward consists of (1) living plants, (2) dead plants, and (3) a dense brownish-black mass, of decayed and condensed vegetable material, in which the vegetable structure is more or less indistinct. Peat consists chiefly of fixed carbon and volatile matter, also of sulphur, moisture, and ash. The volatile matter consists mainly of various combinations of hydrogen and carbon, called hydrocarbons; it goes off in gas or smoke when the peat is heated to a red heat. The fixed carbon is the carbon left after the volatile matter has been driven off. The ash represents the more incombustible mineral matter, usually of the nature of clay or slate. The moisture in peat may be as high as 90 per cent. The essential condition for thick accumulation of peat seems to be abundance of moisture, which favors luxuriant growth and protects the plant remains from complete oxidation or decay. Without moisture the vegetable material would completely oxidize, leaving practically no residue, as it does in dry climates. For the formation of thick peat beds, there seems to be implied some sort of a balance between the slow building up of organic accumulations and the settling of the area to keep it near the elevation of the water table. Present day bog deposits are known in some cases to have a thickness of forty feet. This thickness is not enough to account for some of the great coal seams within the earth; but there seems to be no escape from the conclusion that the same sort of deposits, formed on a larger scale in the past, were the first step in the formation of the coal seams. Flat, swampy coastal plains are believed to furnish the best conditions for thick accumulation of peat. There is good evidence that most of the deposits accumulate essentially in place, without appreciable transportation. In time these surface accumulations of vegetable material may subside and be buried under clay, sand, or other rock materials. The processes of condensation begun in the peat bog are then carried further. They result in the second stage of coal formation, that of _lignite_ or _brown coal_. This is brown, woody in texture, and has a brown streak. It has a higher percentage of fixed carbon, and less volatile matter and water, than peat. Continuation of the processes of induration produces _subbituminous coal_, or _black lignite_, which is usually black and sometimes has a fairly bright luster. It is sometimes distinguished from bituminous coal, where weathered or dried, by the manner in which it checks irregularly or splits parallel to the bedding,--the characteristic feature of bituminous coal being columnar fracture. The next stage in coal formation is _bituminous coal_. It has greater density than the lignites or subbituminous coals, is black, more brittle, and breaks with a cubical or conchoidal fracture. It is higher in fixed carbon, lower in volatile matter and water. A variety of bituminous coal, called _cannel coal_, is characterized by an unusually high percentage of volatile matter, which causes it to ignite easily. This material has a dull luster and a conchoidal fracture. It is composed almost entirely of the spores and spore cases, which are resinous or waxy products, of such plants as lived in the parent coal swamp. There are gradations from bituminous coal into _anthracite coal_. _Semibituminous_ and _semianthracite_ are names used to some extent for these intermediate varieties. The final stage of coal formation is anthracite,--hard, brittle, black, with high luster and conchoidal fracture. It has a higher percentage of fixed carbon and correspondingly less of the volatile constituents, than any of the other coals. The coals form a completely graded series from peat to the hard anthracite. Comparison of the compositions of the coal materials at different stages shows clearly what has happened. Moisture has diminished, certain volatile hydrocarbons have been eliminated as gases, and oxygen has decreased. On the other hand, the residual fixed carbon, sulphur, and usually ash, have remained in higher percentage. This change in composition is graphically represented in Figure 6. During this process volume has been progressively reduced and density increased. Five feet of wood or plant may produce about one foot of bituminous coal, or six-tenths of a foot of anthracite. The exact physical conditions in the earth which determine the progressive changes in coals, above outlined, cannot be fully specified. Time is one of the factors--the longer the time, the greater the opportunity for accomplishing these results. Another factor is undoubtedly pressure, due to the weight of overlying sediments, or to earth movements. In peat condensational changes of this nature are accomplished artificially by the pressure of briquetting machines. Another factor is believed to be the heat developed by earth movements and vulcanism, which presumably facilitates the elimination of volatile materials, and thus accelerates the gradational changes above described. This is suggested by the fact that in places where hot volcanic lavas have gone through coal beds they have locally produced coals of anthracitic and coke-like varieties. In general, however, it has not been possible to determine the degree to which heat has been responsible for the changes. Coals which have been developed in different localities, under what seem to be much the same heat conditions, may show quite different degrees of progress toward the anthracite stage. Another factor that has been suggested as possibly contributing to the change, is the degree of permeability of the rocks overlying the coal to the volatile materials which escape from the coal during its refinement. It is argued that in areas of folding or of brittle rock where the cover is cracked, volatile gases have a better chance to escape, and that the change toward anthracite is likely to advance further here than elsewhere. Bacterial action is an important factor in the earlier stages, in the partial decay of vegetable matter to form peat; accumulation of waste products from this action, however, appears to inhibit further bacterial activity. Coal deposits have the primary shapes of sedimentary beds. They are ordinarily thin and tabular, and broadly lenticular,--on true scale being like sheets of thin paper. At a maximum they seldom run over 100 feet in thickness, and they average less than 10 feet. Seldom is a workable coal bed entirely alone; there are likely to be several superposed and overlapping seams of coal, separated by sandstones, shales, or other rocks. In Illinois and Indiana there are nine workable coal seams, in Pennsylvania in some places about twenty, and in Wales there are over one hundred, many of which are worked. Some of the seams are of very limited extent; others are remarkably persistent, one seam in Pennsylvania having an average thickness of 6 to 10 feet over about 6,000 square miles of its area. Only 2 per cent of the coal-bearing measures of the eastern United States is actually coal. Even where not subsequently disturbed by deformation, coal beds are not free from structural irregularity. They are originally deposited in variable thicknesses on irregular surfaces. During their consolidation there is a great reduction of volume, resulting in minor faults and folds. Subsequent deformation by earth forces may develop further faults and folds, with the result that the convolutions of a coal bed may be very complex. The beds of a coal-bearing series are usually of differing thickness and competency, and as a consequence they do not take the same forms under folding. Shearing between the beds may result in an intricate outline for one bed, while the beds above and below may have much more simple outlines. In short, the following of a coal seam requires at almost every stage the application of principles of structural geology. It is obvious, also, that the identification and location of sedimentary geologic horizons are essential, and hence the application of principles of stratigraphy. The folios of the United States Geological Survey on coal-bearing areas present highly developed methods of mapping and representing the geologic features of coal beds. On the surface map are indicated the topography, the geologic horizons, and the lines of outcrop of the coal seams. In addition, there are indicated the sub-surface contours of one or more of the coal seams which are selected as datum horizons. The sub-surface structure, even though complex, can be readily read from one of these surface maps. With the addition of suitable cross sections and comparative columnar sections, the story is made complete. In the study of the occurrence of coal seams, the reader cannot do better than familiarize himself with one or more of the Geological Survey folios. The high-grade coals of the eastern and central United States are found in rocks of Carboniferous age. The very name Carboniferous originated in the fact that the rocks of this geologic period contain productive coal beds in so many parts of the world. The coal measures of Great Britain, of Germany, Belgium, and northern France, of Russia, and the largest coal beds of China are all of Carboniferous age. Deposits of this period include the bulk of the world's anthracite and high-grade bituminous coal. Coal deposits of more recent age are numerous, but in general they have had less time in which to undergo the processes of condensation and refinement, and hence their general grade is lower. In the western United States there are great quantities of subbituminous coal of Cretaceous age, and of Tertiary lignites which have locally been converted by mountain upbuilding into bituminous and semibituminous coals. Jurassic coals are known in many parts of the world outside of North America, and lignites of Tertiary age are widely distributed through Asia and Europe. PETROLEUM ECONOMIC FEATURES Petroleum is second only to coal as an energy resource. The rapid acceleration in demand from the automobile industry and in the use of fuel oil for power seems to be limited only by the amounts of raw material available. =Production and reserves.= The distribution by countries of the present annual production of petroleum, the past total production, and the estimated reserves, is indicated in terms of percentages of the world's total in the table[19] on the opposite page. This table indicates the great dominance of the United States both in present and past production of petroleum, as well as the concentration of the industry in a few countries. In addition the United States controls much of the Mexican production as well as production in other parts of the world, making its total control of production at least 70 per cent. of the world's total. Notwithstanding its large domestic production, the United States has recently consumed more oil than it produces. Imports of crude oil are about balanced by exports of kerosene, fuel oils, lubricants, etc. The per capita consumption of petroleum in the United States is said to be twenty times greater than in England. On the other hand, the remaining principal producers consume far less than they produce, the excess being exported. The oil from the United States, Russia, the Dutch East Indies, India, Roumania, and Galicia is for the most part treated at refineries near the source of supply or at tidewater, and exports consist of refined products. The Mexican oil is largely exported in crude form to the United States though increasing quantities are being refined within Mexico. The figures shown in the table for oil reserves are of course the roughest approximations, particularly for some of the less explored countries. However, they are compiled from the best available sources and may serve at least to show the apparent relative positions of the different countries at this time. Further exploration is likely to change the percentages and add very greatly to the totals. The significant feature of these figures is the contrast which they indicate between distribution of reserves and distribution of past production. Particularly do they show that the reserves of the United States, which are more closely estimated than those of any other country, are in a far lower ratio to past production than are the reserves in other countries. It was estimated in 1920 that about 40 per cent of the United States reserves are exhausted.[20] PRESENT AND PAST PRODUCTION AND RESERVES OF OIL, BY COUNTRIES, IN TERMS OF PERCENTAGE OF WORLD'S TOTAL -------------------------+--------------+--------------+------------- | | _Per cent | _Per cent Country |_Per cent of | of total | of total | production, | production, | oil | 1918_ | 1857-1918_ | resources_ -------------------------+--------------+--------------+------------- United States and Alaska | 69.15 | 61.42 | 16.26 Mexico | 12.40 | 3.80 | 10.51 Russia (southeastern | | | Russia, southwestern | | | Siberia, region of the | | | Caucasus, northern | | | Russia, and Saghalien) | 7.86 | 24.96 | 15.69 East Indies | 2.58 | 2.51 | 7.00 Roumania, Galicia, and | | | western Europe | 2.79 | 4.07 | 2.64 India | 1.55 | 1.41 | 2.31 Persia and Mesopotamia | 1.40 | .19 | 13.52 Japan and Formosa | .48 | .51 | 2.87 Egypt and Algeria | .40 | .07 | 2.15 Germany | .14 | .22 | -- Canada | .06 | .33 | 2.31 Northern South America, | | | including Peru, | | | Trinidad and Venezuela | .93 | .43 | 13.31 Southern South America, | | | including Bolivia and | | | Argentina | .26 | .06 | 8.24 China | -- | -- | 3.19 Italy | } | | Cuba | } | .02 | Other countries | } | | | ------ | ------ | ------ World total | 100.00 | 100.00 | 100.00 -------------------------+--------------+--------------+------------- Looking forward to the future, it is clear that there will be considerable shifts in the centers of principal production of petroleum in the directions indicated by the reserve figures. In particular, conspicuous development of production may be expected in the immediate future in the countries bordering the Caribbean Sea and the Gulf of Mexico. In the eastern hemisphere production is rapidly increasing in Persia and Mesopotamia; and Russia, with the stabilization of political conditions, may become ultimately the world's leading oil producer. At the now indicated rate of production, world reserves now estimated would be exhausted in eighty-six years and the peak of production would be passed earlier. With continuing acceleration of production, total reserves would be exhausted in considerably less time,--providing physical conditions would allow the oil to be pumped from the ground at the necessary speed, which they probably will not. These figures taken at face value are alarming; but the earth offers such huge possibilities for further discoveries that the life of oil reserves above indicated is likely to be considerably extended. At many times in the history of the mineral industry the end has apparently been in sight for certain products; but with the increased demand for these products has come increased activity in exploration, with the result that as yet no definite end has been approached for any one of them. The more immediate problems of the petroleum industry seem to the writer to be of rather different nature: first, whether the discovery and winning of the oil can be made to keep pace with the enormous acceleration of demand; and second, the adjustment of political and financial control of oil resources, the possession of which is becoming so increasingly vital to national prosperity. In regard to the first question, it is a much more difficult problem today to locate and develop a supply of oil to replace the annual world production (recently half a billion barrels), than it was twenty years ago, when it was necessary for this purpose to find only one-fifth this amount; and if the demand is unchecked, it will be still more difficult to replace the three-quarters of a billion barrels of oil which will doubtless be required in a very few years. Regardless of the amount of oil actually in the ground, it is entirely possible that physical limitations on its rate of discovery and recovery will prevent its being made available as fast as necessary to meet the increasing demand. This fact is likely to make itself felt through increase of price. Other natural results should be the development of substitutes, such as alcohol or benzol for gasoline; the larger recovery of oil from oil shales; and the general speeding up of conservational measures of various kinds. These are all palliatives and not essential remedies. To make enough alcohol to substitute for the gasoline now coming from oil would use a very considerable fraction of the world's food supply. To make enough benzol (a by-product of coke) to replace gasoline would necessitate the manufacture of many times the amount of coke now required by the world's industries. To develop the oil shale industry to a point where it could supply anything like the amount of oil now derived from oil pools would mean the building of great plants, including towns, railroads, and other equipment, equivalent to the plants of the coal mining industry. To apply any one of the various conservational measures discussed on later pages would only temporarily alleviate the situation. The question of political and financial control of oil supplies may be illustrated by particular reference to the United States. On present figures it appears that within three to five years the peak of production in this country will be passed; and at the present rate of production the life of the reserves may not be over seventeen to twenty years. Of course production could not continue to the end at this rate, and the actual life will necessarily be longer. Again the doubtful factor is the possibility for further discoveries. Many favorable structures have been mapped which have not yet been drilled, and there are considerable unexplored areas where the outcrops are so few that there is no clue at the surface to the location of favorable structures. The future is likely to see a considerable amount of shallow drilling for the sole purpose of geological reconnaissance. For upwards of ten years important parts of the public domain have not been available for exploration, but Congress has now enacted legislation which opens up vast territories for this purpose. Even with large allowance for these possibilities, it seems unlikely that production in the United States can increase very long at the accelerating rate of the domestic demand, which is already in excess of domestic production. The supplies of Mexico are in a large part controlled by American capital and are thus made available to the United States (subject, of course, to political conditions); but even with these added, the United States is in a somewhat unfavorable situation as compared with certain other countries. This situation is directing attention to the possibility of curtailment of oil exports, and to the possibility of acquiring additional oil supplies in foreign countries. In this quest the United States is peculiarly handicapped in that most foreign countries, in recognition of the vital national importance of the oil resource, have imposed severe restrictions on exploration by outsiders. Nationals of the United States are excluded from acquiring oil concessions, or permitted to do so only under conditions which invalidate control, in the British Empire, France, Japan, Netherlands, and elsewhere, and the current is still moving strong in the direction of further exclusion. As the United States fields are yet open to all comers, it has been suggested that some restriction by the United States might be necessary for purposes of self-protection, or as an aid in securing access to foreign fields. The activity of England during and since the war has increased the amount of oil controlled by that country from an insignificant quantity to potentially over half of the world's oil reserves. The problem of future oil supplies for the United States presents an acute phase of the general question of government coöperation or participation in mineral industries, which is further discussed in Chapter XVIII. The following table summarizes the distribution of the oil production in the United States, together with the salient features of its geologic distribution and character. This table, in conjunction with Fig. 8 below, shows clearly that the bulk of the United States production of oil comes from two great sources--the Pennsylvanian sandstones of the Mid-Continent field in Kansas and Oklahoma, and the Cretaceous and Tertiary sediments of the southern half of California. Phenomenal development of the Central and North Texas field in 1919 increased its yield to about one-sixth of the country's total. The older Appalachian oil field, extending from New York to West Virginia and Tennessee, was the earliest area discovered; it is still one of the more productive fields, though it has long since passed its maximum production. The other principal sources of oil are the Gulf Coast field in Louisiana and Texas, the North Louisiana field, the southern Illinois field, and the Rocky Mountain region. This last region, containing large amounts of government land recently opened to exploration, bids fair to produce increasing quantities of oil for some time. PAST PRODUCTION OF PETROLEUM IN THE UNITED STATES. (FIGURES FROM U. S. GEOLOGICAL SURVEY) -------------+-----------------+----------+-----------+--------------- | | | | _Total | _Age of | |_Production| production | containing | | for 1918 |including 1918 _State_ | rocks_ | _Base_ |(barrels)_ |(barrels)_ -------------+-----------------+----------+-----------+--------------- Alaska |East-Low. |Paraffin | (a) | (a) | Tertiary | | | | West-Jurassic | | | California |Cretaceous: | | | | Tertiary |Asphalt | 97,531,997| 1,110,226,576 Colorado |Pierre-Cretaceous|Paraffin | 143,286| 11,319,370 Illinois |Mississippian- |Paraffin | 13,365,974| 298,225,380 | Pennsylvanian | | | Indiana |East-Ordovician |Paraffin | 877,558| 106,105,584 | (Trenton) West-| | | | Pennsylvanian | | | Kansas |Pennsylvanian |Par.-Asph.| 45,451,017| 148,450,298 Kentucky, |Mississippian |Paraffin | 4,376,342| 18,213,188 Tennessee | | | | Louisiana |Cretaceous-Quat. |Paraffin | 16,042,600| 150,769,911 | Cretaceous- | | | | Eocene | | | Michigan, |Carboniferous |Paraffin | (a) | (a) Missouri | | | | Montana | -- | -- | 69,323| 213,639 New Mexico |Carboniferous- | -- | (a) | (a) | Cretaceous | | | New York, |Devonian- |Paraffin | 8,216,655| 788,202,717 Pennsylvania| Carboniferous | | | Ohio, East |Ordovician- |Paraffin | 7,285,005| 463,367,386 and West | Carboniferous | | | Oklahoma |Pennsylvanian |Paraffin |103,347,070| 851,320,457 Texas |Pennsylvanian, |Asph.-Par.| 38,750,031| 327,550,005 | Cretaceous-Quat.| | | Utah | -- | -- | (b) | (b) West |Devonian- | -- | 7,866,628| 294,474,710 Virginia | Carboniferous | | | Wyoming |Carboniferous- |Asph.-Par.| 12,596,287| 40,019,573 | Cretaceous | | | Other | -- | -- | 7,943| 112,925 | | |-----------| ------------- | | |355,927,716| 4,608,571,719 -------------+-----------------+----------+-----------+--------------- (a) Included in "Other." (b) Included in Wyoming. [Illustration: FIG. 7. Chart showing the present tendency of the United States in respect to its unmined reserve of petroleum. Data from U.S. Geological Survey. After Gilbert and Pogue.] =Methods of estimating reserves.= It may be of interest to inquire into the basis on which predictions are made of the life of an oil pool. The process is essentially a matter of platting curves of production, and of projecting them into the future with the approximate slopes exhibited in districts which are already approaching exhaustion.[21] While no two wells or two districts act exactly alike, these curves have group characteristics which are used as a rough basis for interpreting the future. [Illustration: FIG. 8. The annual output of the principal oil fields of the United States for the last twenty years. Data from U.S. Geological Survey.] A less reliable method is to calculate from geologic data the volume and porosity of the oil-bearing reservoirs, and to estimate the percentage of recovery on the basis of current practices and conditions. Complete data for this method are often not available; but in the early years of a field, before production curves are established, this method may serve for a rough approximation. [Illustration: FIG. 9. Curve showing the usual decline in oil field production after the period of maximum output is reached. After Ralph Arnold. The Petroleum Resources of the United States, Smithsonian report for 1916, p. 283. Compare this theoretical curve of final decrease with the production curve shown in Fig. 8.] =Classes of oils.= When crude petroleum is distilled, it gives off in succession various substances and gradually thickens until it leaves a solid residue, which may be largely either paraffin wax or asphalt. The two main classes of oils are determined by the nature of this solid residual. The products given off are natural gas and then liquid hydrocarbons of various kinds, which evaporate in the order of their lightness. Petroleum is thus a mixture or mutual solution of different liquids, gases, and solids. Nearly one-fifth of the domestic consumption of crude petroleum is burned directly as fuel, and four-fifths are refined. The several principal primary products of refinement are gasoline, kerosene, fuel oil, and lubricating oil; but these may be broken up into other substances, each the starting point of further refinements, with the result that present commercial practice yields several hundred substances of commercial value. With increasing chemical and technical knowledge these products are being multiplied. The rapidly increasing demand for gasoline has led to the use of processes which extract a large proportion of this substance from the raw material, by "cracking" or breaking up other substances; but while, under the stress of necessity, there is possibility of slight modification of the proportions of principal substances extracted from the crude oil, it is not possible to change these proportions essentially. It is, therefore, a problem to adjust relative demands to supplies of the different products. The domestic demand for gasoline is greater than the supply. On the other hand, the demand for kerosene, which must be produced at the same time, is much less than the domestic supply. Hence the importance of maintaining export markets for kerosene. The nature or grade of the oil of various fields is an important matter in considering reserves for the future. Perhaps half of the United States reserves consist of the asphalt-base oils of the California and certain of the Gulf fields, which yield comparatively small amounts of gasoline and other valuable light products, though they are very satisfactory for fuel purposes. Similarly the large reserve tonnages of oil in Mexico and the Caribbean countries, in Peru, and probably in Russia, are essentially of the heavier, lower grade oils. The oils of the Mid-Continental and eastern fields of the United States, of Ontario, of the Dutch East Indies, of Burma, and of Persia and Mesopotamia are reported to be largely of the paraffin base type, which, because of its larger yield of gasoline and light oils, is at present considerably more valuable. These generalizations are of course subject to qualifications, in that the oils of a given region may vary considerably, and that some oils are intermediate in character, containing both asphalt and paraffin wax. =Conservation of oil.= The rapid increase in demand for oil as compared with discovery of new sources is leading naturally to a more intensive study of the conservational aspects of the industry. This is a complex and difficult subject which we shall not take up in detail, but we may point out some of the phases of the problem which are receiving especial attention. [Illustration: FIG. 10. Chart showing the relative values of the principal petroleum products manufactured in the United States from 1899 to 1914. After Gilbert and Pogue. Note the decreasing importance of kerosene in sustaining the cost of refining, and the necessity of exports for maintaining a balanced outlet of products. Data from Story B. Ladd, Petroleum Refining. Census of Manufactures: 1914, Bureau of Census, Washington, 1917, p. 10.] About 50 per cent of the oil in the porous strata, of oil pools is ordinarily not recovered, because it clings to the rock. Efforts are being made along various lines to increase the percentage of recovery,--as, for instance, in preventing infiltration of water to the oil beds and in the use of artificial pressures and better pumping. "Casing-head gasoline" is being recovered to an increasing extent from the natural gas which was formerly allowed to dissipate in the air. Minute division of the ownership of a pool, with consequent multiplication of wells and unrestricted competition, tends to gross over-production and highly wasteful methods. The more rapid exhaustion of one well than the others may result in the flooding of the oil sands by salt waters coming in from below. Various efforts have been made toward a more systematic and coördinated development of oil fields. In general, the organization and technique involved in the development of an oil field are improving in the direction of extracting a greater percentage of the total available oil. Better methods of refining the oil, and the refining of a larger percentage of the crude oil, make the oil more available for a greater variety of purposes and therefore more valuable. Great advances have been made along these lines, particularly in the application of the "cracking" method for a greater recovery of the more valuable light oils at the expense of the less valuable heavy oils. Similarly, modifications of internal combustion engines will probably permit the use, in an increasing number of cases, of products of lower volatility than gasoline. One of the conservational advances in coming years will probably be a restriction in the amount of crude oil used directly for fuel and road purposes without refining. These crude uses cut down the output of much desired products from the distillation of the oil. Various other restrictions in the use of oil have been proposed, such as the curtailment of the use of gasoline in pleasure cars. The gasless Sundays during the war represented an attempt of this kind. In general, it seems likely that such restrictions will come mainly through increase in the price of oil products. The substitution of oil from oil shales, and of alcohol for gasolene, already mentioned, will be conservational so far as the oil is concerned, though perhaps not so in regard to other elements of the problem. GEOLOGIC FEATURES =Organic theory of origin.= According to this theory, accumulations of organic materials in sedimentary beds, usually muds or marls, have been slowly altered and distilled during geologic ages; the products of distillation have migrated chiefly upward to porous strata like sandstones or cavernous limestones, where, under suitable conditions, they have become trapped. The original organic material is believed to have been plants of low order and animal organisms (such as foraminifera) which were deposited as organic detritus with mud and marl in the bottoms of ponds, lakes, estuaries, and on the sea bottom,--in both salt and fresh waters. Bacteria are supposed to have played a part in the early stage of alteration, sometimes called the biochemical stage. When the organic matter was buried under later sediments and subjected to pressure, physical conditions were responsible for further volatilization or distillation. This stage is called the geochemical stage. There is much in common as to origin between coal, oil shales, and petroleum. According to White,[22] whether the ingredient organic matter, be it plant or animal, will be in part transformed to coal of the ordinary type, to cannel, to oil shale, to the organic residues in so-called bituminous shales and carbonaceous shales, or to petroleum and natural gas, is dependent upon the composition of the ingredient organic débris, the conditions of its accumulation or deposition, and the extent of the microbian action. White has further developed the important principle that, in the geochemical stage of development, both coal and oil react to physical influences in much the same way; and that therefore when both are found in the same geologic series, the degree of concentration of the coal, measured by its percentage of carbon, may be an indication of the stage of development of the oil. More specifically where the coal contains more than 65 to 70 per cent of fixed carbon, chances for finding oil in the vicinity are not good (though commercial gaspools may be found), probably for the reason that the geochemical processes of distillation have gone so far as to volatilize the oils, leaving the solid residues in the rock. White also finds that the lowest rank oils, with considerable asphalt, are found in regions and formations where the coal deposits are the least altered, and the lighter, higher rank oils, on the whole, where the coal has been brought to the correspondingly higher ranks; in other words, up to the point of complete elimination of the oil, improvement in quality of the oil accompanies increased carbonization of the coal. The principle, therefore, becomes useful in exploration in geologic series where oil is associated with coal. Where the coal is in one series and the oil in another, separated by unconformity (indicating different conditions of development), the principle may not hold, even though there is close geographical association. The oil and gas distillates migrate upward under gas pressure and under pressure of the ground-water. If there are no overlying impervious beds to furnish suitable trapping conditions, or conditions to retard the flow, the oil may be lost. The conditions favorable for trapping are overlying impervious beds bowed into anticlines, or other structural irregularities, due either to secondary deformation or to original deposition, which may arrest the oil in its upward course. A dome-like structure or anticline may be due to stresses which have buckled up the beds, or to unequal settling of sediments varying in character or thickness; thus some of the anticlinal structures of the Mid-Continent field may be due to settling of shaley sediments around less compressible lenses of sandstone which may act as oil reservoirs, or around islands which stood above the seas in which the oil-bearing sediments were deposited and on the shores of which sands capable of acting as oil reservoirs were laid down. Favorable conditions for trapping the oil may be furnished by impervious clay "gouges" along fault planes, or by dikes of igneous rock. Favorable conditions may also be merely differences in porosity of beds in irregular zones, determined by differences either in original deposition or in later cementation. The thickness of oil-producing strata may vary between 2 or 3 feet and 200 feet. The porosity varies between 5 and 50 per cent. In sandstones the average is from 5 to 15 per cent. In shales and clays, which are commonly the impervious "cap-rocks," porosity may be equally high, but the pores are too small and discontinuous to permit movement. When the impervious capping is punctured by a drill hole, gas is likely to be first encountered, then oil, and then water, which is usually salty. The gas pressure is often released with almost explosive violence, which has suggested that this is an important cause of the underground pressures. It has been supposed also that the pressures are partly those of artesian flows. The vertical arrangement of oil, gas, and water under the impervious capping is the result of the lighter materials rising to the top. In certain fields, oil and gas have been found in the tops of anticlines in water-saturated rocks, and farther down the flanks of folds or in synclines in unsaturated rocks. The localization of oil pools is evidently determined partly by original organic deposition, often in alignment with old shore lines, and partly by the structural, textural, and other conditions which trap the oil in its migration from the source. =Effect of differential pressures and folding on oil genesis and migration.= Another organic hypothesis proposed somewhat recently[23] is that oil is formed by differential movement or shearing in bituminous shales, which are often in close relationship with the producing sand of an oil field, and that the movement of oil to the adjacent sands is accomplished by capillary pressure of water and not by ordinary free circulation of water under gravity. The capillary forces have been shown to be strong enough to hold the oil in the larger pores against the influence of gravity and circulation. The accumulation of the oil into commercial pools is supposed to take place in local areas where the oil-soaked shale, due to jointing or faulting, is in direct contact with the water of the reservoir rock. This suggests lack of wide migration. This hypothesis is based on experimental work with bituminous shales. The general association of oil pools with anticlinal areas is explained on the assumption that anticlines on the whole are areas of maximum differential movement, resulting in oil distillation, and that they are ordinarily accompanied by tension joints or faults, affording the conditions for oil migration. Data are insufficient, however, to indicate the extent to which the anticlinal areas are really areas of maximum shearing. As regards the exact nature of the process, it is not clear to what extent differential movement may involve increase in temperature which may be the controlling factor in distillation,--although in McCoy's experiment oil was formed when no appreciable amount of heat was generated. The development of petroleum by pressure alone acting on unaltered shale, as shown by these experiments, has been taken by White[24] to have a significant bearing on the geochemical processes of oil formation. Under differential stresses acting on fine-grained carbonaceous strata under sufficient load, there is considerable molecular rearrangement, as well as actual movement of the rock grains,--thus promoting the distillation of oil and gas from the organic matter in the rocks, and the squeezing out of the oil, gas, and water into adjacent rocks, such as coarse round-grained sandstones and porous limestones, which are more resistant to change of volume under pressure. Migration, concentration and segregation of the oil, gas, and water is supposed to be brought about, partly through the effect of capillary forces--the water, by reason of its greater capillary tension, tending to seize and hold the smaller voids, and thus driving the oil and gas into the larger ones--and partly through the action of gravity. White also suggests that the process may go further where the parent carbonaceous strata are of such thickness and under such load of overlying rocks that they undergo considerable interior adjustment and volume change before yielding to stress by anticlinal buckling,--than where the strata yield quickly. It is not clear to the writer that the interior adjustment assumed under this hypothesis is necessarily slowed up or stopped by anticlinal buckling. Interior stresses are inherent in any sedimentary formation, when settling and consolidating and recrystallizing under gravity, and these may be independent of regional thrusts from without. The first oils evolved by pressure from the organic mother substance are probably heavy, the later oils lighter, and the oils from formations and regions where the alteration is approaching the carbonization limit are characteristically of the highest grade. This is the reverse of the order of products obtained by heat distillation. Whether there is also a natural fractionation and improvement of the first heavy oils as they undergo repeated migrations is not known. =Inorganic theory of origin.= Another theory of the source of oil has had some supporters, although they are much in the minority. This is the so-called "inorganic" theory, that oil comes from magmas and volcanic exhalations. In support of this theory attention is called to the fact that igneous rocks and the gases associated with them frequently carry carbides or hydrocarbons; that many oil fields have a suggestive geographic relationship with volcanic rocks; and that certain of the oil domes, as for instance in Mexico, are caused by plugs of igneous rocks from below. It has been suggested that deep within the earth carbon is combined with iron in the form of an iron carbide, and that from the iron carbide are generated the hydrocarbons of the oil, either by or without the agency of water. Iron carbide is magnetic, and significance has been attached to the general correspondence between the locations of oil in the western United States and regions of magnetic disturbance. It seems not unlikely that some inorganic theory of this sort is necessary to explain the ultimate source of oil or of the substances which become oil, but the evidence is overwhelming that organic agencies have been mainly responsible for the principal oil pools now known. =Oil exploration.= A simple geographic basis for oil exploration is the fact that the major oil fields of the world are situated between 20° and 50° north latitude, and that thus far there are no major oil areas within the tropics or within the southern hemisphere. This broad generalization may have little value when exploration is carried further. It has also been suggested that the geographic distribution of oil corresponds roughly with the average annual temperatures, or isotherms, between 40° and 70.°[25] It is thought that this present distribution of temperatures may indicate roughly the temperatures of the past when the oil was accumulated; and the inference is drawn that there was some sort of limitation of areal deposition within these temperature limits. If this be true, the only reasons why the southern hemisphere is not productive are the relatively small size of the land areas and the lack of exploration to date. In approaching broadly the problem of oil exploration, the geologist considers in a general way the kinds and conditions of rocks which are likely to be petroliferous or non-petroliferous. Schuchert[26] summarizes these conditions for North America as follows: 1. The impossible areas for petroliferous rocks. (_a_) The more extensive areas of igneous rocks and especially those of the ancient shields; exception, the smaller dikes. (_b_) All pre-Cambrian strata. (_c_) All decidedly folded mountainous tracts older than the Cretaceous; exceptions, domed and block-faulted mountains. (_d_) All regionally metamorphosed strata. (_e_) Practically all continental or fresh-water deposits; relic seas, so long as they are partly salty, and saline lakes are excluded from this classification. (_f_) Practically all marine formations that are thick and uniform in rock character and that are devoid of interbedded dark shales, thin-bedded dark impure limestones, dark marls, or thin-bedded limy and fossiliferous sandstones. (_g_) Practically all oceanic abyssal deposits; these, however, are but rarely present on the continents. 2. Possible petroliferous areas. (_a_) Highly folded marine and brackish water strata younger than the Jurassic, but more especially those of Cenozoic time. (_b_) Cambrian and Ordovician unfolded strata. (_c_) Lake deposits formed under arid climates that cause the waters to become saline; it appears that only in salty waters (not over 4 per cent?) are the bituminous materials made and preserved in the form of kerogen, the source of petroleum; some of the Green River (Eocene) continental deposits (the oil shales of Utah and Colorado) may be of saline lakes. 3. Petroliferous areas. (_a_) All marine and brackish water strata younger than the Ordovician and but slightly warped, faulted, or folded; here are included also the marine and brackish deposits of relic seas like the Caspian, formed during the later Cenozoic. The more certain oil-bearing strata are the porous thin-bedded sandstones, limestones, and dolomites that are interbedded with black, brown, blue, or green shales. Coal-bearing strata of fresh-water origin are excluded. Series of strata with disconformities may also be petroliferous, because beneath former erosional surfaces the top strata have induced porosity and therefore are possible reservoir rocks. (_b_) All marine strata that are, roughly, within 100 miles of former lands; here are more apt to occur the alternating series of thin and thick-bedded sandstones and limestones interbedded with shale zones. The extent to which marine or brackish water conditions of sedimentation are requisite to the later formation of oil, as is suggested in the above quotation, has long been a debatable question. It may be noted that certain oil shales formed in fresh water basins contain abundant organic matter which is undoubtedly suitable for the generation of oil and gas, and that these shales on distillation yield oil essentially like that obtained from oil shales of marine origin; that certain important oil-bearing sands of the younger Appalachian formations were laid down in waters which are believed to have been only slightly saline; that natural gas is present in fresh water basins; and that it has not been demonstrated that salt in appreciable amounts is necessary for the geologic, any more than for the artificial, distillation of oil. Most of the great oil fields have been in regions of marine or other saline water deposits, but it has not been proved that this is a necessary condition. White[27] says: "At the present stage of our knowledge, fresh-water basins appearing otherwise to meet the requirements should be wildcatted without prejudice." The principal oil-bearing horizons in any locality are comparatively few, and it is ordinarily easy to determine by stratigraphic methods the presence or absence of a favorable geologic horizon. By knowing the succession and thicknesses of the beds in a given region it is possible to infer from surface outcrops the approximate depth below the surface at which the desired horizon can be found. To do this, however, the conditions of sedimentation, the initial irregularities of the beds, the structural conditions, including unconformities, and other factors must be studied. In exploration for oil the determination of the existence and location of the proper horizon is but an initial step. For instance, the oil of the Midcontinent field of the United States is in the beds of the Pennsylvanian, which are known to occupy an enormous area extending from Illinois and Wyoming south to the Gulf of Mexico. This information is clearly not sufficiently specific to limit the location of drill holes. Sometimes seepages of oil or showings of gas near the surface are sufficient basis for localizing the drill holes.[28] Commonly, however, it is necessary to find some structural feature in the nature of a dome or anticline which suggests proper trapping conditions for an oil pool. This is accomplished by geologic and topographic mapping of the surface. Levels and contours are run and outcrops are platted. As the outcrops are usually of different geologic horizons, it is necessary to select some one or more identifiable beds as horizon markers, and to map their elevations at different points as a means of determining the structural contours of the beds. When several key horizons are thus used, their elevations must be reduced to the elevations of one common horizon by the addition or subtraction of the intervals between them. For instance, knowing the succession, an outcrop of a certain sandstone may indicate that the marking horizon is 200 feet below, and the structural contour is then drawn accordingly. Observations of strike and dip at the surface are helpful; but where the beds are but slightly flexed, small irregularities in deposition may make strike and dip observations useless in determining major structures. It is then necessary to have recourse to the elevations of the marking horizons. In the selection of key horizons, knowledge of the conditions of sedimentation is very important. For example, some of the oil fields occur in great delta deposits, where successive advances and retreats of the sea have resulted in the interleaving of marine and land deposits. The land-deposited sediments usually show great variations in character and thickness laterally and vertically; and a given bed is likely to thin out and disappear when traced for a short distance, rendering futile its use as a marker. The marine sediments, on the either hand, show a much greater degree of uniformity and continuity, and a bed of marine limestone may extend over a large area and be very useful as a key horizon. Over large areas outcrops and records of previously drilled water and oil wells may not be sufficient to give an indication of structure; it then becomes necessary to secure cross sections by drilling shallow holes to some identifiable bed, and to determine the structure from these cross sections, in advance of deeper drilling through a favorable structure thus located. The coöperative effort of the Illinois State Survey and private interests, cited on page 306, is a good illustration of this procedure. This method is only in its infancy, because well-drilling has not yet exhausted the possibilities of structures located from surface outcrops. The so-called anticlinal structures, which have been found by experience to be so favorable to the accumulation of oil, are by no means symmetrical in shape or uniform in size. They may be elongated arches with equal dip on the two sides, or one side may dip and the other be nearly flat. In a territory with a general dip in one direction, a slight change in the angle, though not in the direction of dip, sometimes called an arrested dip, may cause sufficient irregularity to produce the necessary trapping conditions. In other cases the anticline may be of nearly equidimensional dome form. The largest anticlines which have been found to act as specific reservoirs are rarely more than a few miles in extent, and in many cases only a mile or two. The "closure" of an anticline is the difference between the height of a given stratum at the highest point and at the edges of the structure. A considerable number of productive anticlines are known in which the beds dip so gently as to give a closure of 20 feet or less. After the structural outlines of beds near the surface have been determined, all possible information should be used in projecting these structures downward to the oil-producing horizons. Where a number of wells have been previously drilled in the vicinity, examination of their records may indicate certain lateral variations in the thickness of the beds between the horizon which has been mapped and the producing horizon. The effect of such lateral variations may be either to accentuate the surface structure, or to cause it to disappear entirely and thus to indicate lack of favorable trapping conditions. The possibility of several oil-producing beds, at different depths--a not uncommon condition in many fields--should also be kept in mind. As already indicated, anticlines are not always essential to make the necessary trapping conditions. In the Beaumont field of Texas, for instance, it has been shown that irregular primary deposition of sediments differing in porosity both vertically and horizontally allowed the oil to migrate upward irregularly along the porous beds and parts of beds, and to be trapped between the more impervious portions of the beds. Further questions to be considered in the exploration of an area are the content of organic matter in the sediments which may have served as a source of oil, the presence of impervious cap-rocks or of variations in porosity sufficient to retain the oil, the thickness of sediments and the extent to which they have undergone differential stresses, the amount of erosion and the possibilities that oil, if formed, has escaped from the eroded edges of porous strata, and, where carbonaceous beds are present, their degree of carbonization, and many other similar matters. Each field in fact has its own "habit," determined by the interaction of several geologic factors. This habit may be learned empirically. Geologists have often gone wrong in applying to a new district certain principles determined elsewhere, without sufficient consideration of the complexity and relative importance of the sundry geologic factors which in the aggregate determine the local habit of oil occurrence. Geographically associated fields characterized by similarity of oil occurrence, age, and origin, are known as _petroliferous provinces_. The factors entering into the classification of fields are so numerous that more precise definition of a petroliferous province is hardly yet agreed upon. The part played by the economic geologist in oil exploration and development is a large one for the obvious reasons given above. Probably no other single division of economic geology now employs so large a number of geologists. Practically no large oil company, or large piece of oil exploration and development, is now handled without geologic advice. Quoting from Arnold:[29] It ought to be as obvious that exploration with the drill should be preceded by careful geologic studies as it is that railroad construction should be based on surveys. These studies should include such subjects as topography, stratigraphy, structure, and surface evidence of petroleum in the regions to be tested. The work divides itself into two stages--preliminary reconnaissances and detailed surveys. The preliminary reconnaissance should consist in procuring all the available published and hearsay evidence regarding the occurrence of oil or gas seepages or hydrocarbon deposits in the region; in making preliminary geologic surveys to determine from which formations the oil is to come and the areal distribution of these formations; in determining those general regions in which the surface evidence is supposed to be most favorable for the accumulation of hydrocarbons; and in determining the best routes and methods of transportation. The second stage includes detailed geologic surveys of those regions where the surface evidence indicates that petroleum is most likely to be found and the location of test holes at favorable points. By working out the surface distribution and structure of the formations it is usually possible to select the areas offering the best chances of success. Geology should always be the dominant factor in determining the location of test holes, although modifications to meet natural conditions must sometimes be made. OIL SHALES One of the sources of oil which is likely to become important in the future is oil shales,--that is, shales from which oil product can be extracted by distillation. These have already been referred to on previous pages. Such shales are now mined only in Scotland and in France to a relatively small extent, but there are immense reserves of these shales in various parts of the world which are likely to be drawn upon when commercial conditions require it. In the United States alone it is estimated that the oil shales are a potential source of oil in amounts far greater than all the natural petroleum of this hemisphere.[30] The solution of the problem of extraction of oil from shales is fairly well advanced technically, and the problem has now become principally one of cost. In order to recover any large amount of oil from this source, operations of stupendous magnitude, approximately on the scale of the coal industry, must be established. As long as there are sufficient supplies of oil concentrated by nature to be drawn upon, it is unlikely that oil shale will furnish any considerable percentage of the world's oil requirements. With the great increase in world demand for oil, however, which may very possibly outstrip the available annual supply in the future, and particularly with the increase in the United States demand relative to domestic supplies, exhaustive surveys of the situation are being made with a view to development of oil shales when warranted by market conditions. Oil shales are sedimentary strata containing decomposed products of plants and animals. Locally they grade into cannel coal, with which they are genetically related. They may be regarded as representing the kinds of sediments from which the oil of oil pools has in the main originated. The most extensive of the oil shales of the United States are found in the Eocene beds of northwestern Colorado, northeastern Utah, and southwestern Wyoming, and in the Miocene beds of northern Nevada. The largest known foreign deposits occur in Brazil and Russia. NATURAL GAS ECONOMIC FEATURES Natural gas is used both for lighting and for fuel purposes. In the United States it has become the basis of a great industry, the value of the product ranging above that of lead and zinc. The United States is the largest producer of natural gas. Other producers are Canada, Dutch East Indies, Mexico, Hungary, Japan, and Italy. Nearly all producing oil fields furnish also some natural gas. In the United States nearly 40 per cent of the total production of natural gas comes from West Virginia, about 17 per cent from Pennsylvania, about 17 per cent from Oklahoma, and less than 10 per cent from each of Ohio, California, Louisiana, Kansas, Texas, and several other states. One of the recent interesting developments in this industry is the recovery of gasoline from the natural gas. This is obtained by compression and condensation of the casing-head gas from oil wells, and also, more recently, by an absorption process which is applied not only to "wet" gas from oil wells but also to so-called "dry" gas occurring independently of oil. It is a high-grade product which in recent years has amounted to about 10 per cent of the total output of gasoline for the United States. GEOLOGIC FEATURES Natural gas, like oil, originates in the distillation of organic substances in sediments, and migrates to reservoirs capped by impervious strata. It is commonly, though not always, associated with oil and coal. The geologic features of its occurrence have so much in common with oil that a description would essentially duplicate the above account of the geologic features of oil. ASPHALT AND BITUMEN ECONOMIC FEATURES Asphalt and bitumen are not used as energy resources, but they have so much in common with oil in occurrence and origin that they are included in this chapter. Asphalt and bitumen find their main use in paving. Other important uses are in paints and varnishes, in the manufacture of prepared roofing, for various insulating purposes, and in substitutes for rubber. Nearly the entire world's supply of natural asphalt comes from the British Island of Trinidad and from Venezuela. Both of these deposits are under United States commercial control probably affiliated with Dutch-English interests. Prior to the war about half the product went to Europe and half to the United States. Large amounts of asphaltic and bituminous rock, used mainly in paving, are normally produced in Alsace, France, and in Italy. Prior to the war both the Alsatian and Italian deposits were under German commercial control. Their output is practically all consumed in Europe. The United States takes a large part in the world's trade in natural asphalt, by importation from Trinidad and Venezuela, and by some reëxportation chiefly to Canada and Mexico. The United States also produces some natural asphalt and bituminous rock for domestic consumption. Deposits of natural asphaltic material are widely distributed through the United States, but commercial production is limited to a few localities in Kentucky, Texas, Utah, Colorado, Oklahoma, and California. The asphalt manufactured from petroleum constitutes a much larger tonnage than natural asphalt though it does not enter so largely into world trade. The manufactured product is largely but not exclusively in American control. Large amounts are made in this country and will no doubt be made for the next decade, from oil produced in the southwestern states and in Mexico. At the present time as much or more asphalt is made in the United States from Mexican as from domestic crude oil. The refineries are located near the Gulf coast so that exports can avoid overland shipments. The relative merits of natural asphalt and asphalt manufactured from oil may be subject to some discussion; but it is perfectly clear that the manufactured material is sufficient, both in quantity and variety, to make the United States entirely independent and have an exportable surplus. GEOLOGIC FEATURES Natural asphalt and similar products are in the main merely the residuals of oil and gas distillation accumulated by nature under certain conditions already described in connection with oil (pp. 140-144). In some cases the asphaltic material is found as impregnations of sediments, and appears to have remained in place while the lighter organic materials were volatilized and migrated upward. In other cases it occurs in distinct fissure veins; the fissures and cavities apparently were once filled with liquid petroleum, which has subsequently undergone further distillation. The original liquid character of some of these bitumens is shown by occasional fragments of unworn "country rock" imbedded in the veins. The effect of surface waters, carrying oxidizing materials and sulphuric acid, is believed to have contributed to the drying out and hardening of these veins or dikes. Asphalts and bitumens include a wide variety of hydrocarbon materials, such as gilsonite, grahamite, elaterite, ozokerite, etc., which are used for somewhat different purposes. The deposits of the United States show much variety in form, composition, age, and geologic associations. The important Kentucky deposits occur as impregnations of Carboniferous sandstones at the base of the Coal Measures of that state. The Trinidad asphalt comes from the famous "pitch lake," which is a nearly circular deposit covering about a hundred acres 150 feet above sea level, and which is believed to fill the crater of an old mud volcano. The so-called pitch consists of a mixture of bitumen, water, mineral and vegetable matter, the whole inflated with gas, which escapes to some extent and keeps the mass in a state of constant ebullition. The surface of the lake is hard, and yet the mass as a whole is plastic and tends to refill the excavations. The lake is believed to be on the outcrop of a petroleum-bearing stratum, and the pitch to represent the unevaporated residue of millions of tons of petroleum which have exuded from the oil-sands. The pitch is refined by melting,--the heat expelling the water, the wood and other light impurities rising, and the heavy mineral matter sinking to the bottom. The asphalt of Venezuela is similar in nature, but the pitch "lake" is here covered with vegetation and the soft pitch wells up at certain points as if from subterranean springs. FOOTNOTES: [17] For more detailed treatment of international coal movements before the war and of coal movements within the United States, see the U. S. Geological Survey's _World Atlas of Commercial Geology_, Pt. 1, 1921, pp. 11-16. [18] Campbell, Marius R., The coal fields of the United States: _Prof. Paper 100-A, U. S. Geol. Survey_, 1917, pp. 5, 6, 7. [19] Compiled from tables quoted by White, David, The petroleum resources of the world: _Annals Am. Acad. Social and Political Sci._, vol. 89, 1920, pp. 123 and 126. [20] White, David, _loc. cit._, p. 113. [21] See Arnold, Ralph, Petroleum resources of the United States: _Econ. Geol._, vol. 10, 1915, p. 707. [22] White, David, Late theories regarding the origin of oil: _Bull. Geol. Soc. Am._, vol. 28, 1917, p. 732. [23] McCoy, A. W., Notes on principles of oil accumulation: _Jour. Geol._, vol. 27, 1919, pp. 252-262. [24] White, David, Genetic problems affecting search for new oil regions: _Mining and Metallurgy_, _Am. Inst. of Min. Engrs._, No. 158, Sec. 21, Feb., 1920. [25] Mehl, M. G., Some factors in the geographic distribution of petroleum: _Bull. Sci. Lab._, _Denison Univ._, vol. 19, 1919, pp. 55-63. [26] Schuchert, Charles, Petroliferous provinces: _Bull. 155_, _Am. Inst. Mining and Metallurgical Engrs._, 1919, pp. 3059-3060. [27] Loc. cit., p. 20. [28] Seepages or residual bituminous matter near the surface may be due to upward escape of oil material through joints in the rocks capping a reservoir, and productive pools may be found directly below such showings. In other regions similar surface indications may mean that the stratum in the outcrop of which they are found is oil-bearing; but accumulations of oil, if present, may be several miles down the dip, at places where the structural conditions have been favorable. In still other cases the seepage may have been in existence for such a long time as to exhaust the reservoir. It must also be remembered that gas seeps are common in sloughs and marshes where vegetation is decaying, and may be of no significance in the search for petroleum. [29] Arnold, Ralph, Conservation of the oil and gas resources of the Americas: _Econ. Geol._, vol. 11, 1916, pp. 321-322. [30] Oil shales may also be made to yield large quantities of fuel and illuminating gas, and of ammonia (see pp. 101-102). CHAPTER IX MINERALS USED IN THE PRODUCTION OF IRON AND STEEL (THE FERRO-ALLOY GROUP) GENERAL FEATURES Iron and steel and their alloys are the most generally used of the metals. The raw materials necessary for their manufacture include a wide variety of minerals. Iron is the principal element in this group; but in the manufacture of iron and steel, manganese, chromium, nickel, tungsten, molybdenum, vanadium, zirconium, titanium, aluminum, uranium, magnesium, fluorine, silicon, and other substances play important parts, either as accessories in the furnace reactions or as ingredients introduced to give certain qualities to the products. Nearly all parts of the world are plentifully supplied with iron ores for an indefinite period in the future, but their abundant use has thus far been confined mainly to the countries bordering the North Atlantic,--the United States, Germany, and England,--which, possessing ample coal supplies, have had the initiative to develop great iron and steel industries. China has abundant coal, moderate quantities of iron ore, and a large population, but a low per capita consumption of iron and steel products. Development of its iron and steel industry is just beginning. Japan has neither coal nor iron in sufficient quantities, and hence the Japanese effort in recent years to control the mineral resources of China and other countries. As a result of the war Germany has been largely deprived of its iron ores, and France may assume somewhat the rank in iron ore production once held by Germany. Sweden and Spain have been considerable producers of iron ore, but both lack coal, with the result that their ores have been largely exported to England and Germany. With increase of per capita consumption in outlying parts of the world, iron and steel industries are beginning to develop locally on a small scale, as in India, South Africa, and Australia. Russia has had sufficient supplies of coal and iron, but the stage of industrial development in that country has not called for great expansion of its iron and steel industry. There has been a tendency for iron and steel manufacture to become concentrated at a comparatively few places on the globe favored by the proper combinations of coal, iron, transportation, proximity to consuming populations, initiative and capacity to take advantage of a situation, and other factors. Even though on paper conditions may seem to be favorable in outlying territories for the development of additional plants, this development is often held back by competition from the established centers. On the west coast of the United States, there are raw materials for an iron and steel industry and there has been discussion for years as to the possibilities of starting a successful large scale steel industry. The consuming power of the local population for all kinds of iron and steel would seem to be great enough to warrant such action. However, the demand is for an extremely varied assortment of iron and steel products; and to start an industry, making only a few of the cruder products such as pig iron and semi-finished forms, would not meet this demand. All varieties of finishing plants and associated factories would also need to be started in order to meet the situation. This would require large capital. Furthermore the local demand for some of the accessory finished products might not warrant the establishment of the accessory plants. Throughout the history of the iron and steel business there has been a marked tendency for the iron ore to move to regions of coal production rather than for the coal to move to the iron ore regions. The coal or energy factor seems ultimately to control. This is due in considerable part to the fact that coal furnishes the basis of a great variety of industries for which iron ore is only one of the feeders, and which are so interrelated that it is not always easy to move the iron and steel industry to a spot near the sources of iron ore where iron and steel alone could be produced. In regard to iron ore supplies of proper grade and quantity, the United States is more nearly self-sufficing than any of its competitors. It imports minor amounts of ore from Cuba and Canada, and even from Chile and Sweden, to border points, in the main merely because these imported ores can compete on a price basis with the domestic ores. The entire exclusion of these ores, however, would make comparatively little difference in the total volume of our iron and steel industry; though it would probably make some difference in distribution, to the disadvantage of plants along the coast. There is only one kind of iron ore in which the United States has anything approaching deficiency, and that is ore extremely low in phosphorus, adapted to making the so-called low-phosphorus pig which is needed for certain special steels. Ordnance requirements during the war put a premium on these steels. While some of these extremely low-phosphorus ores are mined in the United States, additional quantities have been required from Spain and Canada and to a lesser extent from North Africa and Sweden. Also the Spanish pyrite, imported ordinarily for its sulphur content, on roasting leaves a residue of iron oxide extremely low in phosphorus which is similarly used. The elimination of pyrite imports from Spain during the war, therefore, was a considerable contributing factor to the stringency in low-phosphorus iron ores. War experience showed that the United States was dependent on foreign sources for 40 per cent or upwards of its needs in this regard. Certain developments in progress, notably the project for concentration of siliceous eastern Mesabi Range ores, make it likely that future domestic production will more nearly be able to meet the requirements. The equivalent of 15 per cent of the iron ore mined in the United States is exported as ore to Canadian ports on the Great Lakes and in the form of crude iron and steel products to many parts of the world. England and Germany are almost the sole competitors in the export trade. When we turn to the minerals used for making the alloys of iron and as accessories in the manufacture of iron, it appears that no one of the principal iron and steel producing countries of the world is self-supporting, but that these "sweeteners" must be drawn in from the far corners of the earth. The importance of these minor constituents is altogether out of proportion to their volume. For instance, only fourteen pounds of manganese are necessary in the making of a ton of steel, yet a ton of steel cannot be made without manganese. The increasing specialization in iron and steel products, and the rapidly widening knowledge of the qualities of the different alloys, are constantly shifting the demand from one to the other of the ferro-alloy minerals. Each one of the ferro-alloy minerals may be regarded as being in the nature of a key mineral for the iron and steel industry, and the control of deposits of these minerals is a matter of international concern. Control is not a difficult matter, in view of the fact that the principal supplies of practically every one of the alloy minerals are concentrated in comparatively few spots on the globe,--as indicated on succeeding pages. Nature has not endowed the United States, nor in fact the North American continent, with adequate high-grade supplies of the principal ferro-alloy minerals,--with the exception of molybdenum, and with the exception of silica, magnesite, and fluorspar, which are used as accessories in the process of steel making. With plenty of iron ore and coal, and with an iron and steel capacity amounting to over 50 per cent of the world's total, the United States is very largely dependent on other countries for its supplies of the ferro-alloy minerals. The war brought this fact home. With the closing of foreign sources of supplies, it looked at one time as if our steel industry was to be very greatly hampered; and extraordinary efforts were made to keep channels of importation open until something could be done in the way of development, even at excessive cost, of domestic supplies. The result of war efforts was a very large development of domestic supplies of practically all the ferro-alloy minerals; but in no case, with the exceptions noted above, did these prove sufficient to meet the total requirements. This development was at great cost and at some sacrifice to metallurgical efficiency, due to the low and variable grades of the raw materials. With the post-war reopening of importation much of the domestic production has necessarily ceased, and large amounts of money patriotically spent in the effort to meet the domestic requirements have been lost. These circumstances have resulted in the demand in Congress from producers for direct financial relief and in demand for protective tariffs, in order to enable the new struggling industries to exist, and to permit of development of adequate home supplies. Such tariffs might be beneficial to these particular domestic industries if wisely planned; but also, in view of the limited amounts of these particular ores in this country, their general low grade, and the high cost of mining, tariffs might very probably hasten exhaustion of our limited supplies and might handicap our metallurgical industries both in efficiency and cost (see pp. 365-366, 393-394). IRON ORES ECONOMIC FEATURES =Technical and commercial factors determining use of iron ore minerals.= Popularly, an iron ore is an iron ore, and there is little realization of its really great complexity of composition and the difficulty of determining what is or is not a commercial ore. Percentage of iron is of course an important factor; but an ore in which the iron is in the mineral hematite is more valuable than one with an equivalent percentage of iron which is in the form of magnetite. Substances present in the ore in minor quantities, such as phosphorus, sulphur, and titanium, have a tendency to make the iron product brittle, either when it is cold or when it is being made, so that excessive amounts of these substances may disqualify an ore. Excessive quantities of silica, lime, or magnesia may make the ore undesirable. Where an acid substance, like silica, is balanced by basic constituents like lime and magnesia, considerable amounts of both may be used. Excessive moisture content may spoil an ore because of the amount of heat necessary to eliminate it in smelting. The metallurgical processes of the iron and steel industry are essentially adapted to the principal grades of ore available. The cheapest of the steel-making processes, called the acid Bessemer process, requires a very low-phosphorus ore (usually below .050 per cent in the United States and below .030 per cent in England.) The basic open-hearth processes, making two-thirds of the steel in the United States, allow higher percentages of phosphorus, but not unlimited amounts. The basic Bessemer (Thomas) process, used for the "minette" ores of western Europe and the Swedish magnetites, may use an ore with any amount of phosphorus over 1.5 per cent. The phosphatic slag from this process is used as fertilizer. The supply of low-phosphorus Bessemer ore in the United States is at present limited as compared with that of the non-Bessemer ores, with the result that steel-plant construction for many years past has been largely open-hearth. The open-hearth process is favored also because it allows closer control of phosphorus content in the steel. Small but increasing amounts of steel are also made in the electric furnace; for the most part, however, this process is more expensive than the others, and it is used principally for special alloy steels. Iron ores are seldom so uniform in quality that they can be shipped without careful attention to sampling and grade. In the Lake Superior region the ores are sampled daily as mined, and the utmost care is taken to mix and load the ore in such a way that the desired grades can be obtained. Ordinarily a single deposit produces several grades of ore. When ores are put into the furnace for smelting the mixtures are selected with great care for the particular purpose for which the product is to be used. The mixture is compounded as carefully as a druggist's prescription. An ore salesman, after ascertaining the nature of the iron and steel products of a plant, has to use great skill in offering particular ores for sale which not only will meet the desired grade in regard to all elements, but also will meet competition in price. In some respects, the marketing of different grades of iron ore is as complex as the marketing of a miscellaneous stock of merchandise. With ores, as with merchandise, custom and sentiment play their part,--with the result that two ores of identical grade mineralogically and chemically may have quite a different vogue and price, simply because of the fact that furnace men are used to one and not to the other and are not willing to experiment. The geologist is ordinarily concerned merely with finding an ore of as good a general grade as possible; but he often finds to his surprise that his efforts have been directed toward the discovery of something which, due to some minor defect in texture, in mineralogical composition, or in chemical composition, is difficult to introduce on the market. There is here a promising field, intermediate between geology (or mineralogy) and metallurgy, for the application of principles of chemistry, metallurgy, and mineralogy, which is occupied at the present time mainly by the ore salesman. Both the mineralogist and metallurgist touch the problem but they do not cover it. With increasingly precise and rapidly changing metallurgical requirements, this field calls for scientific development. =Geographic distribution of iron ore production.= Iron ores are widely distributed over the world, but are produced and smelted on a large scale only in a few places where there is a fortunate conjunction of high grades, large quantity, proximity of coal, cheap transportation to markets, and manufacturing enterprise. Over 90 per cent of the iron ore production of the world is in countries bordering the North Atlantic basin. The United States produces about 40 per cent, France about 12 per cent, England about 10 per cent, Germany before the war 15 to 20 per cent, and Spain, Russia, and Sweden each about 5 per cent. Lesser producing countries are Luxemburg, Austria-Hungary, Cuba, Newfoundland, and Algeria; and insignificant amounts are produced in many other parts of the world. Of the world's iron and steel manufacturing capacity, the United States has about 53 per cent, Germany 16 per cent, England 14 per cent, France 10 per cent, the remainder of Europe (chiefly Russia, Austria-Hungary, and Belgium) 7 per cent. The absence of important iron ore production and of iron and steel manufacture either in the southern hemisphere or in any of the countries bordering the Pacific is a significant feature, when we remember what part iron plays in modern civilization. Japan, however, is beginning to develop a considerable iron and steel industry, which promises to use a large amount of ore from China, Manchuria, and Korea, and possibly to compete in American Pacific Coast markets. In the United States about 85 per cent of the production, or one-third of the world's production, comes from the Lake Superior region, a large part of the remainder from the Birmingham district, Alabama, and smaller quantities from the Adirondacks. For the rest of the North American continent, the only largely producing deposit is that at Belle Isle, Newfoundland, which is the basis of the iron industry of eastern Canada. Cuba supplies some ore to the east coast of the United States. In Europe there are only three large sources of high-grade iron ore which have heretofore been drawn on largely,--the magnetite deposits of northern Sweden, the hematites and siderites of the Bilbao and adjacent districts of northern Spain, and the magnetite-hematite deposits of southern Russia. The first two of these ores have been used to raise the percentage of iron in the low-grade ores which are the principal reliance of western Europe. The Swedish ores have also been necessary in order to raise the percentage of phosphorus and thus make the ores suitable for the Thomas process; on the other hand the Spanish ores and a small part of the Swedish material have been desired because of their low phosphorus content, adapted to the acid Bessemer process and to the manufacture of low-phosphorus pig. The Russian ores have largely been smelted in that country. The largest of the western European low-grade deposits is a geographic and geologic unit spreading over parts of Lorraine, Luxemburg, and the immediately adjacent Briey, Longwy, and Nancy districts of France. The ores of this region are called "minette" ores. This unit produces about a fourth of the world's iron ore. Low-grade deposits of a somewhat similar nature in the Cleveland, Lincolnshire, and adjacent districts of England form the main basis for the British industry. There is minor production of iron ores in other parts of France and Germany, in Austria-Hungary, and in North Africa (these last being important because of their low phosphorus content). Comparison of figures of consumption and production of iron ores indicates that the United States, France, Russia, and Austria-Hungary are self-supporting so far as quantity of materials is concerned. Certain ores of special grades, and ores of other minerals of the ferro-alloy group required in steel making, however, must be imported from foreign sources; this matter has been discussed above. Great Britain and Germany appear to be dependent on foreign sources, even under pre-war conditions, for part of the material for their furnaces. During the war there was considerable development of the low-grade English ores, but this does not eliminate the necessity for importing high-grade ores for mixture. Belgium produces a very small percentage of her ore requirements and is practically dependent on the Lorraine-Luxemburg field. The principal effect of the war on iron ore production was the occupation of the great French mining and smelting field by the Germans, thereby depriving the French of their largest source of iron ore. Since the war the situation has been reversed, France now possessing the Lorraine field, which formerly supplied Germany with 70 per cent of its iron ore. As the German industrial life is largely based on iron and steel manufacture, the problem of ore supplies for Germany is now a critical one. It has led to German activity in Chile and may lead to German developments in eastern Europe and western Asia, particularly in the large and favorably located reserves of southern Russia. It seems likely, however, that arrangements will also be made to continue the export of ore from the Lorraine field down the Rhine to the principal German smelting centers. France needs the German coal for coking as badly as Germany needs the French iron ore. The Rhine valley is the connecting channel for a balanced movement of commodities determined by the natural conditions. These basic conditions are likely in the long run to override political considerations. The Lake Superior deposits, the Swedish magnetites, the Spanish hematites, and the Russian ores carry 50 to 65 per cent of metallic iron. The Birmingham deposits of southeastern United States, the main British supplies, and the main French and German supplies contain about 35 per cent or less. It is only where ores are fortunately located with reference to consuming centers that the low-grade deposits can be used. For outlying territories only the higher-grade deposits are likely to be developed, and even there many high-grade deposits are known which are not mined. The largest single group not yet drawn on is in Brazil. Others in a very early stage of development are in North Africa and Chile. =World reserves and future production of iron ore.= The average rate of consumption of iron ore for the world in recent years has been about 170 million tons per year. At this rate the proved ore reserves would last about 180 years. If it be assumed that consumption in the future will increase at about the same rate as it has in the past, the total measured reserve would still last about a century. These calculations of life, however, are based only on the known reserves; and when potential reserves are included the life is greatly increased. And this is not all; for beyond the total reported reserves (both actual and potential), there are known additional large quantities of lower-grade ores, at present not commercially available, but which will be available in the future,--to say nothing of expected future discoveries of ores of all grades in unexplored territories. Both geological inference and the history of iron ore exploration seem to make such future discoveries practically certain. Iron ore constitutes about 4 per cent of the earth's shell and it shows all stages of concentration up to 70 per cent. Only those rocks are called "iron ores" which have a sufficiently high percentage of iron to be adapted to present processes for the extraction of iron. When economic conditions demand it, it may be assumed that iron-bearing rocks not now ordinarily regarded as ores may be used to commercial advantage, and therefore will become ores. Not only is an indefinitely long life assured for iron ore reserves as a whole, but the same is true of many of the principal groups of deposits. The question of practical concern to us, therefore, is not one of total iron ore reserves, but one of degrees of _availability_ of different ores to the markets which focus our requirements for iron. The annual production of ore from a given district is roughly a measure of that ore's ability to meet the competitive market, and therefore, of its actual immediate or past availability. Annual production is the net result of the interaction of all of the factors bearing on availability. It may be argued that there are ores known and not yet mined which are also immediately available. On the whole, they seem to be less available than ores actually being produced; otherwise general economic pressure would require their use and actual production. In considering the future availability of iron ores, it is obvious that tables of past production afford only a partial basis for prediction. Presumably districts which have produced largely in the past may be expected to continue as important factors. In these cases production has demonstrated availability. Continued heavy production may thus be expected from the ores of the Lake Superior region, from the Clinton hematites of Alabama, from the ores of the Lorraine-Luxemburg-Briey district, from the Cleveland ores of England, from the Bilbao ores of Spain, from the high-grade magnetites of northern Sweden, and (assuming political stability) from the ores of southern Russia. Similarly, also, recent increases in production from certain districts are probably significant of increased use of such ores in the future. Among these developments are the increasing production of Swedish ores and their importation into England and Germany, and the increasing use of Clinton hematites and Adirondack magnetites in the United States. Low-grade ores from the great reserves of Cuba are being mined and brought to the east coast of the United States in increasing amounts, and it is highly probable that they will take a larger share of the market. A similar project in Chile, which lay dormant during the war because of restricted shipping facilities, is expected in the near future to yield important shipments to the United States. In none of these cases will production be limited in the near future by ore reserves. Increased production and use of iron ores are also to be looked for in Newfoundland, North Africa, China, India, Australia, and South Africa. On the commercial horizon are ores of still newer districts, the availability of which may not be read from tables of production. Their availability must be determined by analysis and measurement of the factors entering into availability. Availability of iron ore is determined by percentage of iron, percentages of impurities, percentages of advantageous or deleterious minor constituents, physical texture, conditions for profitable mining, adaptability to present furnace practice, distance from consuming centers, conditions and costs of transportation, geographical and transportational relation to the coal and fluxes necessary for smelting, trade relations, tariffs and taxes, inertia of invested capital, and other considerations. All of these factors are variable. A comparison of ores on the basis of any one of these factors or of any two or three of them is likely to be misleading. A comparison based on the quantitative consideration of all of the several factors seems to be made practically impossible by the difficulty of ascertaining accurately the quantitative range and importance of each factor, and by the difficulty of integrating all of the factors even if they should be determined. However, their combined effect is expressed in the cost of bringing the product to market; and comparison of costs furnishes a means of comparing availability of ores. A high-grade ore, cheaply mined and favorably located with reference to the points of demand, will command a relatively high price at the point of production. The same ore so located that its transportation costs are higher will command a lower price; or it may be so located that the costs of mining and bringing it to places where it can be used are so high that there is no profit in the operation. There are known high-grade iron ores which, because of cost, are not available under present conditions. The availability of an ore, then, depends on its relation to a market,--whether, after meeting the cost of transportation, it can be sold at prevailing market prices at the consuming centers, and can still leave a fair margin of profit for the mining operation. The price equilibrium between consuming centers affords a reasonably uniform basis against which to measure availability of ores. Figures of cost are obtainable as a basis for comparison of availability of iron ores of certain of the districts, but not enough are at hand for comparison of the ores of all districts. Careful study of costs has demonstrated the availability in the near future of the Brazilian high-grade Bessemer hematites; and projects which are now under way for exportation to England and the United States will doubtless make this enormous reserve play an important part in the iron industry. Iron ore is known but not yet mined in many parts of the western United States and western Canada. With the increasing population along the west coast of North America, projects for smelting the ore there are becoming more definite. Establishment of smelters on the west coast would make available a large reserve of ore (see also, however, p. 155). The list of changes now under way or highly probable for the future might be largely extended. The use of iron and steel is rapidly spreading through populous parts of the world which have heretofore demanded little of these products. This increased use is favoring the development of local centers of smelting, which will make available other large reserves of iron ore. The growth of smelting in India, China and Australia illustrates this tendency. Iron ore reserves are so large, so varied, and so widely distributed over the globe, that they will supply demands upon them to the remote future. Reserves become available and valuable only by the expenditure of effort and money. Ores are the multiplicand and man the multiplier in the product which represents value or availability. Iron ore can be made available, when needed, almost to any extent, but at highly varying cost and degree of effort. The highest grade ores, requiring minimum expenditure to make them available, are distinctly limited as compared to total reserves. Any waste in their utilization will lead more quickly to the use of less available ores at higher cost. One of the significant consequences of the exhaustion of the highest grade reserves will be an increased draft upon fuel resources for the smelting of the lower grade ores. Availability of iron ores is limited, not by total reserves, but by economic conditions. GEOLOGIC FEATURES Iron rarely exists in nature as a separate element. It occurs mainly in minerals which represent combinations of iron, oxygen, and water, the substances which make up iron rust. Very broadly, most of the iron ores might be crudely classified as iron rust. In detail this group is represented by several mineral varieties, principal among which are hematite (Fe_{2}O_{3}), magnetite (Fe_{3}O_{4}), and limonite (hydrated ferric oxide). Iron likewise combines with a considerable variety of substances other than oxygen; and some of these compounds, as for instance iron carbonate (siderite), iron silicate (chamosite, glauconite, etc.), and iron sulphide (pyrite), are locally mined as iron ores. While an ore of iron may consist dominantly of some one of the iron minerals, in few cases does it consist exclusively of one mineral. Most ores are mixtures of iron minerals. Fully nine-tenths of the iron production of the world comes from the so-called hematite ores, meaning ores in which hematite is the dominant mineral, though most of them contain other iron minerals in smaller quantities. About 5 per cent of the world's iron ores are magnetites, and the remainder are limonites and iron carbonates. Iron ores are represented in nearly all phases of the metamorphic cycle, but the principal commercial values have been produced by processes of weathering and sedimentation at and near the surface. =Sedimentary iron ores.= Over 90 per cent of the world's production of iron ore is from sedimentary rocks. The deposits consist in the main either of beds of iron ore which were originally deposited as such and have undergone little subsequent alteration, or of those altered portions of lean ferruginous beds which since their deposition have been enriched or concentrated sufficiently to form ores. A minor class of iron ores in sediments consists of deposits formed by secondary replacement of limestones by surface waters carrying iron in solution. 1. Deposits of the first class,--originally laid down in much their present form,--are usually either oölitic, _i. e._, containing great numbers of flat rounded grains of iron minerals like flaxseeds, or consist in large part of fossil fragments of sea shells, replaced by iron minerals. The Clinton ores of the Birmingham district, the Wabana ores of Newfoundland, the minette ores of the Lorraine district in central Europe, and the oölitic ores of northern England are all of these types. Their principal iron mineral is hematite, although the English ores also contain considerable iron carbonate or siderite. The cementing or gangue materials are chiefly calcite and quartz, in variable proportions. The large reserves of high-grade hematite in the Minas Geraes district of Brazil are also original sediments, but lack the oölitic texture. An insignificant proportion of the world's iron is obtained from "bog ores," which are sedimentary deposits of hydrated iron oxide in swamps and lakes. These ores have been used only on a small scale and chiefly in relatively undeveloped countries. They are of particular interest from a genetic standpoint in that they show the nature of some of the processes of iron ore deposition as it is actually going on today. None of the ores of this class, with the exception of the iron carbonates, have undergone any considerable surface enrichment since their primary deposition. Neither, with the exception of the Brazilian ores, have they undergone any deep-seated metamorphism. The shapes, sizes, and distribution of the deposits may be traced back to the conditions of original deposition. In England and western Europe the principal deposits have been only slightly tilted by folding. In the United States the Clinton ores have partaken in the Appalachian folding. In Brazil, the ores have undergone close folding and anamorphism. 2. Deposits of the second class, which owe much of their value to further enrichment since deposition, are represented by the hematite ores of the Lake Superior district. These may be thought of as the locally rusted and leached portions of extensive "iron formations," in which oxidation of the iron, and the leaching of silica and other substances by circulating waters, have left the less soluble iron minerals concentrated as ores. The Lake Superior iron formations now consist near the surface mainly of interbanded quartz (or chert) and hematite, called _jasper_ or _ferruginous chert_ or _taconite_. These are similar in composition to the leaner iron ores of Brazil, called _itabirite_, but differ in that the silica is in the form of chemically deposited chert, rather than fragmental quartz grains. [Illustration: FIG. 11. Alteration of Lake Superior iron formation to iron ore by the leaching of silica.] When originally deposited the iron was partly hematite (perhaps some magnetite) and largely in the form of iron carbonate (siderite) and iron silicate (greenalite), interbanded with chert. The original condition is indicated by the facts that deep below the surface, in zones protected from weathering solutions, siderite and greenalite are abundant, and that they show complete gradation to hematite in approaching the surface. The ore has been concentrated in the iron formation almost solely by the process of leaching of silica by surface or meteoric waters, leaving the hematite in a porous mass. Figure 11 illustrates this change as calculated from analyses and measurements of pore space. During this process a very minor amount of iron has been transported and redeposited. In short, the Lake Superior iron ores are residual deposits formed by exactly the same weathering processes as cause the accumulation of clays, bauxites, and the oxide zones of sulphide deposits. The development of an iron ore rather than of other materials as an end-product is due merely to the peculiar composition of the parent rock. The solution of silica on such an immense scale as is indicated by these deposits has sometimes been questioned on the general ground that silica minerals are insoluble. However, there is plenty of evidence that such minerals _are_ soluble in nature; and the assumption of insolubility, so often made in geologic discussions, is based on the fact that most other minerals are _more_ soluble than silica minerals, and that in the end-products of weathering silica minerals therefore usually remain as important constituents. Iron oxide, on the other hand, is _less_ soluble even than silica,--with the result that when the two occur together, the evidence of leaching of silica from the mixture becomes conspicuous. The fact that these deposits are almost exclusively residual deposits formed by the leaching of silica has an important bearing on exploration. If they have been formed by the transportation and deposition of iron from the surrounding rocks, there is no reason why they should not occasionally be found in veins and dikes outside of the iron formation. As a matter of fact they do not transgress a foot beyond the limits of the iron formation. Failure to recognize the true nature of the concentration of these ores has sometimes led to their erroneous classification as ores derived from the leaching and redeposition of iron from the surrounding rocks. The distribution and shapes of ore deposits of this class are far more irregular and capricious than those of the primary sediments, as would be expected from the fact that their concentration has taken place through the agency of percolating waters from the surface, which worked along devious channels determined by a vast variety of structural and lithological conditions. The working out of the structural conditions for the different mines and districts constitutes one of the principal geologic problems in exploration. These conditions have been fully discussed in the United States Geological Survey reports, and are so various that no attempt will be made to summarize them here. One of the interesting features of the concentration of Lake Superior iron ores is the fact that it took place long ago in the Keweenawan period, preceding the deposition of the flat-lying Cambrian formations, at a time when the topography was mountainous and the climate was arid or semi-arid. These conditions made it possible for the oxidizing and leaching solutions to penetrate very deeply, how deeply is not yet known, but certainly to a depth below the present surface of 2,500 feet. At present the water level is ordinarily within 100 feet of the surface, and oxidizing solutions are not going much below this depth. This region, therefore, furnishes a good illustration of the intermittent and cyclic character of ore concentration which is now coming to be recognized in many ore deposits. Subsequent changes far beneath the surface have folded, faulted, and metamorphosed some of the Lake Superior iron ores but have not enriched them. The same processes have recrystallized and locked together the minerals of some of the lean iron formations, making them hard and resistant, so that subsequent exposure and weathering have had little effect in enriching them to form commercial ores. The weathering of limestones containing minor percentages of iron minerals originally deposited with the limestones may result in the residual concentration of bodies of limonite or "brown ores" associated with clays near the surface. This process is similar in all essential respects to the concentration of the Lake Superior ores. Such limonitic ores are found rather widely distributed through the Appalachian region and in many other parts of the world. Because of the ease with which they can be mined and smelted on a small scale they have been used since early times, but have furnished only a very small fraction of the world's iron. 3. In a third class of sedimentary ores, the iron minerals are supposed to have been introduced as replacements of limestones subsequent to sedimentation. Such ores are not always easy to discriminate from ores resulting primarily from sedimentation. This class is represented by the high-grade deposits of Bilbao, Spain, Austrian deposits, and by smaller deposits in other countries. The Bilbao ores consist mainly of siderite, which near the surface has altered to large bodies of oxide minerals. They occur in limestones and shales and are not associated with igneous rocks. The deposits are believed to have been formed by ordinary surface waters carrying iron in solution, and depositing it in the form of iron carbonate as replacements of the limestones. The original source of the iron is believed to have been small quantities of iron minerals disseminated through the ordinary country rocks of the district. The action of surface waters, in thus concentrating the iron in certain localities which are favorable for precipitation, is similar to the formation of the lead and zinc ores of the Mississippi valley, referred to in the next chapter. Deposits formed in this manner may be roughly tabular and resemble bedded deposits, or they may be of very irregular shapes. The sedimentary iron ores in general evidently represent an advanced stage of katamorphism, and illustrate the tendency of this phase of the metamorphic cycle toward simplification and segregation of certain materials. The exact conditions of original sedimentation present one of the great unsolved problems of geology, referred to in Chapter III. =Iron ores associated with igneous rocks.= About five per cent of the world's production of iron ore is from bodies of magnetite formed in association with igneous rocks. These are dense, highly crystalline ores, in which the iron minerals are tightly locked up with silicates, quartz, and other minerals, suggestive of high temperature origin. The largest of these deposits is at Kiruna in northern Sweden; in fact this is the largest single deposit of high-grade ore of any kind yet known in the world. Here the magnetite forms a great tabular vertical body lying between porphyry and syenite. In the Adirondack Mountains of New York and in the highlands of New Jersey, magnetites are interbedded and infolded with gneisses, granites, and metamorphic limestones. In the western United States there are many magnetite deposits, not yet mined, at contacts between igneous intrusives and sedimentary rocks, particularly limestones (so-called "contact-metamorphic" deposits). The ores of the Cornwall district of Pennsylvania and some of the Chilean, Chinese, and Japanese ores are of the same type. Magnetites containing titanium, which prevents their use at the present time, are known in many parts of the world as segregations in basic igneous rocks. They are actually parts of the igneous rock itself (p. 34). Among the large deposits of this nature are certain titaniferous ores of the Adirondacks, of Wyoming, and of the Scandinavian peninsula. In all of these cases, it is clear that the origin of the ores is in some way related to igneous processes, and presumably most of the ores are deposited from the primary hot solutions accompanying and following the intrusion of the igneous rocks; but thus far it has been difficult to find definite and positive evidence as to the precise processes involved. None of these deposits have undergone any important secondary enrichment at the surface. Their sizes, shapes, and distribution are governed by conditions of igneous intrusion, more or less modified, as in the Adirondacks, by later deformation. =Iron ores due to weathering of igneous rocks.= A small part of the world's iron ores, less than 1 per cent of the total production, are the result of surface alteration of serpentine rocks. These ores are mined principally in Cuba (Fig. 12). Here they have been developed on a plateau-like area on which erosion is sluggish. The process of formation has been one of oxidation of the iron minerals and leaching of most of the other constituents, leaving the iron concentrated near the surface in blanket-like deposits. The minerals of the original rock contained alumina, which, like the iron, is insoluble under weathering conditions, and hence the Cuban iron ores are high in alumina. They also contain small quantities of nickel and chromium which have been concentrated with the iron. A large part of the iron minerals, especially where close to the surface, have been gathered into small shot-like nodules called _pisolites_. It is thought that the solution and redeposition of the iron by organic acids from plant roots may be at least a contributing cause in the formation of this pisolitic texture. [Illustration: FIG. 12. Representing in terms of weight the mineralogical changes in the katamorphism of serpentine rock to iron ore, on the assumption that alumina has remained constant, eastern Cuba.] The Cuban iron ores are similar in their origin to _laterites_, which are surface accumulations of clay, bauxite, and iron oxide minerals, resulting from the weathering of iron-bearing, commonly igneous, rocks. The typical laterites carry more clay and bauxite than the Cuban iron ores, but this is due merely to the fact that the original rocks commonly carry more materials which weather to clay. In fact the Cuban iron ores are themselves, broadly speaking, laterites. =Iron ores due to weathering of sulphide ores.= A relatively minute portion of the world's iron ore comes from the "gossans" or "iron caps" over deposits of iron sulphides. The gossans are formed by oxidation and leaching of other minerals from the deposits, leaving limonite or hematite in concentrated masses (see pp. 46-47). MANGANESE ORES ECONOMIC FEATURES Manganese ores are used mainly in the manufacture of steel, the alloys spiegeleisen and ferromanganese being added to the molten steel after treatment in the Bessemer converter and open-hearth furnace in order to recarburize and purify the metal. The alloy ferromanganese is also used in the production of special manganese steels. Manganese ore is used in relatively small amounts in dry batteries, in the manufacture of manganese chemicals, in glass making, and in pigments. Steel uses 95 per cent of the total manganese consumed, batteries and chemicals 5 per cent. On an average each ton of steel in the United States requires 14 pounds of metallic manganese, equivalent to 40 pounds of manganese ore. With manganese ores, as with iron ores, the percentage of minor constituents,--phosphorus, silica, sulphur, etc.,--determines to a large extent the manner of use. Low-grade manganese ores, ranging from 10 to 35 per cent in manganese, 20 to 35 per cent in iron, and containing less than 20 per cent of silica, are used mainly in the production of the low-grade iron-manganese alloy called _spiegeleisen_ or _spiegel_ (16 to 32 per cent manganese). The higher-grade ores, ranging from 35 to 55 per cent in manganese, are used mainly in the production of the high-grade alloy called _ferromanganese_ or _ferro_, in which the manganese constitutes 65 to 80 per cent of the total. To a very limited extent manganese is smelted directly with iron ores, thus lessening the amount to be introduced in the form of alloys; this, however, is regarded as wasteful use of manganese, since its effectiveness as so used is not very great. Steel makers usually prefer to introduce manganese in the form of ferromanganese rather than as spiegel. On the other hand, the ores of the United States as a whole are better adapted to the manufacture of spiegel. With the shutting off of foreign high-grade supplies during the war, resulting in the increased use of local ores, it became necessary to use larger amounts of the spiegel which could be made from these ores. Metallurgists stated that it was theoretically possible to substitute spiegel for the higher grade alloy up to 70 per cent of the total manganese requirement, but in actual practice this substitution did not get much beyond 18 per cent. The principal manganese ore-producing countries in normal times are Russia, India, and Brazil. Relatively little ore is used in these countries, most of it being sent to the consuming countries of Europe and to the United States. The Indian ore has been used largely by British steel plants, but much of it also has gone to the United States, Belgium, France, and Germany. The Russian ore has been used by all five of these countries, Germany having a considerable degree of commercial control and receiving the largest part; a small quantity is also used in Russia. Brazilian ore has gone mainly to the United States, and in part to France, Germany, and England. Smaller amounts of manganese ore have been produced in Germany, Austria-Hungary, Spain, and Japan. This production has had little effect on the world situation. That produced in Austria-Hungary and Germany is used in the domestic industry. That from Spain and Japan is in large part exported. The highest grade of manganese ore comes from the Russian mines, especially those in the Caucasus region. Most of the ore used for the manufacture of dry batteries and in the chemical industry, where high-grade ores are required, has come from Russia. By far the larger part of the Russian production, however, has gone into steel manufacture. Indian and Brazilian ores have likewise been used mainly in the steel industry. Some Japanese ore also is of high grade and is used for chemical and battery purposes. Nature has not endowed the United States very abundantly with manganese ores, and such as are known are widely scattered, of relatively small tonnage, and of a wide range of grade. The principal producing districts are the Philipsburg district of Montana and the Cuyuna Range of Minnesota; there are also scattering supplies in Virginia, Arizona, California, and many other states. The use of domestic ores has sometimes been unsatisfactory, because of frequent failure of domestic producers to deliver amounts and grades contracted for. It has been, on the whole, cheaper, easier, and more satisfactory for the large consumers to purchase the imported ore, which is delivered in any desired amount and in uniform grades, rather than to try to assemble usable mixtures from various parts of the country. Before the European War, the United States produced only 1 to 2 per cent of its needed supply of manganese, the rest being imported mainly from India, Russia, and Brazil, in the form of ore, and from England in the form of ferromanganese (about half of the total requirement). The partial closing of the first two and the fourth of these sources of supply under war conditions made it necessary to turn for ore to Brazil and also to Cuba, where American interests developed a considerable industry in medium-grade ores. At the same time steps were taken to develop domestic resources; and with the high prices imposed by war conditions, the domestic production, both of high- and low-grade ore, was increased largely, but still was able to supply only 35 per cent of the total requirements of manganese. At the close of the war sufficient progress had been made--in the discovery of many new deposits in the United States, in the use of low-grade domestic ores, which before had not been able to compete with imported ores, and in the increased use of spiegel, allowing wider use of low-grade ores,--to demonstrate that, if absolutely necessary, and at high cost, the United States in another year or two could have been nearly self-sufficing in regard to its manganese requirements. The release of shipping from war demands resulted immediately in larger offerings of foreign manganese ore and of ferromanganese from England, at prices which would not allow of competition from much of the domestic or Cuban ore production or from the domestic manufacture of alloys. The result was a rather dramatic closing down of the manganese industry, with much financial loss, the passage of a bill for reimbursement of producers, and a demand on the part of the producers, though not of consumers, for a protective tariff. In the questions thus raised it is desirable that geologists and engineers professionally connected with the industry thoroughly understand the basic facts; for they are liable to be called upon for advice, not only on questions relating to domestic supplies affected by possible future foreign policies, but on the formulation of the policies themselves. Conservation, cheaper steel, and future trade relations of the United States all require consideration, before action is taken to protect this one of several similarly situated mineral industries, in the effort to make the country self-supporting. These questions are further dealt with in Chapters XVII and XVIII. Manganese production was also developed during the war in the Gold Coast of West Africa, in Costa Rica, in Panama, in Java, and elsewhere; but with the possible exception of Java and Chile, none of these sources are likely to be factors in the world situation. The war-developed manganese production of Italy, France, Sweden, and United Kingdom is also unlikely to continue on any important scale. GEOLOGIC FEATURES Like iron ores, manganese ores consist principally of the oxides of manganese (pyrolusite, psilomelane, manganite, wad, and others), and rarely the carbonate of manganese (rhodochrosite). They are similar in their geologic occurrence to many of the iron ores and are often mixed with iron ores as manganiferous iron ores and ferruginous manganese ores. The higher grade manganese ores are of two general types. Those of the Caucasus district in Russia are sedimentary beds, oölitic in texture, which were originally deposited as rather pure manganese oxides, and which have undergone little secondary concentration. They are mined in many places in much the same manner as coal. Those of India and Brazil are chiefly surface concentrations of the manganese oxides, formed by the weathering of underlying rocks which contain manganese carbonates and silicates. The origin of the primary manganese minerals in the Indian and in some of the Brazilian deposits is obscure. In others of the Brazilian ores, the manganese was deposited in sedimentary layers interbedded with siliceous "iron formations," and the whole series has subsequently been altered and recrystallized. The manganese ores of Philipsburg, Montana, the principal large high-grade deposits mined in the United States, were derived by surface weathering from manganese carbonates which form replacements in limestone near the contact with a great batholith of granodiorite. The primary manganese minerals probably owe their origin to hot magmatic solutions, as suggested by the close association of the ores with the igneous rock, the presence of minerals containing chlorine, fluorine, and boron, and the development in the limestone of dense silicates and mineral associations characteristic of hot-water alteration. The manganese ores are mined principally in the oxidized zone. Rich silver ores are found below the water table, but mainly in veins independent of the manganese deposits. At Butte, Montana, a little high-grade manganese material has been obtained from the unoxidized pink manganese carbonate, which is a common mineral in some of the veins. It is associated with quartz and metallic sulphides and is similar in origin to the copper ores of the same district (pp. 201-202). The lower-grade and the more ferruginous manganese ores are of a somewhat similar origin to the principal high-grade ores, in that they represent surface concentrations of the oxides from smaller percentages of the carbonates and silicates in the rocks below. Deposits of this nature have been derived from a wide variety of parent rocks--from contact zones around igneous intrusions, from fissure veins of various origins, from calcareous and clayey sediments, and from slates and schists. The manganese and manganiferous iron ores of the Cuyuna district of Minnesota, the largest source of low-grade ores in this country, were formed by the action of weathering processes on sedimentary beds of manganese and iron carbonates constituting "iron formations." The process is the same as the concentration of Lake Superior iron ores described elsewhere. Manganese, like iron, is less soluble than most of the rock constituents, and tends to remain in the outcrop under weathering conditions. To some extent also it is dissolved and reprecipitated, and is thus gathered into concretions and irregular nodular deposits in the residual clays. In some cases it is closely associated with iron minerals; in others, due to its slightly greater solubility, it has been separated from the iron and segregated into relatively pure masses. With manganese, as with iron, katamorphic processes are responsible for the concentration of most of the ores. The ores are in general surface products, and rarely extend to depths of over a hundred feet. CHROME (OR CHROMITE) ORES ECONOMIC FEATURES The principal use of chrome ores is in the making of the alloy ferrochrome (60 to 70 per cent chromium), used for the manufacture of chrome, chrome-nickel, and other steels. These steels have great toughness and hardness, and are used for armor-plate, projectiles, high-speed cutting tools, automobile frames, safe-deposit vaults, and other purposes. Chrome ore is used also both in the crude form and in the form of bricks for refractory linings in furnaces, chiefly open-hearth steel furnaces; and as the raw material for bichromates and other chemicals, which are used in paints and in tanning of leather. In the United States in normal times about 35 per cent of the total chromite consumed is used in the manufacture of ferrochrome, and about 35 per cent for bichromate manufacture, leaving 30 per cent for refractory and other purposes. In the higher commercial grades of chrome ore the percentage of chromic oxide is 45 to 55 per cent, but under war conditions ore as low as 30 per cent in Cr_{2}O_{3} was mined. Recovery of chrome from slags resulting from the smelting of chromiferous iron ores was one of the war-time developments. The principal chromite-producing countries in normal times are New Caledonia, and Rhodesia (controlled by French and British interests), and to a somewhat lesser extent Russia and Turkey (Asia Minor). Small amounts of chromite are mined in Greece, India, Japan, and other countries. The Indian deposits in particular are large and high-grade but have been handicapped by inadequate transportation. The production of chrome ore in New Caledonia, Rhodesia, Russia, and Turkey has usually amounted to more than 90 per cent of the total world's production. The ore from New Caledonia has been used by France, Germany, England, and to some extent by the United States. Rhodesian ore has been used by the United States and the principal European consumers. Latterly more Rhodesian ore has gone to Europe and more Caledonian ore to the United States. The Russian ore has been in part used in Russia and in part exported, probably going mainly to France and Germany. The Turkish ore has been exported to the United States, England, and Germany; it probably supplied most of Germany's chromite requirements during the war. During the war the United States was temporarily an important producer, as were also Canada, Brazil, Cuba, and to a minor degree Guatemala. The richest chrome ore mined at present comes from Guatemala, but the mines are relatively inaccessible. The New Caledonian, Rhodesian, Russian, Turkish, and Indian ores are also of high grade. The ores mined in the United States, Canada, Brazil, Cuba, Greece, and Japan are of lower grade. The use of domestic chromite supplies in the United States presents much the same problem as does manganese. The ore bodies are small, scattered, and of a generally law grade. War-time experience showed that they could be made to meet a large part of the United States requirements, but at high cost and at the risk of early exhaustion of reserves. California and Oregon are the principal sources, and incidental amounts have been produced in Washington, Wyoming, and some of the Atlantic states. With the resumption of competition from foreign high-grade ores at the close of the war, the domestic mining industry was practically wiped out; the consequences being financial distress, partial direct relief from Congress, and consideration of the possibilities of a protective tariff,--which in this case would have to be a large one to accomplish the desired results (see Chapters XVII and XVIII). GEOLOGIC FEATURES The principal chrome mineral is chromite, an oxide of chromium and iron. Chromite is a common minor constituent of basic igneous rocks of the peridotite and pyroxenite type. In these rocks it occurs both as disseminated grains, and as stringers, and large irregular masses which probably represent magmatic segregations. Alteration, and weathering of the parent rock, forming first serpentine and then residual clays, make the chromite bodies progressively richer and more available, by leaching out the soluble constituents of the rock leaving the chromite as residual concentrates. All the important chromite deposits of the world are associated in somewhat this manner with serpentine or related rocks. They are formed in the same way as the lateritic iron ores of Cuba, and from the same sort of rocks (pp. 171-173). Chromite is very insoluble, and the mechanical breaking down of deposits and transportation by streams frequently forms placers of chrome sands and gravels. Such placers have not been worked to any extent. Katamorphic processes give the important values to chromite deposits. NICKEL ORES ECONOMIC FEATURES The principal use of nickel is in the manufacture of nickel steel, the most important of all alloy steels. Ordinary nickel steels carry about 3-1/2 per cent nickel. Nickel is used in all gun and armor-plate steels, and in practically all other good steels except tool steels. It is also extensively alloyed with other metals, particularly with copper to form the strong non-corrosive metal (monel metal) used for ship propellers and like purposes. Nickel is also used for electroplating, for nickel coins, for chemicals, etc. Of the total production about 60 per cent is used in steels, 20 per cent in non-ferrous alloys and 20 per cent in miscellaneous uses. The ores mined range from 2 to 6 per cent in metallic nickel. Canada (Sudbury, Ontario) produces over three-fourths of the world's nickel and is likely to have an even greater share of the future production. The French supply from New Caledonia is second in importance, and minor amounts are produced in Norway and in several other countries. The control and movement of the Canadian and New Caledonian supplies are the salient features of the world nickel situation. Nickel leaves the producing countries mostly as matte. Canadian matte has been refined mainly in the United States, but the tendency is toward refining a larger proportion in Canada. In Europe there are refineries in France, England, Belgium, Germany, and Norway, which normally treat the bulk of the New Caledonian and some of the Canadian production. Small quantities of New Caledonian matte or ore are also refined in Japan, and during the war considerable amounts came to the United States. The United States now produces perhaps 10 per cent of its normal requirements of nickel from domestic sources, principally as a by-product of copper refining. However, the United States has a large financial interest in the Canadian deposits, and refines most of the matte produced from Sudbury ores in a New Jersey refinery. Shipments to Europe of Canadian nickel refined in the United States have been a feature of the world's trade in the past. The nickel-bearing iron ores of Cuba, consumed in the United States, constitute a potential nickel supply of some importance, if processes of preparation become commercially perfected. Known supplies of nickel in Canada and New Caledonia are ample for a considerable future, and geologic conditions promise additional discoveries at least in the former field. The probable reserves of the Sudbury district have been estimated to be fully 100,000,000 tons, which would supply the world's normal pre-war requirements for about a hundred years. In recent years the British and Canadian governments have taken an active interest in the nickel industry. They organized a joint commission for its investigation, the report[31] of which furnishes the most comprehensive view of the world nickel situation yet available. The British government has directly invested in shares of the British-American Nickel Company, and has negotiated European contracts for sale of nickel for this company. The Canadian government has exerted some pressure toward larger refining of nickel matte in Canada. GEOLOGIC FEATURES The principal ore minerals are the nickel sulphides and arsenides (particularly pentlandite, but also millerite, niccolite, and others), which are found at Sudbury intergrown with the iron and copper sulphides, pyrrhotite and chalcopyrite; and the hydrated nickel-magnesium silicates (garnierite and genthite), which are products of weathering. The richer ores of Canada contain about 5 or 6 per cent of nickel, the New Caledonian ores less than 2 per cent. The Sudbury ores carry also an average of about 1.5 per cent of copper. Nickel, while present in the average igneous rock in greater amounts than copper, lead, or zinc, is apparently not so readily concentrated in nature as the other metals and is rarely found in workable deposits. The few ore bodies known have been formed as the result of unusual segregation of the nickel in highly magnesian igneous rock of the norite or gabbro type, at the time of its solidification or soon after; and in some cases, in order to produce the nickel ore, still further concentration by the agency of weathering has been necessary. Thus there are two main types of deposits. The first, the sulphide type, is represented by the great ore bodies of the Sudbury district. These are situated in the basal portions of a great norite intrusive, and are ascribed to segregation of the sulphides as the rock solidified. To some extent the segregation was aided by mineralizing solutions following the crystallization of the magma, but in general there is little evidence that the ores were deposited from vagrant solutions of this kind (see pp. 34-35). These ores owe their value to primary concentration; secondary transportation and reprecipitation by surface waters has not been important. A small amount of the green arsenate, annabergite or "nickel bloom," has been developed by oxidation at the surface. The second, the garnierite or "lateritic" type of nickel ores, is somewhat more common and is represented by the deposits of New Caledonia. In this locality the original rock is a peridotite, relatively low in nickel, which has been altered to serpentine. Weathering has concentrated the more resistant nickel at the expense of the more soluble minerals, and has produced extensive blanket deposits of clay, which in their lower portions contain nickel in profitable amounts. Similar processes, working on material of a somewhat different original composition, have produced the nickel-bearing and chrome-bearing iron ores of Cuba (pp. 171-173). TUNGSTEN (WOLFRAM) ORES ECONOMIC FEATURES The principal use of tungsten is in the making of high speed tool steels. It is added either as the powdered metal or in the form of ferrotungsten, an alloy containing 70 to 90 per cent of tungsten. Tungsten is also used for filaments in incandescent lamps, and in contacts for internal combustion engines, being a substitute for platinum in the latter use. Of late years tungsten alloys have also been used in valves of airplane and automobile engines. The average grade of tungsten ores mined in the United States is less than 3 per cent of the metal; before smelting they are concentrated to an average grade of 60 per cent tungsten oxide. Germany through its smelting interests controlled the foreign tungsten situation prior to the war; two-thirds of its excess output of ferrotungsten was consumed by England and the balance principally by the United States and France. Other consumers in the main satisfied their requirements by imports of tool steel from these four countries. The bulk of the tungsten ore consumed in Europe prior to 1914 came from British possessions; these were principally the Federated Malay States, Burma, Australia, and New Zealand. The United States, Portugal, Bolivia, Japan, Siam, Argentina, and Peru were also producers. The great demand for tungsten created by the war added China to the list of important producers and greatly increased the production from Burma and Bolivia. Smelting works were established in England and those of the United States and France were greatly enlarged. England is at present in a position to dominate the world tungsten situation. The question of control of the ores obtainable in China, Korea, Siam, Portugal, and western South America is likely to be an important one for the future. Of the annual pre-war world production, the United States used about one-fifth. Three-fourths of this requirement was met by domestic production. The balance was obtained by importation, chiefly from Germany, from Portugal and Spain, and from England, both of concentrates and of ferrotungsten. To the considerable demand for high speed tool steels occasioned by munitions manufacture, production in the United States responded quickly. Supplies of tungsten came chiefly from California, Colorado, Arizona, Nevada, and South Dakota. At the same time importation largely increased, chiefly from the west coast of South America and the Orient. Consumption reached a half of the world's total. Considerable amounts of ferrotungsten were exported to the Allies. The end of the war created a possible tungsten shortage in this country into a tungsten surplus. In so far as actual domestic consumption is concerned there has been a return to something like pre-war conditions, as the only known new use to which tungsten may be put--the manufacture of die steel--does not involve the use of any large amount of ferrotungsten. The richer mines of the two chief tungsten-producing districts in the United States have shown impoverishment and at present no important new deposits are known. The grade of the producing deposits is on an average low. The domestic production of tungsten ore will doubtless decrease, owing to the importation of cheaper foreign ores, unless a high tariff wall is erected. Importation from the Orient and the west coast of South America should continue in reduced amounts, depending upon the ability of domestic manufacturers to obtain and hold foreign markets for ferrotungsten and high speed tool steel. In the commercial control of tungsten ores the United States has at present a strong position, second only to that of England. GEOLOGIC FEATURES Tungsten ores contain tungsten principally in the form of the minerals scheelite (calcium tungstate), ferberite (iron tungstate), hübnerite (manganese tungstate), and wolframite (iron-manganese tungstate). All these minerals are relatively insoluble and have high specific gravity, and as a consequence they are frequently accumulated in placers, along with cassiterite and other stable, heavy minerals. A large part of the world's tungsten production in the past has been won from such deposits. Placers are still important producers in China, Siam, and Bolivia, although in these countries vein deposits are also worked. With the exhaustion of the more easily worked placer deposits, increasing amounts of tungsten are being obtained from the primary or fixed deposits. These are found almost exclusively in association with granitic rocks, and have a variety of forms. The most productive deposits are in the form of veins, cutting the granites and the surrounding rocks into which the granites were intruded, and containing quartz, metallic sulphides, and in some cases minerals of tin, gold, and silver. The deposits of the two most important districts in the United States, in Boulder County, Colorado, and at Atolia, California, are of this general nature. The close association of such deposits with plutonic igneous rocks, and the characteristic mineral associations (see pp. 37-41) suggest strongly that the deposits were formed by hot solutions deriving their material from a magmatic source. Other tungsten deposits, which only recently became of importance, are of the contact-metamorphic type--in limestones which have been invaded by hot aqueous and gaseous solutions near the borders of granitic intrusions. In these occurrences the tungsten mineral is almost invariably scheelite, and is associated with calcite, garnet, pyroxene, and other silicates. A magmatic origin of the tungsten is probable. Some of the deposits of the Great Basin area and of Japan are of this nature, and it is believed that important deposits of this type may be discovered in many other countries. Tungsten is likewise found in original segregations in igneous rocks and in pegmatite dikes, but these deposits are of comparatively small commercial importance. In some tungsten deposits a hydrated oxide called tungstite has been formed as a canary-yellow coating at the surface. On the whole, however, tungsten minerals are very resistant to weathering, and in all their deposits secondary concentration by chemical action at the surface has not played any appreciable part. The disappearance of tungsten minerals from alluvial materials which are undergoing laterization, which has been described in Burma,[32] seems to indicate that the tungsten is dissolved in surface waters to some extent; but in the main it is probably carried completely out of the vicinity and not reprecipitated below. MOLYBDENUM ORES ECONOMIC FEATURES The main use of molybdenum is in the manufacture of high-speed tool steels, in which it has been used as a partial or complete substitute for tungsten. Its steel-hardening qualities are more effective than those of tungsten, but it is more difficult to control metallurgically. It has been used in piston rods and crank shafts for American airplanes. Its use in tool steel is mainly confined to Europe, where its metallurgical application is in a more advanced stage than in the United States. Molybdenum is added to steel either as powdered molybdenum or in the form of ferromolybdenum, an alloy containing 60 to 70 per cent of the metal. Molybdenum chemicals are essential reagents in iron and steel analysis and other analytical work; they are also used as pigments. Molybdenum metal has been used to a small extent in incandescent lamps and as a substitute for platinum in electric contacts and resistances. Molybdenum ores range from considerably less than 1 per cent to about 5 per cent in molybdenum. The world's principal sources of molybdenum ores in approximate order of importance are the United States, Canada, Norway, Australia, Korea, Austria, Peru, and Mexico. About half of the world's supply is produced in the United States. Production of molybdenum in this country practically began in 1914. Most of the production has come from Colorado and Arizona. It is believed that the United States contains reserves more than sufficient to meet any possible future demand. Thus far the demand has not kept up with capacity for production. The principal consuming countries are England, France, and Germany. GEOLOGIC FEATURES The chief ore minerals are molybdenite (molybdenum sulphide) and wulfenite (lead molybdate). The larger part of the world's production is from the molybdenite ores. Molybdenite occurs principally in association with granitic rocks,--in pegmatite dikes, in veins, and in contact-metamorphic deposits,--in all of which associations its origin is traced to hot solutions from the magma. It is frequently present as an accessory mineral in sulphide deposits containing ores of gold, copper, silver, lead, and zinc. At Empire, Colorado, one of the principal producing localities, it is found in veins, associated with pyrite, and filling the interstices between brecciated fragments of a wall rock composed of alaskite (an acid igneous rock). In molybdenite deposits secondary concentration has not been important. Wulfenite is rather common in the upper oxidized zone of deposits which contain lead minerals and molybdenite. It is probably always secondary. Deposits of wulfenite have been worked on a small scale in Arizona. VANADIUM ORES ECONOMIC FEATURES Vanadium is used mainly in steel, to which it gives great toughness and torsional strength. Vanadium steels are used in locomotive tires, frames, and springs, in those parts of automobiles that must withstand special bending strains, in transmission shafts, and in general in forgings which must stand heavy wear and tear. Vanadium is also used in high-speed tool steels, its use materially reducing the amount of tungsten necessary. It is added in the form of ferrovanadium, carrying 35 to 40 per cent of vanadium. Another use of vanadium is in chrome-vanadium steels for armor-plate and automobiles. Minor amounts are used in making bronzes, in medicine, and in dyeing. The low-grade ores of the United States range from 1 to 8 per cent of vanadium oxide, the general mean being nearer the lower figure. The high-grade ores of Peru contain from about 10 to as high as 50 per cent of the oxide; the roasted ore as shipped averages about 35 to 40 per cent. Two-thirds of the world's supply of vanadium comes from Peru, where the mines are under American control. The concentrates are all shipped to the United States and some of the ferrovanadium is exported from this country to Europe. The Germans during the war supplied their needs for vanadium from the minette iron ores in the Briey district in France, and presumably the French will in the future utilize this source. An unrecorded but small quantity is obtained by the English from lead-vanadate mines in South Africa. There are some fairly large deposits of vanadium minerals in Asiatic Russia, which may ultimately become an important source. The United States supplies less than one-half of its normal needs of vanadium, from southwestern Colorado and southeastern Utah. The grade of these deposits is low and the quantity in sight does not seem to promise a long future. Through its commercial control of the Peruvian deposits, the United States dominates the world's vanadium situation. GEOLOGIC FEATURES The Minasragra vanadium deposit of Peru contains patronite (vanadium sulphide) associated with a peculiar nickel-bearing sulphide and a black carbonaceous mineral called "quisqueite," in a lens-shaped body of unknown depth, enclosed by red shales and porphyry dikes. The origin is unknown. The patronite has altered at the surface to red and brown hydrated vanadium oxides. The deposits of Colorado and Utah are large lens-shaped bodies containing roscoelite (a vanadium-bearing mica) in fissures and brecciated zones and replacing the cementing materials of flat-lying sandstones. Locally the sandstones contain as much as 20 per cent of the roscoelite. The deposits contain small amounts of fossil wood which may have been an agent in the precipitation of the vanadium. There is considerable doubt as to their origin, but it is generally supposed that they represent concentrations by surface waters of minute quantities of material originally scattered through the surrounding sediments; it has also been suggested that certain igneous dikes in this region may have had some connection with the mineralization. Deposits of carnotite, a potassium-uranium vanadate, which have been worked for their content of uranium and radium and from which vanadium has been obtained as a by-product, are found as impregnations of the sandstone in these same localities (p. 265). There are other deposits containing small amounts of vanadium which are not at present available as ores. Vanadinite, a lead-vanadate, and descloizite, a vanadate of copper or lead, are found in the oxide zones of a number of lead and copper deposits in the southwestern United States and Mexico. Titaniferous iron ores, extensive deposits of which are known in many places, usually contain a small percentage of vanadium. Outside of the Peruvian deposit, the affiliations of which are doubtful, the vanadium deposits of economic importance owe their positions and values mainly to the action of surface processes, rather than to igneous activity. ZIRCONIUM ORES ECONOMIC FEATURES The oxides of zirconium have high refractory properties which make them useful for refractory bricks and shapes for furnace linings, for chemical ware, and for other heat, acid, and alkali resisting articles. For these purposes they find a limited market. Experimental work seems to show possibilities of a very considerable use of zirconium as a steel alloy; indeed, results are so suggestive that during the war the government conducted an active campaign of investigation with a view to using it in ordnance and armor steel. For such purposes the alloy ferrozirconium is used, which carries 25 to 35 per cent zirconium metal. The principal known deposits of zirconium ores, in order of commercial importance, are in Brazil, in India, and in the United States (Pablo Beach, Florida). The Brazilian and Indian deposits are also the principal sources of monazite (pp. 288-289). The United States controls one of the important Brazilian deposits. Germany before the war controlled the Indian deposits, and is reported to have taken much interest in the development of zirconium steels. During the war German influence in India was effectively broken up. The use of zirconium has been in an experimental state, and known sources of supply have been ample for all requirements. GEOLOGIC FEATURES The zirconium silicate, zircon, is a fairly common accessory constituent of granitic rocks and pegmatite veins. From these rocks it is separated by weathering, disintegration, and stream transportation, and, having a high specific gravity, it becomes concentrated in placers. The deposits of southern India, of the coast of Brazil, and of Pablo Beach, Florida, all contain zircon along with ilmenite, garnet, rutile, monazite, and other insoluble, heavy minerals, in the sands of the ocean beaches. Smaller deposits of zircon-bearing sands exist in rivers and beaches in other parts of the United States and in other countries, but none of these deposits has thus far proved to be of commercial importance. The largest and most important zirconium deposits are on a mountainous plateau in eastern Brazil and are of a unique type, entirely different from those just described. They contain the natural zirconium oxide, baddeleyite or brazilite, mixed with the silicate, the ore as produced carrying about 80 per cent zirconia (ZrO_{2}). The ores consist both of alluvial pebbles and of extensive deposits in place. The latter are associated with phonolite (igneous) rocks, and seem to owe their origin to the agency of hot mineralizing solutions from the igneous rocks. TITANIUM ORES ECONOMIC FEATURES Titanium is sometimes used in steel manufacture to take out occluded gases and thus to increase the strength and wearing qualities. Its effect is to cure certain evils in the hardening of the molten steel, and it is not ordinarily added in amounts sufficient to form a definite steel alloy. Aluminum is frequently used in place of titanium. Titanium is added in the form of ferrotitanium, containing either about 15 per cent titanium and 6 to 8 per cent carbon, or about 25 per cent titanium and no carbon. Titanium compounds are also used in pigments, as electrodes for arc-lights, and by the army and navy for making smoke-clouds. The United States has domestic supplies of titanium sufficient for all requirements. Production has come chiefly from Virginia. Additional quantities have been imported from Canada and Norway. The recently developed deposits of Pablo Beach, Florida, may produce important amounts of titanium minerals along with the output of zircon and monazite. GEOLOGIC FEATURES The principal titanium minerals are rutile (titanium oxide) and ilmenite (iron titanate). These minerals are formed mainly under high temperatures, either during the original solidification of igneous rocks, or as constituents of the pegmatites which follow the crystallization of the main igneous masses. The Virginia production comes from pegmatite dikes cutting through gabbros, syenites, and gneisses. The deposits contain rutile in amounts as high as 30 per cent of the mass, but averaging 4 or 5 per cent, in addition to varying amounts of ilmenite. Titaniferous magnetites, formed in many basic igneous rocks by the segregation of certain iron-bearing materials into irregular masses, contain large quantities of ilmenite which are not commercially available under present metallurgical processes. Rutile and ilmenite both have high specific gravity and are little affected by weathering. Consequently they are not decomposed at the surface, but when carried away and subjected to the sorting action of streams and waves, they form placer deposits. Both of these minerals are recovered from the sands at Pablo Beach, Florida. MAGNESITE ECONOMIC FEATURES The most important use of magnesite is as a refractory material for lining furnaces and converters. It is also used in the manufacture of Sorel cement for stucco and flooring, in making paper, in fire-resisting paint, in heat insulation, and as a source for carbon dioxide. Small amounts are used in Epsom salts and other chemicals. As taken from the ground the ore consists principally of the mineral magnesite or magnesium carbonate, with minor impurities (1 to 12 per cent) of lime, iron, silica, and alumina. In making magnesite bricks, it is calcined or "dead-burned" to drive out the carbon dioxide. Austria-Hungary and Greece are the large European producers of magnesite and Scotland supplies a little. Most of the European production is consumed in England and the Central European countries, but part has been sent to America. Outside the United States there are American supplies in Canada, and recent developments in Venezuela and Mexico (Lower California). Magnesite is produced in considerable quantities in the United States, in California and Washington. Some material is imported from Canada, and a small amount comes from Scotland as return cargo for ballast purposes. Before the war only about 5 per cent of the United States requirements of magnesite were met by domestic production. The country was practically dependent on imports from various European countries; chiefly from Austria-Hungary and Greece The Austrian magnesite (controlled in large part by American capital) was considered especially desirable for lining open-hearth steel furnaces, because of the presence of a small percentage of iron which made the material slightly more fusible than the pure mineral. When the shipments from this source were discontinued during the war and prices rose to a high figure, experiments were made with American magnesite, and the deposits on the Pacific Coast were developed on a large scale. A process of treatment was perfected by which the Washington magnesite was made as desirable for lining furnaces as the Austrian material. At the same time large amounts were imported from Canada and Venezuela and lesser amounts from Lower California. Under the high prices which prevailed during the war, dolomite was to some extent substituted for magnesite. Dolomite, which may be thought of as a magnesite rock high in lime, occurs in large quantities close to many points of consumption. It is cheaper but less satisfactory than magnesite, and is not likely to be used on any large scale. While the United States has undoubtedly sufficient reserves of magnesite to supply the domestic demands for many years, the mines are far from the centers of consumption and it is expensive to transport the material. Since the war, magnesite shipped from Canada and overseas has again replaced the American product in the eastern market to some extent. The Canadian magnesite is of lower grade than the domestic and European magnesite and is consequently less desirable. Deposits in Venezuela are also expected to furnish some material for the eastern furnaces, in competition with those of Austria and Greece. Austrian magnesite, however, will be likely to dominate the market in the future if delivered at anything like pre-war prices. This situation has led to agitation for a protective tariff on magnesite. GEOLOGIC FEATURES Magnesite, as noted above, is the name of a mineral, the composition of which is magnesium carbonate. The principal magnesite deposits are of two types, of different modes of origin and of somewhat different physical characteristics. The large magnesite deposits of Austria and of Washington, as well as those of Quebec, occur as lenses in beds of dolomite (calcium-magnesium carbonate). They are in fairly close proximity to igneous rocks, and magnesia-bearing solutions issuing from these rocks are believed to have dissolved out the calcium carbonate of the dolomite and replaced it with magnesium carbonate. In these deposits the material is coarsely crystalline and forms fairly large, continuous bodies, which are worked by quarrying. The Washington deposits closely resemble marble, and had sometimes been mistaken for that rock until war-time needs resulted in their more thorough investigation. The commoner type of magnesite deposits is represented by those of Greece, California, Venezuela, and many other countries. These consist of veins and replacements in serpentine. The original rock was a highly magnesian igneous rock of the peridotite type, which is very unstable under weathering conditions, and rapidly alters to serpentine. Magnesite is formed both by this process and by the further breaking down of the serpentine itself. The processes are those of katamorphism. Under these circumstances the magnesite is characteristically fine-grained or massive, and occurs in veins, lenses, and irregular bodies in cavities and fractured zones. It is usually worked by open cuts. Magnesite is also reported to occur in sedimentary beds in which it was primarily deposited in its present form and has not undergone later alteration. Such deposits are not important commercially. FLUORSPAR ECONOMIC FEATURES The chief use of fluorspar is as a flux in the manufacture of open-hearth steel. Minor uses are in chemical and enameling industries, in the smelting of copper, lead, and iron, and in the manufacture of the ferro-alloys in the electric furnace. In order to be used in steel-making, the fluorspar after being concentrated should contain at least 85 per cent calcium fluoride and less than 4 per cent silica. Chemical and enameling industries require material with 95 to 98 per cent calcium fluoride and less than 1 per cent silica. The chief foreign producer of fluorspar is Great Britain, and much of this product comes to the United States. Canada produces a small amount, some of which also comes to the United States. Several thousand tons are produced yearly in Germany and France, and are largely consumed there. The production of fluorspar in the United States is several times that of any other country. The ore mined comes principally from the southern Illinois and western Kentucky field, and is used largely for fluxing purposes in open-hearth steel furnaces. Minor amounts are produced in Colorado, New Mexico, and other states. The United States has sufficient supplies of fluorspar to meet all its own demands for this material. Small amounts, however, are imported for use in eastern furnaces because the material can be brought over from England very cheaply. The domestic fluorspar is suitable for practically all purposes for which fluorspar is used except for lenses in optical instruments. For this use very small quantities of material imported from Japan have been used, but recently fluorspar of a grade suitable for optical purposes has been found in Illinois, Kentucky, New Hampshire, and other states. For fluxing purposes domestic fluorspar is superior to the foreign product. GEOLOGIC FEATURES Fluorspar is the trade name for the mineral fluorite, which is composed of calcium fluoride. This is a common mineral in veins and replacements which carry ores of zinc, lead, silver, gold, copper, and tin. It is formed under a variety of conditions, but is always ascribed to solutions coming from nearby igneous rocks. The large fluorspar deposits of Illinois and Kentucky contain fluorite with calcite, barite, and metallic sulphides, in wide veins filling fissures in limestones and sandstones and replacing the fissure walls. Into these sediments there are intruded certain peridotite dikes. The fluorite and associated minerals were probably deposited by hot solutions bringing the material from some large underlying igneous mass of which the dikes are off-shoots. In the western United States many metalliferous deposits carry large amounts of fluorite, which is treated as a gangue or waste mineral, but which could be profitably extracted if there were local markets. In England, fluorite is obtained in this manner as a by-product from lead and zinc mines. SILICA ECONOMIC FEATURES Silicon and its oxide, silica, find important applications in the manufacture of iron and steel. Silicon, like manganese, is an important constituent of many steels, the alloy ferrosilicon being added to deoxidize and purify the metal and thus to increase its tensile strength. Like titanium, it is added chiefly for its curative effect rather than as a useful ingredient. On an average 4 pounds of 50 to 55 per cent ferrosilicon are used in the United States for each ton of steel produced. A higher grade of ferrosilicon (80 to 85 per cent) is used for certain special steels, and during the war considerable quantities were used in making hydrogen gas for balloons. Lower grades (10 to 15 per cent silicon) are practically a high silicon pig iron. Silica has an important use in the form of silica brick or "ganister" for lining furnaces and converters in which acid slags are formed. For this purpose siliceous rocks, chiefly quartzites and sandstones, are ground up, mixed with lime as a binder, and fused and pressed into bricks and shapes. For the most satisfactory results the rock should contain 96 per cent or more of silica, and very little of the alkali materials, which increase the fusibility. In addition to its applications to the iron and steel industry, silica finds an almost universal use in a wide variety of structural and manufacturing operations. The extensive use of sand and gravel--composed chiefly of silica--for road materials and railway ballast is well known. In construction work silica is used in the form of stone, sand-lime brick, cement, mortar, concrete, etc. Large quantities of sand, or silica, are used for molds in foundries, for abrasives, for the manufacture of glass, for filters, and for a great variety of other purposes which readily suggest themselves (see pp. 84, 267). For most uses of silica there are local supplies available. For certain purposes requiring material of a particular chemical composition or texture, however, satisfactory deposits are known in only a few places. For example, the material for silica refractories is obtained in the United States chiefly from certain regions in Pennsylvania, Missouri, and Wisconsin. The United States has ample domestic supplies of silica for practically all requirements. Ferrosilicon of the higher grades is manufactured principally in electric furnaces at Niagara Falls. The capacity is ample to meet all demands, but cheap ferrosilicon from Canada also enters United States markets. GEOLOGIC FEATURES Silicon and oxygen, making up the compound silica, are the two most abundant elements in the earth's crust, and quartz (SiO_2) is a very abundant mineral. The processes of weathering and transportation everywhere operative on the surface of the earth tend to separate quartz from other materials, and to concentrate it into deposits of sand. Katamorphism is primarily responsible for most of the deposits of silica which are commercially used. Anamorphism--cementing and hardening the sands into sandstones and quartzites--has created additional value for certain uses, as in refractories, building stones, and abrasives (see pp. 84, 267). FOOTNOTES: [31] Report of the Royal Ontario Nickel Commission. Printed by order of the Legislative Assembly of Ontario, Toronto, 1917. [32] Campbell, J. Morrow, Tungsten deposits of Burma and their origin, _Econ. Geol._, vol. 15, 1920, p. 511. CHAPTER X COPPER, LEAD, AND ZINC MINERALS COPPER ORES ECONOMIC FEATURES The electrical industry is the largest consumer of copper. The manufacture of brass, bronze, and other copper alloys constitutes another chief use for the metal. Considerable quantities of copper sheets, tubes, and other wares are used outside of the electrical industry, as for instance in roofing, plumbing, and ship bottoms. Copper is also used in coinage, particularly in China, where it is the money standard of the working population. The average grade of all copper ores mined in the United States in recent years has been about 1.7 per cent metallic copper. Ores containing as low as 0.6 per cent have been mined in the Lake Superior country, and bonanza deposits containing 20 to 60 per cent have been found and worked in some places, notably in Alaska and Wyoming. The lower-grade ores, carrying 1 to 3 per cent copper, are usually concentrated before smelting, while the richer ores, carrying 3 to 5 per cent or more, are generally smelted direct. Many of the ores contain values in gold and silver, and also in lead and zinc. An average of about 40c. worth of gold and silver per ton is obtained from all the copper ores of the United States. In other countries the average grade of copper ores mined is somewhat higher than in the United States,--where large scale operations, particularly the use of steam-shovel methods on extensive bodies of disseminated or "porphyry" copper ores, as well as improvements in concentrating and metallurgical processes, have made possible the use of low-grade ore. The principal sources of copper are the North American continent, Chile and Peru, Japan, south and central Africa, Australia, and Spain and Portugal. Smaller quantities are produced in Russia, Germany, Norway, Cuba, Serbia, and a number of other countries. The United States normally produces nearly two-thirds of the world's copper and consumes only about one-third. In addition the great bulk of the South American, Mexican, and Canadian crude copper comes to the United States for refining. Through financial interests abroad and by means of refining facilities, the United States controls a quantity of foreign production which, together with the domestic production, gives it control of about 70 per cent of the world's copper. No other country produces one-sixth as much copper as the United States. England, because of production in the British Empire (mainly Africa and Australia) and British financial control of production in various foreign countries, is not dependent upon the United States for supplies of raw copper. Japan, Spain, Portugal, and Norway are able to produce from local mines enough copper for their own needs and for export. But France, Italy, Russia, Germany, and the rest of Europe normally are dependent upon foreign sources, chiefly the United States. South America, Mexico, Canada, Africa, and Australia are exporters of copper. The control of these countries over their production in each case is political and not financial, except in the case of Canada, where about half the financial control is also Canadian. It is in these countries and in Spain that the United States and England have financial control of a large copper supply. Before the war German interests had a considerable control over the American copper industry through close working arrangements with electrolytic refineries. Germany was the largest foreign consumer of copper, and German companies bought large quantities of the raw copper in the United States, Canada, Mexico, and South America, had it refined, and sold the finished material in both the American and foreign markets. During the war this control was broken up. In view of the importance of copper metal as a raw material, particularly in the electrical industry, the strength of the United States in copper as a key resource ranks even above its control of petroleum. In the United States in recent years about 40 per cent of the annual production of copper has come from Arizona, chiefly from the Bisbee, Globe, Ray-Miami, Jerome, and Morenci-Metcalf districts; about 18 per cent has come from the Butte district of Montana; about 12 to 15 per cent from Keweenaw Point, Michigan; and about 12 per cent from Bingham, Utah. From 3 to 5 per cent of the country's output comes from each of the states of New Mexico, Nevada, Alaska, and California. All other states together produce only a little over 2 per cent of the total. The so-called "porphyry" coppers in Utah, Arizona, Nevada, and New Mexico, described below, are the source of about 35 per cent of the present production of the United States. The deep mines of Butte and Michigan are responsible for about 30 per cent of the production, and the ore bodies of Arizona (other than porphyry) and of Alaska produce about 25 per cent. Reserves of copper ore are such as to give no immediate concern about shortage, nor to indicate any large shift in the distribution of production in the near future. Development is on the whole considerably in advance of present demands. The principal measured reserves are in the so-called porphyry coppers of the United States and Chile. In the United States the life of these reserves now estimated is approximately 25 years. The reserves of the Chile Copper Company are the largest of any known copper deposit in the world, and the Braden copper reserve (also in Chile) is among the largest. For the deep mines of the United States, the developed reserves have a life of perhaps only five years, but for most of these mines the life will be greatly extended by further and deeper development. The porphyry coppers, because of their occurrence near the surface and the ease with which they may be explored by drilling, disclose their reserves far in advance. The deep mines are ordinarily developed for only a few years in advance of production. GEOLOGIC FEATURES The principal copper minerals may be classified into the sulphide group, the oxide group, and native copper. Native copper, mined in the Lake Superior region, is the source of 8 to 10 per cent of the world's copper supply. The oxide group of minerals--including the copper carbonates, azurite and malachite; the silicate, chrysocolla; the oxide, cuprite; the sulphates, chalcanthite and brochantite; and some native copper associated with these minerals--probably supplies another 5 per cent. The remaining 85 per cent is derived from the sulphide group. Of the sulphide group by far the most important mineral is chalcocite (cuprous sulphide), which supplies the bulk of the values in the majority of the mining camps of the western hemisphere. Locally, as at Butte, enargite (copper-arsenic sulphide) is of great value. Other minerals of considerable importance in some districts are chalcopyrite and bornite (copper-iron sulphides), tetrahedrite (copper-antimony sulphide), and covellite (cupric sulphide). Very commonly the copper sulphides are associated with large quantities of the iron sulphide, pyrite, as well as with varying amounts of lead and zinc sulphides and gold and silver minerals. The principal copper ores originate in the earlier stages of the metamorphic cycle, in close association with igneous activity. Katamorphism or weathering, in place, has played an important part in enriching them. The processes of transportation and sedimentary deposition, which have done so much toward making valuable iron ore deposits, have contributed little to the formation of copper ores. =Copper deposits associated with igneous flows.= The copper ores of the Lake Superior district, and of a few small deposits in the eastern United States, contain small percentages of native copper in pre-Cambrian volcanic flows or in sediments between the flows. The ore bodies have the form of long sheets parallel to the bedding, the copper and associated minerals filling amygdaloidal openings and small fissures in the flows, and replacing conglomeratic sediments which lie between the flows. The copper was probably deposited by hot solutions related to the igneous rocks, either issuing from the magmas or deriving heat and dissolved material from them. Secondary concentration has not been important. There is practically none of it near the present erosion surface; but it appears in one part of the district near an older erosion surface covered by Cambrian sediments, suggesting a different climatic condition at that time. The Kennecott copper deposits of Alaska have a number of resemblances to the Lake Superior copper deposits, suggesting similarity in origin. The Kennecott deposits occur exclusively in limestone, which rests conformably on a tilted surface of igneous flows ("greenstones") not unlike those of Lake Superior. The flows carry native copper and copper sulphides in minutely disseminated form and in amygdules, but apparently not in quantities sufficiently concentrated to mine. The flows are supposed to be the original source of the copper now in the limestone. The primary copper mineral in the limestone is chalcocite, in exceptionally rich and solid masses, showing no evidence of having replaced earlier sulphides. It is regarded as a product of primary deposition, under the influence of hot solutions related in some way to the igneous flows; but whether the solutions were magmatic, originating in the lavas or below, or whether they were meteoric waters rendered hot by contact with the extrusives, and thereby made effective in leaching copper from them, is not clear. The oxidation of the Kennecott copper ores is not extensive. It presents an interesting feature, in that since glacial time the ground has been frozen and the moisture is now present in the form of ice. The oxidation clearly took place before glacial time. Abundant fragments of both the oxide and the sulphide ores are mined from the lateral moraine of a nearby glacier. This is a good illustration of the cyclic nature of secondary concentration which is coming to be recognized in so many camps. The Boleo copper deposits of Lower California occur in volcanic tuffs and associated conglomerates of Tertiary age. They have certain peculiar mineralogic associations--the ores containing large quantities of all the common copper oxide minerals, and a number of rare oxide minerals of copper, lead, silver, and cobalt, together with gypsum, sulphur, and much iron and manganese oxide. The copper oxides and carbonates are in places gathered into rounded concretions called "boleos" (balls). Sulphides are present in the lowest beds and may represent the form in which the copper was originally deposited. The copper-bearing beds have been much silicified, and it has been suggested that mineralization was accomplished by hot-spring waters, probably of igneous origin. These deposits have a few marked similarities to the Lake Superior copper ores. =Copper veins in igneous rocks.= A second group of copper ores in igneous rocks is made up of deposits in distinct fissure veins and as replacements along such veins. The chief deposits of this type are at Butte, Montana--which is, from the standpoint of both past and present production, the greatest single copper district in the world. Here a large batholith of Tertiary granite was intruded by porphyry dikes; and faulting, accompanying and following the intrusions of the dikes, developed numerous fissures. The fissures were mineralized with copper sulphides and arsenides, iron sulphides, and locally with zinc sulphide and manganese carbonate,--all in a matrix of quartz. At the same time the wall rocks were extensively mineralized and altered; the fissure veins grade off into the wall rock, and in fact the larger part of the ore is simply altered granite with disseminated sulphides. The solutions which deposited the ores are inferred to have been hot from the nature of the wall-rock alterations, from the presence of hot-water minerals like fluorite, cassiterite, and others, and from the general association of the ores in time and place with the porphyry intrusions. The solutions are believed to have originated from the porphyry and possibly from other intrusives. In the Butte district, and in the great majority of copper sulphide vein ores throughout the world, secondary concentration by surface waters has played a considerable part in developing ores of commercial value. Near the surface the copper is leached out and carried down by waters containing various solvents, particularly sulphuric acid from the oxidation of pyrite. A leached zone is formed containing the ordinary products of rock weathering,--rusty quartz and clay, sometimes black with manganese oxides. A small part of the copper remains in this zone as oxides, carbonates, and silicates. Below the oxidized and leached zone there is evidence of deposition of a large amount of secondary copper sulphide in the form of chalcocite. This is supposed to have been formed by the leaching of copper from above as soluble copper sulphate, and its precipitation below by iron and other sulphide minerals which the solutions meet on their downward course--a reaction which has been demonstrated experimentally. It was formerly supposed that most of the chalcocite was of this origin; but as chalcocite is found in important amounts with enargite and chalcopyrite to great depths (now 3,500 feet), where the veins are still rich and strong, it begins to appear that much of the chalcocite is of primary origin. The fissures along which the Butte ores occur are in three main sets, which in order of age strike roughly east-west, northwest-southeast, and northeast-southwest. Two-thirds of the ore is in the first set, about 30 per cent in the second, and the remainder in the third. The mineralization of the several vein systems cannot be discriminated, and it is thought that it was accomplished as a more or less continuous and progressive process. There is some evidence, also, that the fracturing in the several fracture systems was likewise a nearly continuous progressive process, contemporaneous with the ore deposition, and perhaps developing under a single great shear which caused more or less simultaneous and overlapping systems of fractures in the various directions. ="Porphyry coppers."= Another type of copper deposits in igneous rocks is the disseminated or "porphyry" deposits. The term "porphyry" as commonly used includes true porphyries, monzonites, granites, and other igneous rocks. Ores of this type are represented by the great deposits of Bingham, Utah; Ray, Miami, and the New Cornelia mine of Arizona; Ely, Nevada; Santa Rita, New Mexico; Cananea, Sonora, Mexico; northern Chile; and many other districts of importance. They form the greatest known reserves of copper ore. These deposits contain copper minerals, usually in the marginal portions of acid porphyries, in many irregular, closely spaced veins, and in minute seams and spots disseminated through the mass of the rock. In the Ray and Miami and other districts the mineralization has spread largely through adjacent schists, but these deposits are included with the porphyry copper deposits in commercial parlance. The porphyry deposits are of an undulating blanket form of considerable areal extent and shallow depth. At the surface is a leached and weathered zone, often containing more or less of the oxides, carbonates, and silicates of copper, ranging in thickness up to 1,000 feet, but averaging 200 feet or less. Below this is a zone carrying copper in the form of chalcopyrite, enriched by chalcocite deposition from above, ranging in thickness up to 400 feet. The ore in this zone varies from one-half of 1 per cent to 6 per cent of copper and ordinarily averages between 1 and 2 per cent. The use of ore of this grade is made possible by the large quantities and by the cheap and efficient mining and metallurgical practices. The ore body grades below into a zone characterized by lean chalcopyrite, which is supposed to represent original or primary deposition from hot waters associated with the porphyry intrusion. This primary ore, or protore, was clearly formed after the solidification of the igneous rocks, though soon after, by solutions from igneous sources which followed fractured and shattered zones. =Copper in limestone near igneous contacts.= Another great group of copper deposits occurs as replacements of limestone adjacent to porphyry or granitic intrusives. This type is illustrated by some of the deposits at Bingham, Utah, and at Bisbee, Arizona. The primary deposition was of chalcopyrite and other copper sulphides, together with garnet, diopside, and other minerals known to have required high temperature in their formation. The ore fills fissures and replaces extensive masses of the limestone. It is likely to show a fairly sharp contact on the side toward the intrusive, and to grade off into the country rock on the other side with numerous embayments and irregularities. These deposits have been enriched by weathering in the same manner as indicated above for the porphyry coppers, but to highly varying degrees. In the Bisbee deposits large values were found in the weathered zone, and secondary sulphide enrichment below this zone is also important. In the Bingham camp, on the other hand, the weathered zone is insignificant and most of the ore beneath is primary. The weathering of the silicated limestone gangue results in great masses of clay which are characteristic features of the oxide zones of these deposits. =Copper deposits in schists.= Other copper deposits, as at Jerome, Arizona, in the Foothill and Shasta County districts of California, at Ducktown, Tennessee, etc., are irregular lenticular bodies in schists and other rocks, but all show relationship to igneous rocks. The Rio Tinto ores of Spain and Portugal, which belong in this group, have been referred to on page 108. In the Jerome or Verde district of central Arizona, folded pre-Cambrian greenstones and sediments were invaded by masses of quartz-porphyry, and after further deformation, rendering many of the rocks schistose, were intruded by an augite-diorite. Contact metamorphism along both the quartz-porphyry and the diorite contacts was practically lacking. The ore bodies were formed as irregular pipe-like replacements of the schists, being localized in one case by a steeply pitching inverted trough of impervious diorite, and in other cases by shear zones which favored vigorous circulation. A later series of small diorite or andesite dikes cut the ore bodies. The primary ores consist of pyrite, chalcopyrite, and other sulphides, with large amounts of jaspery quartz and some calcite and dolomite. They were clearly formed by replacement of the schists particle by particle, as shown by the frequent preservation of the schist structure in a banding of the sulphide minerals, the residual shreds of unreplaced schist material in the ores, and the usual gradual transition from unreplaced schists to those completely replaced by massive sulphides. The localization of the most important mineralization in an inverted trough is good evidence that the solutions came from below, and the nature of the mineral associations suggests an origin through the work of hot waters associated with igneous intrusives. The diorite, being most closely related in time and space with the ore bodies, seems the most logical source of the ore materials. Secondary concentration of the Jerome ores has proceeded along the general lines previously outlined (pp. 46-50, 202). Here again the evidence is clear that the ores were concentrated in an earlier period, in this case in pre-Cambrian times, probably during the long interval required for the base-leveling of the pre-Cambrian mountains. Since Cambrian times the deposits have been for the most part buried by later sediments. Some of the deposits are still protected by this overlying blanket and mining has not yet reached the zone of altogether primary sulphides. Others have been faulted up and again exposed by erosion; but since being uncovered, steep slopes and rapid erosion have apparently favored the scattering of the copper rather than its concentration and enrichment. In the United Verde Mine, oxidizing conditions at present prevail to the bottom of the chalcocite zone. The very large reserves of the Katanga copper belt of the Belgian Congo are in the form of tabular masses in schistose and highly metamorphosed Paleozoic sediments. The ore bodies are roughly parallel to the bedding, but in instances follow the schistosity which cuts across the bedding. They consist dominantly of the oxide minerals, though in several ore bodies sulphides have been shown by diamond-drilling. The ores have a high content of cobalt and also carry precious metals. The origin of the deposits is not known, but has been ascribed to granitic masses intrusive into the schists. =Sedimentary copper deposits.= In the later phases of the metamorphic cycle, the agencies of transportation (in solution) and sedimentary deposition have resulted in some low-grade deposits of copper sulphides in sedimentary rocks. Deposits of this type are found in the Rocky Mountain region, where they are referred to as the "Red Beds" coppers, but are of no commercial importance. Similar deposits in Germany, the Mansfield copper-bearing shales, have been worked for some time, and during the war were Germany's main source of copper. On Keweenaw Point, Michigan, deposits of native copper formed in this manner in the "Nonesuch" beds have been worked on a commercial scale. Other copper ores on Keweenaw Point are replacements of conglomerate beds between igneous flows, and are of a different origin already described (p. 200). While much of the copper of sedimentary beds gives evidence that it was deposited from solution in cracks and as replacements of the wall rocks, often through the agency of abundant organic material in the beds, and while also comparatively little of this copper can be identified as having been deposited in detrital flakes or fragments along with the other mineral fragments, there is, nevertheless, considerable evidence that some of these deposits were formed essentially during the sedimentation of the enclosing beds and as incidents to this process. Such evidence consists of a close limitation of the copper to certain beds, its wide and uniform distribution within these beds, its absence in similar beds near at hand, the absence of evidence of feeding and escape channels of the kind which would be necessary in case the solutions were introduced long afterward, and often a minute participation of the copper minerals in the minor structures of bedding, false-bedding, and ripple-marks, which would be difficult to explain as due to secondary concentration. The Corocoro copper deposits of Bolivia occur in beds of sandstone with no igneous rocks in the vicinity. However, they are all closely associated with a fault plane, igneous rocks occur at distances of a few miles, and the general mineralization is coextensive with the belt of igneous rocks; the deposits are therefore ascribed to a magmatic source rather than to sedimentary processes. Toward the surface the copper is in part in the form of sulphides, somewhat altered to oxide minerals, and farther down it is entirely native copper, associated with gypsum. This is the only district outside of Lake Superior where native copper has been mined on an important scale. =General comments.= In general, the commercially prominent copper deposits show a close relationship to igneous rocks in place, time, and origin. Seldom do the ores extend more than 1,000 feet away from the igneous rock. The common downward order in sulphide deposits is: first, a weathered zone, originally formed mainly above the water table, consisting above of a leached portion and below of oxides and carbonates of copper in a gangue of quartz or clay; second, a zone of secondary sulphide enrichment, characterized by chalcocite coatings, chalcopyrite, and pyrite, with a gangue of quartz and igneous rock or limestone; and third, a zone of primary deposition with similar gangue, characterized by chalcopyrite, and at Butte by enargite and chalcocite. The oxide zone as a whole may be rich or lean in values, depending on the nature of the associated gangue material and country rock. When these are more soluble than the copper--as is commonly the case in limestone--the copper may be residually concentrated, notwithstanding the fact that much copper originally present has been carried off in solution. When the associated gangue and country rock are less soluble than the copper--as is common with quartz and igneous rocks--the oxide zone is likely to be depleted of values. The zones formed by weathering and secondary enrichment are extremely irregular, both in distribution and depth, in any one deposit, and they overlap and grade into one another in a very complex fashion. In many places the primary zone is too lean to be mined to commercial advantage; but in other places, as at Butte, and in the limestone deposits of Bingham, the primary ores are of considerable importance. When evidence of secondary sulphide enrichment was first recognized there was a tendency to magnify its effectiveness, and to assume that in most cases the values were due to this process; that the primary zones would be found to be valueless. In recent years the emphasis is being somewhat changed because of the recognizing in many camps of rich primary zones. While some chalcocite is clearly the result of secondary enrichment from above, other chalcocite seems to have been related closely to the primary deposition. The quantitative discrimination of the two is a matter of great difficulty. It has come to be recognized that the zonal arrangement caused by enrichment from the surface has been imposed usually on a zonal arrangement caused by the primary hot solutions and not related to the surface but to the source of the solutions. In some districts, as illustrated by Butte and Bingham, the copper-bearing minerals seem to have been deposited nearest the igneous source, while the lead, zinc, gold, and silver minerals have been deposited farther away,--suggesting the cooling of the solutions with increasing distance from the igneous source. The further investigation of this primary zonal arrangement promises interesting results with a practical bearing on exploration and development. One of the newer features of the investigation of copper deposits has been the recognition of the cyclic nature of the secondary concentration. This process has been related not only to the present erosion surface, but to older surfaces now partly buried under later rocks. Ransome's[33] summary of conditions at the Ray-Miami camp has a somewhat general application. Supergene enrichment has generally been treated as a continuously progressive process. There is considerable probability, however, that it is essentially cyclic, although the cyclic character may not be patent in all deposits. A full development of the cycle can take place only under a certain equilibrium of a number of factors, including climate, erosion, topography, and character of rock. The essential fact appears to be that as enrichment progresses and chalcocite increases the process of enrichment becomes slower in action, and erosion may, in some circumstances, overtake it. With the removal of some of the protecting zone of chalcocite the protore is again exposed to oxidation and a second cycle of enrichment begins. Although much of the enriched ore is now below ground-water level, it probably was once above that level, and enrichment is believed to have taken place mainly in the zone of rock above any general water table. Where the old erosion surface roughly coincides with the present erosion surface, the deposits follow more or less the topography. Where the old erosion surface pitches below later sediments, the ores pitch with it, and therefore do not follow the present topography. The recognition of the cyclic nature of secondary concentration is obviously of great significance in exploration and development. Although a vast amount of study has been devoted to the origin and enrichment of copper deposits, and although the general conditions and processes are pretty well understood, the results thus far have been largely qualitative rather than quantitative. LEAD ORES ECONOMIC FEATURES The most prominent uses of lead are in the manufacture of alloys, such as type-metal, bearing metal, shot, solder, and casting metal; as the oxide, red lead, and the basic carbonate, white lead, in paints; for lead pipe, cable coverings, and containers of acid active material; and in lead compounds for various chemical and medical uses. Of the lead consumed in the United States before the war about 38 per cent was utilized in pigments, 30 per cent in alloys other than shot, 15 per cent in pipe, 10 per cent in shot, and 7 per cent in all other uses. During the war much larger quantities were used in munitions, such as shot and shrapnel. The lead content of commercial ores varies widely. It ranges from as low as .25 per cent in the Joplin district of Missouri, to about 15 per cent in the Broken Hill deposits of Australia, and over 20 per cent in the Bawdwin mines of Burma. In the Coeur d'Alene district of Idaho and the southeastern district of Missouri, the two greatest lead producers in the United States, the average grades are about 10 per cent and about 3-1/2 per cent respectively. The grade of ore which may be profitably worked depends not only upon the economic factors,--such as nearness to consuming centers, and the price of lead,--but also upon the amenability of the ore to concentration, the content of other valuable metals, and the fact that lead is very useful in smelting as a collector of gold and silver. Most lead ores contain more or less zinc, and lead is obtained as a by-product of most zinc ores. Argentiferous lead ores form one of the principal sources of silver, and also yield some gold. Lead and copper are produced together from certain ores. Thus the separation of many ores into hard and fast classes, as lead, or zinc, or copper, or silver, or gold ores, cannot be made; in some of the mineral resource reports of the United States Geological Survey the statistics of these five metals are published together. The main sources of lead ore, named in order of their importance, are the United States, Australia, Spain, Germany, and Mexico, which account for over 80 per cent of the world's production. Most of the countries of Europe outside of Spain and Germany produce small amounts of lead, but are largely dependent on imports. Spain exports argentiferous lead and pig lead mainly to England and France, with minor quantities to other countries of Europe and to Argentina. Before the war Germany, which was the largest European consumer, utilized all its own production of lead ores and imported an additional 10 per cent of the world's ores for smelting, as well as considerable amounts of pig lead. Its principal deposits were those of Silesia; under the Peace Treaty they may possibly be lost to Poland, leaving German smelters largely dependent on imports. Australia before the war normally shipped lead concentrates and pig lead to England and also to Belgium, Germany, and Japan. England, the second largest European consumer, before the war had insufficient smelting capacity within the British Empire and was partly dependent on foreign-smelted lead. During the war, however, England contracted for the entire Australian output, and enlarged its smelting capacity accordingly. This may mean permanent loss to Belgium, which had depended mainly on the Australian ores for its smelting industry before the war. In North Africa there is a small but steady production of lead, most of which goes to France. Recent developments in Burma have shown large reserves of high-grade lead-zinc-silver-copper ores, and this region may be expected to become an important producer. There are also large reserves of lead in the Altai Mountains of southwestern Siberia and in the Andes Mountains of South America. England, through control of Australian and Burman lead mines and smelters, domestic smelting facilities, and some financial control in Spain, Mexico, and elsewhere, and France, through financial control of Spanish and North African mines and Spanish, Belgian, and domestic smelters, have adequate supplies of lead. The United States produces about a third of the world's lead and twice as much as any other country. Normally the domestic production is almost entirely consumed in this country. Mexico sends large quantities of bullion and ore to the United States to be smelted and refined in bond. Mexican lead refined and exported by the United States equals in amount one-sixth of the domestic production. Small quantities of ore or bullion from Canada, Africa, and South America are also brought into the United States for treatment. Through domestic production, smelting facilities for Mexican ore, and commercial ownership in Mexico and elsewhere, the United States controls over 45 per cent of the world's lead production. Before the war Germany, through the "Lead Convention" or International Sales Association, and through smelting and selling contracts with large producing mines, practically controlled the European lead market as well as exports from Mexico and the United States and from Australia. During the war German foreign influence was practically destroyed. In the United States about a third of the production of lead comes from southeastern Missouri and about a fourth from the Coeur d'Alene district of Idaho. The five states, Missouri, Idaho, Utah, Colorado and Oklahoma, produce about nine-tenths of the country's total output. Reserves of lead ore are not large in proportion to demand, contrasting in this regard with zinc ore. GEOLOGIC FEATURES The principal lead mineral is the sulphide, galena, from which the great bulk of the world's lead is derived. Cerussite (lead carbonate) and anglesite (lead sulphate) are mined in some places in the upper part of sulphide deposits, and supply a small fraction of the world's output. The ores of lead are of two general classes: The first class, the so-called "soft" lead ores, nearly free from copper and precious metals, and commonly associated with zinc ores, are found in sedimentary beds independent of igneous intrusion. They are of world-wide distribution, were the first to be extensively exploited, were at one time the dominant factor in world production of lead, and at present produce about 30 per cent of the world's total. They are represented by the deposits of the Mississippi Valley, of Silesia, and some of the Spanish deposits. The general description of the origin of the zinc ores of the Mississippi Valley on pp. 216-218 applies to this class of lead ores. It should be noted, however, that in the principal United States lead-producing district, that of southeastern Missouri, the lead ores occur almost to the exclusion of the zinc ores, and are more disseminated through the limestone than is characteristic of the zinc ores. Ores of this type have been found extending only to shallow depths (not over a few hundred feet), and because of the absence of precious metals their treatment is comparatively simple. The second class consists of ores more complex in nature, which are found in association with igneous rocks, and which usually contain some or all of the metals, zinc, silver, gold, copper, iron, manganese, antimony, bismuth, and rare metals, with various gangue minerals among which quartz, siderite, and silicates are important. Today these ores are the source of about 70 per cent of the world's lead. They are represented by the lead deposits of the Rocky Mountain region (Coeur d'Alene, Idaho; Leadville, Colorado; Bingham, Utah; etc.); of Broken Hill, New South Wales; of Burma; and of many other places. They are all related to the earlier stages of the metamorphic cycle and occur in close genetic association with igneous activity. They include deposits in the body of igneous rocks,--in the form of well-defined veins, replacements along zones of fissuring and shearing, and disseminated masses,--as well as veins and replacements in the rocks, particularly in limestones, adjoining igneous intrusions. The deposits present a wide variety of shapes depending on the courses of the solutions by which they were formed. The materials of the ore minerals are believed to have been derived from the igneous rocks and to have been deposited by hot solutions. The source of the solutions--whether magmatic or meteoric--presents the same problems which have been discussed elsewhere (pp. 41-42). The ores are frequently mined to great depths. Because of their complexity they require involved processes of treatment to separate out the values. Ores of this nature have already been referred to in the discussion of the copper ores of Bingham and Butte, and will be referred to in connection with the zinc-lead-silver ores of Leadville, Colorado. Special reference may be made here to the Coeur d'Alene district of Idaho, which is the second largest producer of lead in the United States. The Coeur d'Alene deposits are almost unique in that they contain galena as vein-fillings and replacements in quartzite, with a gangue of siderite (iron carbonate). Quartzite (instead of limestone) is an unusual locus of replacement ores, and siderite is an unusual gangue. These ores are believed to owe their origin to acid igneous intrusives, because of the close association of the ores with some of these intrusives, and because of the content of high-temperature minerals. Some of the ore bodies are found far from intrusives, but it is supposed that in such cases further underground development may disclose the intrusives below the surface. Secondary concentration has been insignificant. In general, weathering of lead ores at the surface and secondary sulphide enrichment below are not so extensive as in the case of copper and zinc. Galena is fairly stable in the oxide zone, and even in moist climates it is found in the outcrop of many veins. Weathering removes the more soluble materials and concentrates the lead sulphide with the residual clay and other gangue. In some districts cerussite and a little anglesite are also found in the oxide zone. The carrying down of lead in solution and its deposition below the water table as a secondary sulphide is not proved on any extensive scale. In this respect it contrasts with zinc; and when the two minerals occur together, lead is likely to be more abundant in the oxide zone, and zinc in the sulphide zone below. Such a change in composition with depth is also found in some cases as the result of primary vertical variations in the mineralization. ZINC ORES ECONOMIC FEATURES Zinc metal has commonly gone under the name of "spelter." Brass and galvanized iron contain zinc as an essential ingredient. Of the total United States zinc consumption in normal times, about 60 per cent is used in galvanizing iron and steel objects to protect them from rust, 20 per cent is used in the manufacture of brass and other alloys, 11 per cent goes into the form of rolled sheets for roofing, plumbing, etc., 1 per cent is employed in desilverizing lead bullion, and the remaining 8 per cent is used for pigments, electrodes, and other miscellaneous purposes. During the war the use in brass-making was greatly increased. The zinc content of the ores mined today ranges from a little over 1-1/2 per cent in the Joplin district of Missouri, to 25 per cent and higher in some of the deposits of the Coeur d'Alene and other western camps, and over 40 per cent in certain bonanzas in British Columbia and Russia. The ores usually contain both zinc and lead in varying proportions, and sometimes gold, silver, and copper are present. Of the zinc produced in the United States, about 73 per cent is obtained from ores containing zinc as the principal element of value, about 25 per cent from zinc-lead ores, and 2 per cent from copper-zinc and other ores. The average grade of the straight zinc ores is about 2-1/2 per cent. Of the world's zinc ore, the United States produces in normal times about one-third, Germany about one-fifth, Australia about 15 per cent, Italy, North Africa and Spain each about 5 per cent. The remaining 15 to 20 per cent comes from a large number of scattered sources, including Japan, East Asia, Norway and Sweden, Canada, Mexico, Austria, France, Greece, Siberia, and Russia. In the near future the Bawdwin mines of Burma will probably be increasingly important producers. Large reserves of zinc also exist in the Altai Mountains of southwestern Siberia, and in the Cordilleran region of South America. In short, zinc is one of the most widely distributed of metallic resources; there is consequently less necessity for great international movements than in the case of many other commodities. The smelting of zinc concentrates is in general carried on close to the points of consumption and where skilled labor is available, rather than at the mines,--although smelters to handle part of the output have recently been built in Australia and in Burma. In Europe the great smelting countries have been Germany and Belgium, and to a lesser extent England and France. Before the war these four countries with the United States produced over nine-tenths of the world's spelter. Belgium did principally a custom business, and a large part of its exports went to England. Australian and Tasmanian zinc ores were the basis of the Belgian and English smelting industries, and also supplied about one-third of the German requirements. Since the war England has contracted to take practically the entire Australian output. This fact, in connection with war-time destruction of Belgian smelters, leaves the future of the Belgian zinc industry in some doubt. Germany may possibly lose to Poland its richest zinc mines, those of Silesia. German activity in the rich deposits of Mexico is to be expected. France controls the deposits of North Africa and satisfies a considerable part of its requirements from that source. Smaller movements of zinc include exports from Italy to England, and a complex interchange among the lesser producers of Europe. English and French zinc-smelting capacity was expanded during the war, and the industry in these countries is in a strong position. Japan also developed a considerable smelting industry during the war, importing ores from eastern Asia and Australia. The United States normally smelts and consumes all its large production of zinc ores and does not enter foreign markets to any extent. Small amounts of zinc concentrates are brought in from Mexico and Canada to be smelted in bond. During the war,--when the Allies were cut off by enemy operations from the customary Belgian and German supplies of spelter, and by shortage of ships from Australian zinc ores,--Australian, Spanish, Italian, and other ores were imported into the United States, and large quantities of spelter were exported from this country to Europe. Mine and smelter capacities were greatly increased, over-production ensued, and with the cessation of hostilities many plants were obliged to curtail or cease operations. The United States has now about 40 per cent of the zinc-smelting capacity of the world. For the present at least the capacity is far in excess of the domestic requirements. Before the war German control of the international zinc market was even stronger than in the case of lead. The German Zinc Syndicate, through its affiliations, joint share-holdings, ownership of mines and smelters, and especially through smelting and selling contracts, controlled directly one-half of the world's output of zinc and three-fourths of the European production. It regulated the Australian exports by means of long-term contracts, and had considerable influence in the United States. To some extent it was able to so manipulate the market that zinc outside the syndicate was also indirectly controlled. During the war political jurisdiction was used by the Allied countries to destroy this German influence. In the United States the principal zinc-producing regions are the Joplin and adjacent districts of Missouri, Oklahoma, Kansas, and Arkansas, furnishing about one-third of the country's output; the Franklin Furnace district of New Jersey, and the Butte district of Montana, each yielding about one-sixth of the total supply; and the Upper Mississippi Valley district of Wisconsin, Iowa, and Illinois, the Leadville district of Colorado, and the Coeur d'Alene district of Idaho, each producing between one-tenth and one-twentieth of the total. Smaller quantities are produced in Tennessee, New Mexico, Nevada, and several other states. Reserves of zinc are ample for the future. They are now developed considerably in advance of probable requirements, a fact which causes keen competition for markets and renders zinc-mining more or less sensitive to market changes. GEOLOGIC FEATURES The most important mineral of zinc is the sulphide, sphalerite or "zinc blende." The minerals of the oxide zone are smithsonite (zinc carbonate) and calamine (hydrous zinc silicate), which yield minor amounts of zinc and are especially productive at Leadville, Colorado. Zincite (zinc oxide) and willemite (zinc silicate) are the important minerals in the deposits of Franklin Furnace, New Jersey. The association of most deposits of zinc with more or less lead has been noted. The ores of zinc are of two general classes, corresponding to the two classes of lead ores (pp. 211-212). Zinc ores of the first type are in veins and replacements in sedimentary rocks at shallow depths, independent of igneous association, and are supposed to have been formed by cold solutions. They are found in the Mississippi Valley, in Silesia, and in many of the smaller European deposits. They were formerly the leading zinc-producers, and now produce about 45 per cent of the world's total. Zinc ores of the second type consist of veins and replacements related to intrusive rocks, sometimes extending to considerable depths, and of more complex composition. They include most of the deposits of the American Cordilleran region (Butte, Coeur d'Alene, Leadville, etc.), of Franklin Furnace, of Australia, of Burma, and of many other places. The zinc-lead ores of the type found in the Mississippi Valley are of special interest, in that they are sulphide ores of an origin apparently independent of igneous agencies. These ores occur as fissure-fillings and replacements, mainly in nearly flat-lying Paleozoic limestones and dolomites--the Bonne Terre dolomitic limestone of southeastern Missouri, the Boone formation of southwestern Missouri and Oklahoma, the Galena dolomite of Wisconsin and Illinois. They are variously associated with a gangue of dolomite, calcite, quartz, iron pyrite, barite, and chert. Not infrequently they are spread out both in sheets and in disseminated form along carbonaceous layers within or at the base of the limestone. The source of the primary sulphides has been a subject of much discussion. All are agreed that they were first deposited with the sediments in minutely dispersed form, through the agency of the organic contents of the sediments, and that such deposition was somewhat generally localized by estuarine conditions which favored the accumulation of organic remains. Many years ago, before the evidence of estuarine deposition was recognized, Chamberlin suggested an ingenious hypothesis for the northern Mississippi Valley,--that the organic material had been localized by ocean's currents forming something in the nature of a Sargasso sea. Differences of opinion become acute, however, when the attempt is made to name the precise sedimentary horizon, out of several available horizons, in which for the most part this primary concentration occurred. Judging from the organic contents of the several beds, the primary source may have been below, within, or above the present ore-bearing horizons. If the ore came from the lower horizons, it was introduced into its present situation by an artesian circulation, for which the structural conditions are favorable. If the ore was derived from overlying horizons, downward moving solutions accompanying erosion did the work. If the primary source was within the horizon of present occurrence of the ores, both upward and downward moving waters may have modified and transported them locally. For each of these hypotheses a plausible case can be made; but much of the evidence can be used interchangeably for any one of them. In spite of the wealth of data available, it is astonishingly difficult to arrive at a conclusion which is exclusive of other possibilities. Without attempting to argue the matter in detail the writer merely records his view, based on some familiarity with these districts, that, on the whole, the evidence favors the accumulation of these deposits by downward moving meteoric solutions during the weathering of overlying strata; but that it is by no means certain that a part of the ores has not been derived from lower horizons. The great area of the producing districts in comparison with their depth, the uniform association of the ore-bearing zone with the surface regardless of geologic horizon uncovered by erosion, the failure of the ores to extend in quantity under cappings of later formations, and the known efficacy of oxidizing waters in local downward transfers of zinc and lead, seem to suggest concentrating agencies which are clearly related