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    [Illustration: Cover Picture. View of the entrance to Calvin
    Coolidge State Forest Park. Photograph was taken looking north along
    State Highway 100A.]




                             THE GEOLOGY OF
                      CALVIN COOLIDGE STATE FOREST


                                  _By_
                          HARRY W. DODGE, JR.


                    DEPARTMENT OF FORESTS AND PARKS
                      Perry H. Merrill, _Director_

                     VERMONT DEVELOPMENT COMMISSION

                       VERMONT GEOLOGICAL SURVEY
                   Charles G. Doll, _State Geologist_


                                  1959




                             THE GEOLOGY OF
                   CALVIN COOLIDGE STATE FOREST PARK


                                  _By_
                          HARRY W. DODGE, JR.




                              INTRODUCTION


Each summer hundreds of eager campers, picnickers, hikers and sportsmen
visit the Pinney Hollow-Bradley Hill recreational area of the Calvin
Coolidge State Forest Park. This area is located a few miles north of
Plymouth, Vermont, and is easily reached via State Route 100A from
either Plymouth or Bridgewater Corners (See index map of Vermont,
geological map). The excellent camping and recreational facilities
coupled with the natural scenic beauty of this region provide many
visitors with an irresistible urge to return, summer after summer, to
this same spot. This pamphlet is designed for all those who visit
Coolidge Forest Park and especially for those who possess questioning
minds and a general desire to learn more about the world around them.




                                GEOLOGY


Have you ever wondered why the present mountains and valleys are where
they are and how and when they got there? Has the thought passed through
your mind that the very rocks on which you stand or see nearby have a
definite story to tell? The geologist not only wonders about such
things, but through his training attempts to answer questions of this
nature. His everyday job includes the reconstruction of ancient land and
sea areas through a careful study of the rock record. He looks at the
rock layers as you might the pages of a history book. The professional
geologist, however, commands many basic geological principles and “tools
of the trade” which permit him to read more accurately the records
preserved in stone.

First, the basic geological principles will be explained and you will be
given the proper “tools” for your adventure. Then, you are invited to
travel back through 550 million years of time to see just why Coolidge
State Forest Park is as it is today and what it was like in the distant
past.




                      BASIC PRINCIPLES AND “TOOLS”


To many of you rocks are just “rocks” and very little thought has been
given to any history which might be gained from their study. Probably
even fewer of you realize that literally billions of years of Earth
history can be derived directly from the rock record. Most geologists
consider the Earth to be nearly four billion years old with man entering
the picture a mere million or so years ago. No human was present to
record the events of billions of years of changing land and seas,
violent earth movements or the gradual evolution of life through the
last 500 million years. The study of rocks and their contained evidence
of past life offers the only clarification for the extremely long past
history of the Earth. In order to unravel this past history the
geologist accepts, with some reservations, three basic principles or
laws.

The Law of Uniformitarianism provides an extremely important link with
the past. This law states that the physical and chemical forces which
are attacking or building up the surface of the earth today have
operated in much the same way during past geological time. This means
that our observations of present day environments, such as streams,
deltas, lakes and oceans can be applied, after slight modification, to
the past as recorded in the rocks. For example, study of present day
deltas, such as the Nile or Mississippi River delta, has led to the
discovery of many past deltaic deposits now preserved as layers and
lenses of rock. This distribution pattern of sands and muds, the nature
of the life forms within each environment and many other such criteria
aid the geologist in his interpretations.

The Law of Superposition provides a physical order to the many layers of
rock which form the geological record. Compare, for the moment, the vast
numbers of rock layers (strata) with the layers of a layercake. As a
layercake is built up each individual layer is placed one over another,
with the bottom or base layer the first to be positioned and followed by
successively overlying layers. If you think of the development of this
cake in terms of time, the base layer is the oldest, the uppermost the
youngest. The Law of Superposition follows this example and states that
the lowermost stratum in a sequence of rock strata is the oldest or the
first to form, while the stratum above is younger and formed at a later
time. Some reservations do exist; however, in the Forest Park under
immediate consideration this general law does apply.

    [Illustration: Figure 1. Igneous dike cutting the Pinney Hollow
    formation. The dike is the darker rock which trends toward the upper
    left of the photograph. This dike is located approximately 100 yards
    south of the Pinney Hollow Historical Monument and on the east side
    of State Highway 100A.]

The third basic principle to command is known as the Law of Faunal
Succession. In generalized form this law states that each stratum of
rock contains its own distinct group of animal or plant remains, termed
fossils, and that these same remains can be recognized throughout the
world wherever they occur. Since plants and animals changed through time
and because their remains are found throughout the world, it is possible
to erect a worldwide time scale based upon animal and plant evolution.
In short, the fossils found in particular rock are characteristic
representatives of the life at the time that rock originated and the
fossils found could have been entombed only at that time. One stratum,
therefore, would have a total fossil assemblage quite different from the
stratum above or below. Without this time reference chart it would be
impossible to reconstruct what did happen during any one time interval
in the past.

In addition to these three basic laws it is necessary to mention the
rudiments of rock classification. The geologist divides _rocks_[1] into
three major groups which are termed Igneous, Sedimentary and Metamorphic
rocks. Igneous rocks are those formed by the solidification of molten
material. This molten material was thrust into the outer crust of the
earth from below and after cooling became a solid igneous rock such as
granite, or in other cases it flowed out over the surface of the earth
in the form of volcanic lava. Some small igneous bodies, termed dikes,
can readily be seen along State Highway 100A adjacent to Calvin Coolidge
State Forest Park (See photograph, Fig. 1).

Sedimentary rocks are formed in quite a different manner and differ in
general appearance. These are what might be considered second-hand
rocks. They are composed of particles derived from other rocks, igneous,
metamorphic or older sedimentary, which have been carried by streams,
wind or ice to a place of rest and there cemented into rock. Perhaps you
can visualize a river which, throughout its course, runs over rocks of
many types. This river would pick up particles of rock from its bed and
banks and transport these to a lake or perhaps the sea, where the
various transported materials would settle to the bottom in distinct
layers. The first layer deposited would become buried under thousands of
tons of overlying layers of sediment whose weight and resultant
pressure, together with the presence of adequate rock-cementing material
such as calcium carbonate or silica, would cause the bottom layer to
harden into rock. The layered appearance of sedimentary rocks is one of
their most characteristic features and these rocks are said to be bedded
or composed of many individual beds of sedimentary rock. Sandstone,
composed of sand size particles; shale, originally mud; and limestone,
once lime-rich mud, are examples of sedimentary rocks.

Metamorphic rocks result when igneous or sedimentary rocks are subjected
to abnormal heat and pressure. Folding or _faulting_[2] of rocks within
the earth’s crust or deep burial beneath overlying rocks or sediments
commonly produce metamorphism. The introduction of hot fluids during
folding and faulting greatly increase the speed and degree of
metamorphic conversion. When igneous or sedimentary rocks are subjected
to metamorphism they tend to lose their original appearance as some
minerals are completely changed or altered into new minerals and most of
the original minerals are oriented in one or more preferred directions.
The degree of heat and pressure, type and amount of hot fluids provided
and the type of rock undergoing metamorphism will determine the nature
of the metamorphic rock to develop. Perhaps the most unfortunate effect
caused by the metamorphism of sedimentary rocks, especially when
considering the history recorded in the rocks, is that in the majority
of cases all fossils originally present are either destroyed or
distorted beyond recognition. The absence of fossils makes age
determination quite difficult and hinders definition of previous
environments.

The rocks seen in and adjacent to Coolidge State Forest Park, with very
few exceptions, are metamorphic rocks which were originally sedimentary
rocks. Luckily the metamorphism is slight and several pages of geologic
history can still be read. Schists, phyllites and _quartzites_[3], all
metamorphic rocks, are well displayed in the Forest Park region.

The parallel arrangement of mica plates and segregation of the darker
minerals into distinct layers in the schists and phyllites together with
the inherited sedimentary layering in the quartzites, impart a
distinctly visible orientation to the rocks seen in the Forest Park. The
parallelism of the mica and segregation of the darker minerals in the
schists and phyllites is directly related to metamorphic processes and
the measurable orientation is called foliation. The original sedimentary
layering of the quartzites, little changed through the metamorphism, is
referred to as bedding. From all indications the foliation and bedding
are practically parallel in this region and since most of the rocks
represented are of the metamorphic type, all orientation features will
be referred to as foliation.

    [Illustration: Figure 2. Block diagram illustrating the dip and
    strike of foliation. The top of the block is considered an
    “imaginary horizontal plane.”]

The geologist uses the terms dip and strike to describe foliation and
uses conventional symbols for plotting purposes (See geologic map). The
dip of the foliation is the angle between an imaginary horizontal line
and the tilt or downward slope of the foliation. The strike is the
compass direction of a line formed by the dipping foliation plane and
its intersection with an imaginary horizontal plane (See block diagram,
Fig. 2). A glance at the geological map will show that the rocks of the
Park area consistently strike north to northwest and dip to the east. If
you look at the foliation symbols which are plotted on the geologic map,
the straight line of the symbol indicates the strike and the black
triangle points in the direction of the dip. The angles of dip have not
been included on the map; however, they average forty-five degrees down
from the horizontal and toward the east or right margin of the
geological map.




 THE ROCKS OF CALVIN COOLIDGE STATE FOREST PARK AND THE STORY THEY TELL


The rocks of Calvin Coolidge State Forest Park can be divided into four
distinct units or _formations_[4]. The geologic map shows these
formations as distinct bands, each band representing a different
formation and its intersection with the ground surface. Each formation
is, so to speak, stacked upon the next older with the oldest forming the
western band and the youngest on the east. Remember that these
metamorphic rock formations were originally sedimentary in nature and
were deposited in a nearly horizontal position on the floor on an
ancient sea. The fact that they are now tilted indicates that earth
movements have taken place sometime after the sedimentary rocks hardened
but before the present time.

The _Pinney Hollow formation_, the oldest formation of rocks seen in the
Forest Park, was named for the excellent exposures of this formation in
Pinney Hollow, a valley located along the western border of Coolidge
Forest. You can readily see this formation, in its most typical
development, along State Highway 100A about three-tenths of a mile south
of the Park entrance road. Here, a historical monument which
commemorates the naming of Pinney Hollow, is embedded in this formation
(See photograph, Fig. 3). The lean-to, tent platform and picnic areas
are situated on this formation and its rocks are easily observed. It is
interesting to note that some of the Pinney Hollow rocks seen in these
camping and picnicking spots differ from the typical rocks seen along
State Highway 100A. In fact, some of the rocks which are dark and are
classed as phyllites, can be confused with the overlying Ottauquechee
rocks. A transition zone, several hundred feet wide, exists between
these two formations and in which the two distinct rock types are
intermingled. This “mixture” zone indicates that there was no break in
the original deposition of these two formations and therefore the rock
record during this time is complete.

    [Illustration:   GEOLOGIC MAP OF CALVIN COOLIDGE STATE
                                  FOREST PARK
                              INDEX MAP OF VERMONT]

  EXPLANATION
    TRAIL
    SECONDARY ROAD (GOOD)
    SECONDARY ROAD (POOR)
    STATE ROUTE 100A
    STATE PROPERTY LINE
    CONTOUR LINE WITH ELEV.
    STREAM
    FOLIATION
  {FORMATIONS}
    MISSISQUOI FORMATION
    BETHEL FORMATION
    OTTAUQUECHEE FORMATION
    PINNEY HOLLOW FORMATION
  TOPOGRAPHY FROM U.S. GEOLOGICAL SURVEY MAP
  GEOLOGY BY H. W. DODGE, JR.

The Pinney Hollow formation, which is an estimated 2,300 or more feet
thick, is primarily composed of pale-green _schist_[5] with abundant
white quartz layers and lenses sandwiched between the dominantly
greenish colored rock. Originally these metamorphic rocks were sand, mud
and sandy mud deposited on a relatively shallow sea floor. After burial
these sediments hardened into thin layers and lenses of sandstone,
shale, and sandy shale. Metamorphism has since converted many of the
shale minerals into mica flakes and some of the sandy shales produced
crystals of the mineral garnet. The pale-green color of the Pinney
Hollow schist is caused by the presence of the green mica-like mineral,
chlorite, which developed during the process of metamorphism.

    [Illustration: Figure 3. The Pinney Hollow Historical Monument
    embedded in the Pinney Hollow formation. The darker rock consists of
    pale-green schist and the white areas are quartz layers and lenses.]




                         NORTH 20 WEST 25 RODS
                             FROM THIS SPOT
                         STOOD THE LOG CABIN OF
                            Jonathan Pinney
                                   &
                        Priscilla Grover Pinney
                            WHO SETTLED HERE A.D.
                         1800. HERE WERE REARED
                          THEIR TEN CHILDREN—
                          AND WHEN THE SECOND
                            GENERATION CAME
                             THEY CALLED IT
                             PINNEY HOLLOW

No fossils have been found in the Pinney Hollow formation of this
region: however, evidence from other parts of Vermont indicate that
these rocks are of Cambrian or perhaps _Ordovician_[6] age. This dates
these rocks as approximately 500 million years old.

The _Ottauquechee formation_, next above the Pinney Hollow formation and
therefore younger in age, was named for the Ottauquechee River along
which this formation is well exposed and was first studied in detail.
The Ottauquechee River is located just north of Calvin Coolidge State
Forest Park where it flows through the towns of Bridgewater and
Bridgewater Corners. You can see this formation along State Highway 100A
approximately nine-tenths of a mile north of the Park entrance road.
Here, the formation crops out on the west side of the road, or on your
left if traveling toward Bridgewater Corners (See photograph, Fig. 4). A
hike along the recently diamond-blazed Coolidge Forest trail, which
leads from behind the Ranger’s cabin to the Picnic area, affords an
almost continuous view of this formation. The highly resistant quartzite
layers within the Ottauquechee formation are very conspicuous features
along this trail as they form several ridges trending across the
footpath.

The Ottauquechee formation, estimated to be 3,000 feet thick, contains
rocks which contrast sharply with the underlying pale-green schists. The
formation consists of alternating black phyllite and layers, up to
several feet thick, of dark- to light-colored quartzite. The black
carbonaceous color of the phyllites and dark quartzites indicate the
presence of life, either animal or plant, during the original deposition
of the sediments forming these rocks. The phyllite was, before
metamorphism, a black shale which formed from black muds rich in organic
matter. From studies done in areas where black muds are accumulating
today it seems quite probable that the ocean waters of Ottauquechee time
were of a restricted nature, that is to say, the waters were stagnant
due to the lack of oxygen. Pinney Hollow shallow marine water conditions
remained, but for some reason, such as a barrier of islands or shallow
water sand bars, the circulation of ocean waters was hindered, leading
to the accumulation of stagnant organic matter on the ocean bottom. This
organic matter gives the Ottauquechee phyllite and some quartzites a
dark to black color.

No fossils have been found in the Ottauquechee formation of the Coolidge
Forest Park, but, as with the Pinney Hollow formation, evidence from
other areas indicates a Cambrian or perhaps Ordovician age for this
formation.

    [Illustration: Figure 4. The Ottauquechee formation as exposed
    approximately nine-tenths of a mile north of the Park entrance road
    and on the west side of State Highway 100A. Note the massive
    quartzite layers above, the black phyllite below.]

Above the Ottauquechee rocks lie those of the _Bethel formation_.
Approximately 2,000 feet thick, this formation is composed mainly of
pale-green schist with interlaminated thin layers of white to greenish
quartzite. The rock presents a distinct banded appearance and contains
conspicuous crystals of garnet and the black mica, biotite. The best
view of this schist is found along State Highway 100A and slightly over
two miles north of the Park entrance road. The relatively steep rock
cliffs which line the west side of this road are composed of the
resistant Bethel formation. In the northeast portion of Coolidge Forest
Park several excellent exposures of this formation in contact with the
underlying Ottauquechee formation can be seen, but, to reach these areas
you would have to leave the beaten trail. If you are interested in
visiting these contact locations it would be best to consult the
geologic map for approximate directions.

    [Illustration: Figure 5. Diagrammatic cross sections illustrating
    the geologic history of the Calvin Coolidge State Forest Park.
    Vertical scale greatly exaggerated. Patterns to designate formations
    in cross section A are repeated in cross sections B and C.

    A. Horizontal layers of early paleozoic sedimentary rocks were
    formed. This cross section extends beyond the limits of the Park
    area. Thicknesses of rock formations are approximately proportional.

    B. During late Ordovician or Devonian time these sedimentary layers
    were folded and the rocks metamorphosed.

    C. Calvin Coolidge State Forest Park as it appears today.]

  A
    MISSISQUOI FORMATION
    BETHEL FORMATION
    OTTAUQUECHEE FORMATION
    PINNEY HOLLOW FORMATION
  B
  C
    SHELTER VILLAGE
    SLACK HILL

The conditions under which the Bethel formation was originally deposited
are similar to those which existed during Pinney Hollow time. Shale,
sandy shale and thin-bedded sandstones formed under shallow water marine
conditions; however, restriction with accompanied organic accumulation
did not occur. This formation is thought to be of either Cambrian or
Ordovician age.

The easternmost, and therefore youngest formation to be found in the
Park area is known as the _Missisquoi formation_. This formation, which
is only partially represented here, is made up of dark phyllite, light
blue-gray schist with conspicuous crystals of garnet and biotite, and
gray quartzite. The rocks of this formation can be seen along Broad
Brook which flows north along the eastern border of Coolidge State
Forest Park.

This formation is considered Ordovician in age with the conditions of
deposition being quite similar to those which existed during
Ottauquechee time. Once again the ocean waters were restricted and
organic-rich black muds accumulated in quantity on the ocean floor.




                          THE GEOLOGIC HISTORY


The early geologic history of Calvin Coolidge State Forest Park is known
from the rock formations discussed in the previous section of this
pamphlet. The Pinney Hollow, Ottauquechee, Bethel and Missisquoi
formations of Cambrian and Ordovician age were deposited, essentially
horizontally, from the shallow seas which covered this area 550 to 450
million years ago (See cross section A, Fig. 5). During some of this
time these seas were at least partially restricted as evidenced by the
Ottauquechee and Missisquoi black phyllites. Other Ordovician formations
were deposited on top of these four formations, but have been completely
stripped away by erosion during the millions of years since their
deposition. Some of these later Ordovician sediments contain great
thicknesses of volcanic rocks which probably covered the Coolidge Park
area.

Sometime after these later Ordovician rocks formed, great stresses
within the earth’s crust folded and in some places faulted these older
rocks. The pressure and resultant heat created during these earth
movements converted the rocks into metamorphic schists, phyllites and
quartzites which are seen today (See cross section B, Fig. 5). Exactly
when these diastropic events took place is still open to question, but
they probably occurred near the end of Ordovician time or perhaps
millions of years later during the last phases of the Devonian Period.

Probably since late Paleozoic time the Coolidge Park area has been
subjected to breaking-down rather than building-up processes. Eventually
this whole region was reduced to a flat plane with only a few hills,
called monadnocks, rising above the general flatness of the landscape.
Still later in the history of Coolidge State Forest Park this flat plane
was lifted high above its former position and streams began to shape the
land into its present form. The more resistant rocks were lowered by the
forces of erosion at a much slower rate than the less resistant types.
The hills and valleys which you see today are primarily the result of
this general uplift followed by the wearing away of the softer rocks at
a more rapid rate than the harder rock types. Since the formations trend
in a north-north-west direction, the hills and valleys are also oriented
in this general direction, as the different resistive qualities of the
rocks are presented in this pattern.

The story is not complete without mention of the last modifying
influence to affect this region. Approximately one million years ago the
first of perhaps three glacial advances began. These large glacial
masses, termed continental glaciers, slowly advanced southward over the
northern sections of the United States and covered a vast region with a
thick sheet of ice. As these ice masses rode over the Coolidge Park
region they tended to round-off irregular features of the landscape and
to fill in certain low regions with their transported rock debris. The
rather smooth outlines of the hills in Coolidge Forest Park and the sand
and gravel deposits so apparent adjacent to nearby streams tell the
story of these glacial advances and subsequent withdrawals. Other
evidence of the overriding ice is seen in the presence of scratch marks,
termed glacial striae, which were produced by fragments of rock as they
were dragged by the moving ice across the underlying rocks. Along the
east bank of Broad Brook and approximately two hundred yards south of
the bridge at Five Corners, abundant glacial striae are clearly
displayed. The orientation of these striae indicates that the glacier
producing them moved from north north-west to the south south-east.

Since the recession of the last ice sheet, which took place several
thousand years ago, the general appearance of the Coolidge Park region
has changed very little. The streams have cut their valleys somewhat
deeper and some of the glacial sands and gravels have been
redistributed. A few thousand years ago Coolidge State Forest Park
looked quite the same as it does today (See cross section C, Fig. 5).




                   SIDE TRIPS OF GEOLOGICAL INTEREST


_Gold Panning_

There are several locations immediately adjacent to the Calvin Coolidge
State Forest Park where some gold can still be obtained from stream
sands and gravels. The abandoned town of Five Corners offers the park
visitor ample chance to “try his luck.” In fact, several local people
still obtain small gold nuggets and a good deal of gold dust from the
streams in the vicinity of Five Corners (See geologic map for geographic
location). Other possible “panning” locations would include Reading Pond
Brook, Buffalo Brook and Kingdom Brook. The gold which is found is
probably derived from nearby gold-bearing quartz veins.

The equipment needed for an afternoon of panning is relatively simple to
obtain. A wide, rather shallow pan with gently sloping sides is really
all that is needed. Gravel and sand, which you find concentrated on the
upstream side of natural rock riffles in the stream bed should be placed
together with water in your pan. Then the pan should be rotated and the
lighter rock materials decanted. The heavy gold will be concentrated in
the bottom of the pan.


_Asbestos and Talc_

For you who might be interested in mineral collecting it is suggested
that a trip to an old pit near Five Corners might prove rewarding. Walk
down Broad Brook from the bridge at Five Corners until you locate the
first small tributary entering on your left. Turn left up this tributary
and continue to walk for approximately one hundred yards. On your left
you will see the remains of a pit dug into the side of the stream bank
which, at this location, is lined with rocks of the Ottauquechee
formation. You will quickly note the fibrous asbestos in the walls of
this pit; however, if you wish to collect specimens of this mineral be
extremely careful and avoid standing beneath any overhanging rock
material. Talc, which feels “soapy” to the touch, is also abundant in
this immediate area. This asbestos and talc occur in a lenticular-shaped
body which is perhaps eight hundred feet wide and of an undetermined
length. It is not certain when these minerals formed; however, it was
probably in the early history of the region.


_Garnet_

Good specimens of garnet can be found in the rocks of the Bethel
formation. These garnets are not of gem quality but still offer good
collecting for the amateur mineralogist. Good collecting localities
exist along State Highway 100A and at several different spots within the
Park itself. A hike along the northern boundary of the Park would
intersect several excellent collecting spots.


_Vermont Marble Company Exhibit, Proctor, Vermont_

If time permits, or a rainy day arrives, a visit to the Vermont Marble
Company Exhibit in Proctor, Vermont, is highly recommended. Marble is a
metamorphic rock which was originally sedimentary in nature. Lime muds
hardened into limestones and then, converted by metamorphism, became
marble. The marble exhibit at Proctor, termed “The largest Marble
Exhibit in the World,” displays over 100 varieties of marble and igneous
rock, granite. Fossils can be seen in some of the varieties of marble,
which demonstrates that under certain conditions recognizable fossils do
occur in metamorphic rocks. Most of the marble seen in this exhibit is
quarried nearby; however, for safety reasons visitors are not allowed to
enter the quarries. It is interesting to note that these marbles are
about the same age as the rocks seen in Coolidge State Forest Park.




                               Footnotes


[1]A rock, for those desiring a more technical definition, is any
    relatively hard, naturally occurring, combination of minerals which
    form an essential part of the earth. Minerals are composed of one or
    more chemical elements, such as oxygen or silicon, that form in
    nature.

[2]A fault is a break or fracture in the rocks of the earth’s crust
    where one side of the fracture moves in some direction relative to
    the other side. Some recent earthquakes in California and Nevada
    have produced visible faults where, for instance, roads were cut and
    one side displaced several yards from the other.

[3]Most of the mineral grains which make up a schist, a metamorphosed
    shale or sandy shale, can be seen with the naked eye, and the
    parallel to subparallel arrangement of platy or flaky minerals, such
    as the abundant mica, is the most obvious feature. In bright
    sunlight a schist presents a very shiny, almost silvery appearance
    which is caused by the reflection of sunlight from the many parallel
    mica plates. A phyllite has much the same mineral composition as
    that of a schist; however, the individual mineral grains are too
    small to be seen without the aid of a magnifying glass. As with the
    schist, a phyllite when viewed in direct sunlight, appears shiny.
    Quartzites are metamorphosed sandstones which have been fused into
    great hardness through the application of heat and pressure.

[4]A geologic formation consists of a group of similar rocks, or
    sequence of rocks, which can be easily distinguished from adjacent
    rocks and whose distribution can be plotted on a map.

[5]More technically this schist is termed a chlorite-sericite-quartz
    schist.

[6]The geologist divides geologic time into four major sections,
    designated Eras. These Eras are in turn subdivided into Geologic
    Periods which are of shorter time duration. The Cambrian Period is
    the oldest of the Paleozoic Era and began some 550 million years
    ago. The Ordovician Period is the next oldest of the Paleozoic Era
    and started about 440 million years ago and ended close to 360
    million years ago.




                          Transcriber’s Notes


—Silently corrected a few typos.

—Retained publication information from the printed edition: this eBook
  is public-domain in the country of publication.

—In the text versions only, text in italics is delimited by
  _underscores_.