ANCIENT EARTH HISTORY
(_Paleozoic Era_)
Beginning with the earliest Paleozoic, the legible records of events
of earth history are far more abundant and less defaced than those
of earlier times. Stratified rocks of the ordinary kinds greatly
predominate over the igneous and metamorphic rocks, and the strata
are in general far less disturbed than those of the Archeozoic and
Proterozoic groups. From the earliest Paleozoic we have also the first
abundant records (fossils) of the life of the earth, so that the
ordinary methods of subdividing and determining the relative ages of
the Paleozoic and later strata, as well as correlating the subdivisions
(formations) in widely separated regions, can be used. From here on in
our discussion of earth history we shall be able to trace the salient
features of the changing outlines of the face of the earth, the coming
and going of the seas over the lands, and the evolution of animals and
plants with a considerable degree of definiteness and satisfaction.
First, we shall trace out, in the regular order of their occurrence,
the main physical history events of Paleozoic time, leaving a
consideration of the evolution of life for other chapters. Because
of limitation of space, our attention will be almost wholly centered
upon the continent of North America, but the reader should bear in
mind that the general principles and facts set forth apply with about
equal force to most other continents. In Europe the wonderful records
of Paleozoic history are found in strata, whose estimated maximum
thickness is about 100,000 feet! It must not be thought, however, that
all these strata are piled up in a single locality, but the figure does
actually represent the sum total of the greatest thickness of the many
subdivisions (formations) of the Paleozoic rocks in different portions
of the continent. In North America the maximum thickness of all
Paleozoic rocks seems to be no less than 50,000 feet. More than 25,000
feet of strata may actually be observed piled layer upon layer in the
highly folded and deeply eroded central Appalachian Mountains. The
great thickness of the strata, combined with the facts that the fossils
show that many marvelous, mostly progressive, changes took place among
living things, that seas came and went repeatedly over many parts of
the continent, and that great changes took place in the configuration
of the land, force us to conclude that Paleozoic time must have lasted
for many millions of years.
Just before the opening of the Paleozoic era practically all of North
America appears to have been dry land, which had undergone so much
erosion that it was low and far less rugged in relief than at present.
This we know, because the rather widespread early Paleozoic (Cambrian)
strata almost everywhere rest upon deeply eroded rocks of either
Archeozoic or Proterozoic age. Considering both the time involved and
the wide area affected, we have no record of anything like such a
profound erosion interval since the beginning of the Paleozoic era. It
seems that the constructive or upbuilding forces within the earth were
then remarkably quiescent, while the destructive forces (erosion) were
almost unhampered in their work of cutting down the land.
Have we any definite idea of the relations of land and water in North
America during the first or Cambrian period of the Paleozoic era? In
the affirmative answer to this question, certain principles will be
brought out which the reader should keep in mind as we trace out the
succeeding great physical changes in the history of North America.
It should, however, be remembered that, in the brief space at our
disposal, only the most general, or the most significant localized,
physical changes in the long and intricate known history of the
continent since the opening of the Paleozoic era can be brought out.
In early Cambrian time a narrow arm of the sea (like a strait) extended
from the Gulf of St. Lawrence southward across eastern New York and
over the site of the present Appalachian Mountains connecting with the
Gulf of Mexico on the south. On the west, a much larger and broader arm
of the sea (like a mediterranean) extended from Alaska southward over
the site of the Rocky Mountains of Canada and across the sites of the
Columbia Plateau to Great Basin of the western United States. All the
rest of the continent was land, apparently almost or wholly devoid of
high mountains.
By what process of reasoning do we conclude that arms of the early
Cambrian sea reached across eastern and western North America? First,
wherever marine strata of definitely determined early Cambrian age now
occur, the early Cambrian sea must have existed because those strata
were obviously deposited in that sea. Second, to those areas we must
add others from which it can be demonstrated that early Cambrian marine
strata have been removed by erosion. Enough field work along these
lines has been done in North America to render it practically certain
that the relations of land and water during early Cambrian time were
essentially as above outlined.
[Illustration: Fig. 33.--Map showing the relations of land and water
in North America during early Cambrian time, at least 25,000,000 years
ago. Lined areas represent land. (Principal data from a map by Willis
published in the Journal of Geology.)]
[Illustration: Fig. 34.--Map showing the relations of land and water
in North America during Middle Ordovician time. Lined areas represent
land. (Principal data from a map published by Willis in the Journal of
Geology.)]
As Cambrian time went on, the marine waters gradually spread from
south to north across most of the Mississippi Valley area, causing the
eastern and western arms of the sea to be connected, thus forming an
interior continental sea. Otherwise the relations of land and water
were much as in early Cambrian time. We know that the sea transgressed
northward across the Mississippi Valley district because, on the
south, the whole Cambrian system of strata (lower, middle, and upper)
is present, while, farther north, only middle and upper Cambrian are
present, and, farthest north, only upper Cambrian strata occur. This
progressive northward overlap of younger and younger (later) Cambrian
strata upon the old rock floor proves, that the Cambrian sea steadily
spread farther and farther northward over the Mississippi Valley area.
That this spreading sea was shallow is amply demonstrated by the
deposits it left, such as shales, conglomerates (i.e., consolidated
gravels) and sandstones, often ripple-marked. The Cambrian strata of
North America vary in thickness from less than 1,000 feet to about
12,000 feet.
In the Mississippi Valley the Cambrian strata are unaltered and almost
undisturbed from their original horizontal position. In the Appalachian
Mountains of the east, and the Rocky Mountains of the west, the strata
are commonly notably folded and faulted. In some places, as in western
New England, the strata have been notably altered (metamorphosed).
The best estimates for the duration of the Cambrian period range from
2,000,000 to 3,000,000 years. It is a remarkable fact that, during this
great lapse of time, North America was unaffected by any great physical
disturbances such as mountain making, emergence of large tracts of
land, or igneous activity. The one great physical event of the Cambrian
was the gradual submergence of a considerable portion of the continent.
That the climate of the earliest Cambrian was at least locally
favorable for the existence of glaciers, is proved by the occurrences
of true glacial deposits in rocks of that age in China, Norway, and
Australia. It is a remarkable fact that the glacial materials of China
occur along the Yangtse River, thus demonstrating that conditions for
glaciers then existed at a latitude as far south as New Orleans. These
evidences of glaciation directly refute the old idea, based upon the
nebular hypothesis, that the climate of the Paleozoic was distinctly
warmer than now. The glacial evidence, added to our knowledge of the
character and world-wide distribution of many identical species of
animals, leads us to conclude that early Paleozoic climate was not
essentially different from that of very recent geologic time, but that
the climate was then much more uniform than at present.
During the second or Ordovician period of the Paleozoic era, the
progressive submergence of Cambrian time continued until a climax
was reached toward the middle of the period when fully four-fifths
of the continent was submerged under shallow sea water. Since middle
Ordovician marine strata are more widespread than the rocks of any
succeeding age, we can be reasonably sure that so much of the continent
was never again covered by the sea. In fact, so far as the records
have been interpreted, this came nearest to being a universal flood in
the whole known history of the continent. By the very character of the
rocks deposited (seldom over a few thousand feet thick), we can be sure
that the middle Ordovician continental sea was everywhere far shallower
than the great ocean abysses of to-day. Because the lands were so low
and restricted, relatively little land-derived sediment washed into the
sea. But the shallow sea water was inhabited by millions of animals,
the shells of many of which slowly accumulated to build up the thick
bodies of limestone strata (Plate 14) which constitute the main bulk of
rock of early and middle Ordovician age. The famous Trenton limestone,
named from a locality in central New York, with its great abundance of
fossils, was formed mostly by the accumulation of shells of animals
during middle Ordovician time.
Later in the Ordovician there was a considerable shift in level between
land and water causing a withdrawal of much of the widespread sea. As
a result of the generally more elevated lands, erosion proceeded more
vigorously, and sands and muds were more abundantly deposited in the
restricted sea, these sediments having consolidated to form the shales
and sandstones which predominate among the upper Ordovician rocks.
A principle above briefly explained in the discussion of the Cambrian
may be reemphasized here. It is as follows: In making a map to show the
relations of land and water, say during middle Ordovician time, the
geologist is by no means dependent only upon actual surface exposures
of middle Ordovician strata. Such exposures fall far short of giving an
adequate conception of the former or even present real extent of such
strata. In many places originally present Ordovician strata have been
removed by erosion. An excellent case in point is the Adirondack region
of northern New York. On the west side of the Adirondacks a great pile
of marine Ordovician strata 1,500 feet thick end abruptly on the gently
sloping flank of the mountains, thus clearly proving that the strata
formerly extended at least twenty to thirty miles eastward. Again, in
the southern Adirondacks a small area of very typical marine middle
Ordovician strata lies fully fifteen miles from the general area of
such rocks to the south. This small body of rock is very clearly only
an erosion remnant of a general sheet of middle Ordovician rock which
once covered the whole intervening district. In many other regions
the middle Ordovician strata are definitely known to be concealed
under cover of later rocks, as in the Mississippi Valley, where the
actual surface exposures constitute only a fraction of the middle
Ordovician strata which underlie nearly all the valley, as proved by
deep well drillings, study of the scattering outcrops, etc. In still
other places, middle Ordovician strata, associated with other rocks,
are highly folded, as in the Appalachians, where such strata outcrop
in only narrow belts following the trend of the folds. In short, then,
wherever it can be proved that middle Ordovician marine strata are
visible at the surface, or are concealed under other rocks, or were
once present, we can be sure that the middle Ordovician sea existed.
Exactly this principle applies to any subdivision of geologic time.
[Illustration: Fig. 35.--Structure section showing rocks representing
three geologic eras separated by millions of years of time. Length
of section 12 miles, vertical scale much exaggerated. At the bottom
are Archeozoic (Precambric) rocks and resting upon them on the left
are early Paleozoic strata 1,500 feet thick. A glacial lake deposit
of late Cenozoic age lies on the Archeozoic rock toward the right. It
is evident that the Paleozoic strata formerly extended much farther
eastward. (By the author as published in a New York State Museum
Bulletin.)]
The Ordovician period closed with a great mountain-making disturbance
in eastern North America, and at the same time all, or nearly all, of
the continent was land. Throughout most of the Cambrian and Ordovician
periods, the strata accumulated to a thickness of thousands of feet in
the marine waters which spread over the eastern border of New York,
the sites of the Green Mountains of Vermont, the Berkshire Hills of
Massachusetts, and southward at least as far as Virginia, over the area
of the Piedmont Plateau. At, or toward the close of the Ordovician
period, a great compressive force in the earth's crust was brought to
bear upon the mass of strata and they were tilted, highly folded, and
raised above sea level into a great mountain range known to geologists
as the Taconic Range. It is quite the rule throughout this region of
Taconic disturbance to find the strata either on edge or making high
angles with the plane of the horizon. Many of the folds were actually
overturned, and in some cases notable thrust faults developed, that is,
the upper strata broke across and great masses were shoved over each
other. These facts all go to show that the mountain-making compressive
force applied to the region was of rather an extreme type. Since the
origin of the Taconic Range a tremendous amount of erosion has taken
place, so that literally only the roots of the range are now exposed in
the Green Mountains, Berkshire Hills, Highlands-of-the-Hudson, and the
northern Piedmont Plateau.
How do we know that the Taconic disturbance took place toward the close
of the Ordovician period? By way of answer to this question two facts
need to be considered. First, relatively late (or young) Ordovician
strata are involved with the folds, thus proving that the folds
formed after those late Ordovician sediments were deposited. Second,
undisturbed strata formed during the middle of the next (Silurian)
period, rest upon the eroded edges of the folds, which proves that the
folds must have developed well before middle Silurian time because the
only time they were subjected to erosion must have been during early
Silurian time.
[Illustration: Fig. 36.--Structure section showing profile
and underground relations of the rocks across part of the
Highlands-of-the-Hudson region in southeastern New York. Length of
section, sixteen miles. The rocks are mostly of Prepaleozoic Age, but
with belts of highly infolded early Paleozoic strata toward the middle
right. (After Berkey, New York Museum Report.)]
Mention should also be made of the profound metamorphism (alteration)
of the Cambrian and Ordovician strata along the main axis of the range,
where the intense compression, aided by heat and moisture, caused the
deeply buried portions of the strata to become plastic, and hence
they became more or less foliated (cleavable) and crystallized into
various metamorphic rock types, the limestone having changed to marble,
the shale to slate or schist, and the sandstone to quartzite. Thus
we explain the rocks of the extensive marble quarries of Vermont and
western Massachusetts, the slate quarries of central eastern New York,
and the Berkshire schist of the Berkshire Hills of Massachusetts.
One of the grandest and most significant of all the profound geological
processes is the birth and history of a great folded mountain range.
Since the Taconic Range affords us such an excellent example of a
large-scale, well-understood folded range of great antiquity we may do
well to consider it in the light of certain other broad relationships.
The great compressive force which folded and upraised the Taconic
Mountains did not accomplish its work rapidly in the ordinary human
history sense of the word. The force was slowly and irresistibly
applied, and the strata well below the surface were gradually bulged
or folded, or fractured where near the surface, the length of time
required for the operation having been, at the very least calculation,
some hundreds of thousands of years, and more than likely a million
years or more. Such a length of time is, however, so short compared
with all known earth history, that we are accustomed to refer to the
formation of such a mountain range as simply an event of geological
history.
Even before such a range attains its maximum height a very considerable
amount of erosion has already taken place. When the first fold appears
above sea level, erosion begins its work and continues with increasing
vigor as the mountain masses get higher and higher. Thus we have
warfare between two great natural processes--the building up and the
tearing down. After a time the building-up process wanes and then
ceases, while the tearing-down process (erosion) continues either
until the whole range has been completely worn down or until some
rejuvenating force causes a renewed uplift. Here is an example of one
of the remarkable procedures of nature. After millions of years of work
causing the deposition of thousands of feet of strata, piled layer upon
layer on the sea floor, a force of lateral pressure is brought to bear
and a mountain range is literally born out of the sea. No sooner is the
range well formed than the destructive processes (erosion) unceasingly
set to work to destroy this marvelous work. But the sediments derived
from the wear of the range are carried into the nearest ocean again to
accumulate and, perchance, after long ages, to be raised into another
range; and so the process may be often repeated. From this we learn
that the mountain ranges of the earth are by no means all of the same
age. The original Adirondacks were formed long before the Taconics,
which originated millions of years before the Appalachians, these
latter having been folded up long before the Sierras. The Rockies,
followed by the Coast Ranges, are each younger than the Sierras as
regards their original folding and uplift. Among foreign countries
special mention should be made of the British Isles, where Ordovician
strata thousands of feet thick were, late in the period, notably folded
and upraised, the crustal disturbance having been accompanied by great
intrusions of molten rocks and vast outpourings of lavas, so that
this region ranks among the greatest of the ancient volcanic areas of
Europe.
[Illustration: Fig. 37.--Map showing the general relations of land and
water in North America during middle and late middle Devonian time
fully 15,000,000 years ago. (After Willis, courtesy of the Journal of
Geology.)]
We shall now turn our attention to a very brief consideration of the
salient points in the physical history of North America during the
next great period (Silurian) of the Paleozoic era. As a result of
the physical disturbance late in the Ordovician the great interior
sea was largely or wholly expelled from the continent, and this was
essentially the condition of the continent at the beginning of the
Silurian. But this condition was of short duration, for early in the
Silurian the sea again began to spread, gradually increasing in extent
to a climax in about the middle of the period. At this time the famous
and extensive Niagara limestone, so named from the rock at the crest
of Niagara Falls, was deposited. Except for the newly formed Taconic
Range, standing out as a bold topographic feature along the middle
Atlantic Coast, and a somewhat wider extent of land, the condition of
the continent during middle Silurian time was very similar to that of
middle Ordovician time.
[Illustration: Fig. 38.--Map showing the general relations of land and
water in North America during middle Mississippian time. (After Willis,
courtesy of the Journal of Geology.)]
Soon after mid-Silurian time the seas became greatly restricted almost
to disappearance as such. In the eastern United States and southeastern
Canada strata of that particular age are found only in parts of
Ontario, New York, Ohio, Michigan, and from Pennsylvania southward
to West Virginia, where they are characterized by red shales and
sandstones, and salt and gypsum deposits. Such materials containing
few fossils very clearly indicate deposition in either extensive
lagoons or more or less cut-off arms of the sea under arid climate
conditions rather than in ordinary marine water.
Still later in the Silurian the interior seas were partially restored,
as shown by the fact that true marine strata corresponding to that
age not only cover the salt and gypsum deposits, but are notably more
extensive than they. About the close of the Silurian period almost all
of the continent was dry land.
Unlike the Ordovician period, the Silurian closed without any
mountain-making disturbance or great uplift of land. The Silurian
period, like the preceding Ordovician and Cambrian, seems to have been
free from any more than slight igneous activity as, for example, in
Maine and New Brunswick. The total thickness of Silurian strata in
North America is seldom more than a few thousand feet.
The salient features of the physical history of the next, or Devonian
period, are much like those of the preceding Silurian. At the beginning
of the Devonian almost all of the continent was dry land, but soon a
long, narrow arm of the sea extended across the eastern side of the
continent from the Gulf of St. Lawrence southward through western
New England, southeastern New York and throughout the Appalachian
district, thus reminding us of the long, narrow sound which occupied
almost exactly the same territory during the early part of the Cambrian
period. In the west the only water was a small embayment reaching
across southern California into Nevada. By middle Devonian time these
water areas had considerably expanded. During relatively late Devonian
time the sea was so expanded as to cover much of the Mississippi
Valley area, the Appalachian Mountains and St. Lawrence Valley areas,
and most of the site of the Rocky Mountains, except for an island
of considerable size reaching from New Mexico through Wyoming. The
main lands were most of northeastern North America, a large land area
extending from Florida to Nova Scotia, and a large area on the western
side of the continent from California to Alaska.
A remarkable formation of late Devonian Age should be briefly
described. In southeastern New York and the northern Appalachian
region there was a tremendous accumulation of sediments which have
consolidated into sandstone, together with some shale and conglomerate.
This so-called "Catskill" formation is from 1,500 to 8,000 feet thick
and is well shown as the main body of rock in the Catskill Mountains.
It is largely a shallow-water deposit of essentially nonmarine origin,
as proved by coarseness of material, ripple marks, and nonmarine
fossils. All evidence points to the origin of this remarkable formation
as a great delta deposit built out into the shallow interior sea.
Notable thinning toward the west, with increasing fineness of grain
of material, shows that the sediment came from the east, no doubt
carried by a large river from the small continental land mass (called
"Appalachia") on the eastern side of North America.
The maximum thickness of the North American Devonian seems to be about
15,000 feet in the northern Appalachian region, but elsewhere it
generally ranges from 1,000 to 4,000 feet thick. In North America the
subdivisions of the Devonian strata of New York are taken as a standard
for comparison, both because of the wonderful completeness and almost
undisturbed character of the rocks there, and because they have been
so carefully studied. The Devonian system is there fully 4,000 feet
thick, with scarcely a minor subdivision missing, and it covers a wide
area (one-third of the State) with many excellent outcrops. There was
practically uninterrupted deposition of Devonian strata in southern
New York. It is doubtful if there is greater refinement of knowledge
regarding the Devonian or any other Paleozoic system of strata anywhere
else in North America.
During middle to late Devonian time the region from southern New
England to Nova Scotia and the St. Lawrence Valley was notably
disturbed by earth movements, the lands having been considerably
elevated and the rocks more or less folded. The great delta deposit of
late Devonian time, already described as being thousands of feet thick
in New York and Pennsylvania, was formed by one or more streams which
carved much sediment from the newly upraised lands. Accompanying the
uplift and folding of the rocks considerable masses of molten granite
were forced into the earth's crust and some molten rock was forced to
the surface, producing volcanoes. Much of the granite may now be seen
at the surface in various portions of the region, while deeply eroded
volcanoes occur near the city of Montreal.
Except for the disturbance of the region from New England to the St.
Lawrence, the Devonian period seems to have closed rather quietly, with
fairly widespread sea water over the land as already outlined. This is
proved by the fact that the early strata of the next period mostly rest
in regular order upon the undisturbed late Devonian strata.
For many years the term "Carboniferous" period was used to
designate a single period of geologic time which, in America at
least, is now divided into two periods--the Mississippian and
Pennsylvanian--corresponding, respectively, to the earliest and latest
Carboniferous. In regard to the relations of land and water during the
Mississippian period, the general statement may be made that the sea,
already fairly extensive in the late Devonian, continued to spread
until during the second half of the Mississippian, when most of the
United States west of the eastern border of the Appalachians (except
the Pacific Northwest), and also the Rocky Mountain region through
Canada, were submerged.
A significant physical change marked the close of the Mississippian.
This was the withdrawal of sea water from nearly all of the continent,
the emergence of the land having been generally sufficient to allow
considerable erosion. The fact that the Mississippian and the next,
or Pennsylvanian, strata are separated by the most extensive distinct
erosion surface in the whole Paleozoic group of rocks is the chief
reason for considering those two sets of strata to have formed during
separate periods of geologic time.
In eastern North America the Mississippian strata vary in thickness
from a few hundred feet to a maximum of about 5,000 feet in eastern
Pennsylvania. In the West, where the thickness is commonly several
thousand feet, limestone greatly predominates. There appears to have
been vigorous volcanic activity during the period from northern
California to Alaska.
Certain profound crustal disturbances marked the close of the period in
western Europe, resulting in upturning and folding of rocks during the
process of mountain forming from Ireland to Germany, and from Bohemia
to southern France. Abundant intrusions and extrusions of molten rocks
accompanied the disturbances.
We turn next to a consideration of the Pennsylvanian period, which is
of very special interest, because within the rocks of that age in North
America, Europe, and China occur the greatest known coal deposits. The
period opened with almost all of North America dry land undergoing
more or less erosion. Early in Pennsylvanian time marine water began
to overspread the western side of the continent, especially most of
the western two-thirds of the area of the United States, where strata
thousands of feet thick piled up. The sea was most widespread before
the middle of the period, when the relations of land and water were
about as shown by Figure 39.
Over the site of the Appalachians and most of the eastern half of the
Mississippi Valley area the land either stood near sea level and was
often swampy or marshy, or at other times it was a little below sea
level, allowing tidewater to overspread the area. Such conditions
alternated repeatedly, usually more or less locally, over different
parts of the districts in which the great coal mines of the east are
located. Under such conditions strata from 1,000 to 8,000 feet thick
accumulated. Remarkable physical geography of this kind resulted in the
growth and accumulation of vast quantities of vegetable matter which
has changed into the world's greatest coal beds. Similar conditions
prevailed over parts of Nova Scotia, New Brunswick, and Rhode Island,
where strata fully 13,000 feet thick accumulated.
"Perhaps the most perfect resemblance to coal-forming condition is
that now found on such coastal plains as that of southern Florida
and the Dismal Swamps of Virginia and North Carolina. Both of these
areas are very level, though with slight depressions in which there
is either standing water or swamp condition. In both regions there is
such general interference with free drainage that there are extensive
areas of swamp, and in both there are beds of vegetable accumulations.
In each of these areas there is a general absence of sediment and
therefore a marked variety of vegetable deposit. If either of these
areas were submerged beneath the sea, the vegetable remains would be
buried and a further step made toward the formation of a coal bed.
Reelevation, making a coastal plain, would permit the accumulation of
another coal bed above the first, and this process might be continued
again and again." (H. Ries.) But it is not necessary to assume repeated
oscillations of a swamp area up and down as the only way of accounting
for a succession of coal beds one above another in a given region,
because a general, but intermittent, subsidence, with possibly some
upward movements, would occasionally cause the prolific plant life
of a swamp to be killed, after which sediment would deposit over the
site. Shoaling of water by accumulation of sediment would permit the
development of more swamp plant life.
[Illustration: Fig. 39.--Map showing the general relations of land and
water, including the great coal-plant swamp areas (vertical lines), in
North America during the Pennsylvanian period at least 10,000,000 years
ago. Lined areas represent land. (After Willis, courtesy of the Journal
of Geology.)]
In most coal-mining districts there are at least several coal beds, one
above another. In Illinois there are nine; in Pennsylvania at least
twenty; in Alabama, thirty-five; and in Nova Scotia seventy-six, but
not all are important commercially. Each coal bed in such a region
represents a swamp which existed in Pennsylvanian time at least ten
or twelve million years ago, and in which there grew a luxuriant
vegetation. Many individual swamps of that time were of wide extent.
The famous Pittsburgh bituminous coal bed represents probably the
largest one of all. It extends from western Pennsylvania into parts
of Ohio and West Virginia over an area of fully 15,000 square miles.
More than 6,000 square miles of it are being worked and the coal bed
averages seven feet in thickness over an area of 2,000 square miles.
Among the various anthracite coal beds of the same age in eastern
Pennsylvania the Mammoth bed is exceptionally thick, reaching a maximum
of fifty feet or more.
In order that the reader may not gain the impression that coal beds
make up a very considerable bulk of the strata in coal-mining regions,
we should state that, on the average, coal actually constitutes less
than 2 per cent of the containing strata.
Some idea of the tremendous length of the geologic ages may be gained
by a consideration of the time which must reasonably be allowed for
the accumulation of so many coal beds and their containing strata. It
has been estimated that a luxuriant growth of vegetation would produce
100 tons of dried organic matter per hundred years. Compressed to
the specific gravity of coal (1.4) this would form a layer less than
two-thirds of an inch deep on an acre. During the chemical alteration
of vegetable matter to coal about four-fifths of the organic matter
disappears in the form of gases. On this basis, then, it would take
about 10,000 years to accumulate the vegetable matter represented in
a coal bed one foot thick. When we realize that the total thickness
of the coal beds of the Pennsylvanian system of strata in the great
mining regions is commonly from 100 to 250 feet, we conclude that the
time they represent is from 1,000,000 to 2,500,000 years. It seems
most reasonable that the time necessary for the deposition of the
containing strata must have been at least as long. It is, therefore, a
fair conclusion that the Pennsylvanian period lasted from 2,000,000 to
5,000,000 years.
That the climate of the great Coal Age was warm (not tropical), very
moist, and uniform, is borne out by such facts as the following,
according to D. White: The succulent nature of the plants with their
spongy leaves indicates prolific growth in moist, mild climate; lack of
yearly rings of growth points to lack of distinct seasons; as in the
case of many existing plants the aerial roots signify a warm, moist
climate; plants of to-day nearest like the coal plants thrive best
in warm, moist regions; vegetable matter at present accumulates best
in temperate rather than tropical climates, because there decay is
not so rapid; and the remarkable uniformity of climate over the earth
is clearly indicated by finding fossil plants of almost or exactly
identical types in rocks of Pennsylvanian Age from the Polar regions to
the Tropics. The more remarkable plants of the great Coal Age time are
described in the chapter on the evolution of plants (Plate 15).
During the last (Permian) period of the Paleozoic era the marine waters
of the west, and the alternating shallow tidewater, swamps, and near
sea level lands of the east gradually gave way to dry lands, so that
by the close of the period marine water covered only a small part of
the Southwest from Oklahoma across central Texas to southern California
and northwestern Mexico, where strata as much as several thousand feet
thick formed. In the middle western part of the area of the United
States, especially from northern Texas to Nebraska and Wyoming, the
climate was arid and red strata (so-called "Red Beds"), salt, and
gypsum were extensively deposited on land and in great salt lakes or
more or less cut-off arms of the sea. Strata commonly from 2,000 to
7,000 feet thick were there deposited. Similar conditions prevailed
in parts of Nova Scotia, New Brunswick, and Newfoundland, where strata
8,000 feet thick accumulated. Over the site of most of the Appalachians
the coal swamp conditions, with local sea incursions, continued from
the preceding period, as shown by the character of the strata (1,000
feet thick) containing some coal.
Vigorous volcanic activity which, as already mentioned, began in the
Mississippian period from northern California to Alaska continued not
only through the Pennsylvanian and Permian but also into the early
Mesozoic era, as shown by the great quantities of volcanic materials
associated with rocks of those ages.
The Permian presents a puzzling combination of climatic conditions
which causes it to stand out in marked contrast against the generally
mild and uniform climates of nearly all of preceding Paleozoic time.
Most remarkable of all are the records of a great Ice Age during early
Permian time. One surprising fact is the widespread distribution of the
glacial deposits in both the north and south temperate zones, and even
well within the torrid zone. They are perhaps most extensive and best
known in Australia, South Africa, India, and Brazil. Glacial deposits
almost certainly of the same age on smaller scales occur in eastern
Massachusetts, southern England, eastern Russia and the Caucasus
region. Although the areas occupied by the Permian glaciers, which in
many cases must have been extensive ice sheets, cannot be accurately
delimited, it is, nevertheless, quite certain that the ice was notably
more extensively developed than it was during the great "Ice Age"
of late (Quaternary) geologic time. Another surprising fact is that
certain of the glaciers must have come down to, or nearly to, sea
level, as shown by the direct association of marine strata with glacial
deposits. Thus, in southern Australia at least eight beds of glacial
materials (some of them 100 to 200 feet thick) occur within true marine
strata 2,000 feet thick. A third remarkable fact is that the Permian
Ice Age, like the Quaternary Ice Age, had interglacial epochs of
relatively mild climate, as proved by the occurrences of beds of coal
between certain of the layers of glacial materials in Australia, South
Africa, and Brazil.
During much of Permian time the climate was arid over large areas as,
for example, much of the western interior of the United States, from
Ireland to central Germany, and in eastern Russia, as proved by great
deposits of salt, gypsum, and red sediments. During late Permian time
the greatest salt beds in the world were deposited in northern Germany,
a well near Berlin having penetrated a practically solid body of salt
associated with certain potash and magnesia salts to a depth of about
4,000 feet without reaching the bottom.
The occurrence of some coal beds, especially in the earlier Permian
rocks shows that, temporarily at least, climatic conditions must have
favored luxurious growths of coal-forming plants in South Africa,
Brazil, Australia, and our own Appalachian district.
From the above facts we see that the Permian represents a remarkable
combination of very extensive glaciation, widespread aridity, and
warmth and moisture favorable to prolific plant growth all in a single
period of geologic time.
The Permian period, and, therefore, the great Paleozoic era, was
brought to a close by one of the most profound physical disturbances
in the known history of North America. This has been called the
Appalachian Revolution because at that time the Appalachian Mountain
range was born out of the sea by folding and upheaval of the strata.
In fact, "the Appalachian Revolution was one of the most critical
periods in the history of the earth, and may have been the greatest
of them all in its results." (C. Schuchert). Mountains were brought
forth in all the continents, including Australia. All of the mountains
which were formed late in the Paleozoic have since been profoundly
affected by erosion, and the only ones (e.g., Appalachians) which now
show considerable altitudes are those which have been rejuvenated by
relatively (geologically) recent earth movements.
We shall now turn our attention to the origin of the Appalachian
Range. All through the vast time (probably fully 20,000,000 years) of
the Paleozoic era a large land mass was remarkably persistent along
the eastern side of North America. This land, which has been called
"Appalachia," had its western boundary approximately along the eastern
border of the sites of the Appalachian Range and the western part of
New England. It extended east of the present coast line at least to the
border of the continental shelf from 100 to 200 miles out. Concerning
the actual altitude and topography of Appalachia we know little or
nothing, but the tremendous quantities of sediment derived from its
erosion show that it was high enough during nearly all of its history
to undergo vigorous erosion.
Barring certain minor oscillations of level, the region just west
of Appalachia was mostly occupied by sea water throughout much of
Paleozoic time, and sediments derived from the erosion of Appalachia
were laid down layer upon layer as strata upon that sea bottom. In
general, the coarsest and greatest thickness of sediments accumulated
relatively near the land, while finer materials, in thinner sheets,
deposited well out over much of the eastern Mississippi Valley area
in the shallow seas which were there so commonly present. By actual
measurement we know that the thickness of strata deposited over the
site of the Appalachians was at least 25,000 feet. Since these latter
strata are mostly of comparatively shallow sea-water origin, as proved
by coarseness of grain of material, ripple marks, fossil coral reefs,
etc., we are forced to conclude that this marginal sea bottom gradually
sank while the process of sedimentation was in progress. Otherwise
we cannot possibly explain the great pile of strata of shallow water
origin. The very weight of accumulating strata may either have aided or
actually caused the sinking of the long, relatively narrow trough.
Finally, toward the close of the Paleozoic era, sinking of the marginal
sea floor and deposition of sediments gave way to a yielding of the
earth's crust by a great force of lateral compression, causing the
strata to be thrown into folds well below the surface and more or less
fractured in their upper portion. Thus, along the eastern side of the
site of the great interior Paleozoic sea, the Appalachian Mountains
rose out of what for millions of years had been a long, narrow, sinking
sea floor. There was more or less folding from the Gulf of St. Lawrence
to central Alabama. Figure 24 diagrammatically represents the principal
stages in the history of the Appalachian Range.
While the most pronounced earth disturbance occurred through the
long Appalachian belt, the whole eastern side of the continent was
profoundly affected. Thus the Mississippi Valley area east of the Great
Plains was considerably upraised never again to be submerged except
along the Gulf Coast, and an eastern interior sea has never since
overspread the region which was repeatedly sea-covered during Paleozoic
time.