Books: The Elements of Geology
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William Harmon Norton >> The Elements of Geology
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Peat Lignite Bituminous Coal
Anthracite
Dismal Swamp Texas Penn.
Penn.
Moisture . . . . 78.89 14.67 1.30 2.74
Volatile matter . 13.84 37.32 20.87 4.25
Fixed carbon . . 6.49 41.07 67.20 81.51
Ash . . . . . . . 0.78 6.69 8.80 10.87
2. The vegetable remains associated with coal are those of land
plants.
3. Coal accumulated in the presence of water; for it is only when
thus protected from the air that vegetal matter is preserved.
4. The vegetation of coal accumulated for the most part where it
grew; it was not generally drifted and deposited by waves and
currents. Commonly the fire clay beneath the seam is penetrated
with roots, and the shale above is packed with leaves of ferns and
other plants as beautifully pressed as in a herbarium. There often
is associated with the seam a fossil forest, with the stumps,
which are still standing where they grew, their spreading roots,
and the soil beneath, all changed to stone. In the Nova Scotia
field, out of seventy-six distinct coal seams, twenty are
underlain by old forest grounds.
The presence of fire clay beneath a seam points in the same
direction. Such underclays withstand intense heat and are used in
making fire brick, because their alkalies have been removed by the
long-continued growth of vegetation.
Fuel coal is also too pure to have been accumulated by driftage.
In that case we should expect to find it mixed with mud, while in
fact it often contains no more ash than the vegetal matter would
furnish from which it has been compressed.
These conditions are fairly met in the great swamps of river
plains and deltas and of coastal plains, such as the great Dismal
Swamp, where thousands of generations of forests with their
undergrowths contribute their stems and leaves to form thick beds
of peat. A coal seam is a fossil peat bed.
GEOGRAPHICAL CONDITIONS DURING THE PENNSYLVANIAN. The
Carboniferous peat swamps were of vast extent. A map of the Coal
Measures (Fig. 260) shows that the coal marshes stretched, with
various interruptions of higher ground and straits of open water,
from eastern Pennsylvania into Alabama, Texas, and Kansas. Some
individual coal beds may still be traced over a thousand square
miles, despite the erosion which they have suffered. It taxes the
imagination to conceive that the varied region included within
these limits was for hundreds of thousands of years a marshy plain
covered with tropical jungles such as that pictured in Figure 304.
On the basis that peat loses four fifths of its bulk in changing
to coal, we may reckon the thickness of these ancient peat beds.
Coal seams six and ten feet thick, which are not uncommon,
represent peat beds thirty and fifty feet in thickness, while
mammoth coal seams fifty feet thick have been compressed from peat
beds two hundred and fifty feet deep.
At the same time, the thousands of feet of marine and freshwater
sediments, with their repeated alternations of limestones,
sandstones, and shales, in which the seams of coal occur, prove a
slow subsidence, with many changes in its rate, with halts when
the land was at a stillstand, and with occasional movements
upward.
When subsidence was most rapid and long continued the sea
encroached far and wide upon the lowlands and covered the coal
swamps with sands and muds and limy oozes. When subsidence
slackened or ceased the land gained on the sea. Bays were barred,
and lagoons as they gradually filled with mud became marshes.
River deltas pushed forward, burying with their silts the sunken
peat beds of earlier centuries, and at the surface emerged in
broad, swampy flats,--like those of the deltas of the Mississippi
and the Ganges,--which soon were covered with luxuriant forests.
At times a gentle uplift brought to sea level great coastal
plains, which for ages remained mantled with the jungle, their
undeveloped drainage clogged with its debris, and were then again
submerged.
PHYSICAL GEOGRAPHY OF THE SEVERAL REGIONS. THE ACADIAN REGION lay
on the eastern side of the northern land, where now are New
Brunswick and Nova Scotia, and was an immense river delta. Here
river deposits rich in coal accumulated to a depth of sixteen
thousand feet. The area of this coal field is estimated at about
thirty-six thousand square miles.
THE APPALACHIAN REGION skirts the Appalachian oldland on the west
from the southern boundary of New York to northern Alabama,
extending west into eastern Ohio. The Cincinnati anticline was now
a peninsula, and the broad gulf which had lain between it and
Appalachia was transformed at the beginning of the Pennsylvanian
into wide marshy plains, now sinking beneath the sea and now
emerging from it. This area subsided during the Carboniferous
period to a depth of nearly ten thousand feet.
THE CENTRAL REGION lay west of the peninsula of the Cincinnati
anticline, and extended from Indiana west into eastern Nebraska,
and from central Iowa and Illinois southward about the ancient
island in Missouri and Arkansas into Oklahoma and Texas. On the
north the subsidence in this area was comparatively slight, for
the Carboniferous strata scarcely exceed two thousand feet in
thickness. But in Arkansas and Indian Territory the downward
movement amounted to four and five miles, as is proved by shoal
water deposits of that immense thickness.
The coal fields of Indiana, and Illinois are now separated by
erosion from those lying west of the Mississippi River. At the
south the Appalachian land seems still to have stretched away to
the west across Louisiana and Mississippi into Texas, and this
westward extension formed the southern boundary of the coal
marshes of the continent.
The three regions just mentioned include the chief Carboniferous
coal fields of North America. Including a field in central
Michigan evidently formed in an inclosed basin (Fig. 260), and one
in Rhode Island, the total area of American coal fields has been
reckoned at not less than two hundred thousand square miles. We
can hardly estimate the value of these great stores of fossil fuel
to an industrial civilization. The forests of the coal swamps
accumulated in their woody tissues the energy which they received
from the sun in light and heat, and it is this solar energy long
stored in coal seams which now forms the world's chief source of
power in manufacturing.
THE WESTERN AREA. On the Great Plains beyond the Missouri River
the Carboniferous strata pass under those of more recent systems.
Where they reappear, as about dissected mountain axes or on
stripped plateaus, they consist wholly of marine deposits and are
devoid of coal. The rich coal fields of the West are of later
date.
On the whole the Carboniferous seems to have been a time of
subsidence in the West. Throughout the period a sea covered the
Great Basin and the plateaus of the Colorado River. At the time of
the greatest depression the sites of the central chains of the
Rockies were probably islands, but early in the period they may
have been connected with the broad lands to the south and east.
Thousands of feet of Carboniferous sediments were deposited where
the Sierra Nevada Mountains now stand.
THE PERMIAN. As the Carboniferous period drew toward its close the
sea gradually withdrew from the eastern part of the continent.
Where the sea lingered in the deepest troughs, and where inclosed
basins were cut off from it, the strata of the Permian were
deposited. Such are found in New Brunswick, in Pennsylvania and
West Virginia, in Texas, and in Kansas. In southwestern Kansas
extensive Permian beds of rock salt and gypsum show that here lay
great salt lakes in which these minerals were precipitated as
their brines grew dense and dried away.
In the southern hemisphere the Permian deposits are so
extraordinary that they deserve a brief notice, although we have
so far omitted mention of the great events which characterized the
evolution of other continents than our own. The Permian fauna-
flora of Australia, India, South Africa, and the southern part of
South America are so similar that the inference is a reasonable
one that these widely separated regions were then connected
together, probably as extensions of a great antarctic continent.
Interbedded with the Permian strata of the first three countries
named are extensive and thick deposits of a peculiar nature which
are clearly ancient ground moraines. Clays and sand, now hardened
to firm rock, are inset with unsorted stones of all sizes, which
often are faceted and scratched. Moreover, these bowlder clays
rest on rock pavements which are polished and scored with glacial
markings. Hence toward the close of the Paleozoic the southern
lands of the eastern hemisphere were invaded by great glaciers or
perhaps by ice sheets like that which now shrouds Greenland.
These Permian ground moraines are not the first traces of the work
of glaciers met with in the geological record. Similar deposits
prove glaciation in Norway succeeding the pre-Cambrian stage of
elevation, and Cambrian glacial drift has recently been found in
China.
THE APPALACHIAN DEFORMATION. We have seen that during Paleozoic
times a long, narrow trough of the sea lay off the western coast
of the ancient land of Appalachia, where now are the Appalachian
Mountains. During the long ages of this era the trough gradually
subsided, although with many stillstands and with occasional
slight oscillations upward. Meanwhile the land lying to the east
was gradually uplifted at varying rates and with long pauses. The
waste of the rising land was constantly transferred to the sinking
marginal sea bottom, and on the whole the trough was filled with
sediments as rapidly as it subsided. The sea was thus kept
shallow, and at times, especially toward the close of the era,
much of the area was upbuilt or raised to low, marshy, coastal
plains. When the Carboniferous was ended the waste which had been
removed from the land and laid along its margin in the successive
formations of the Paleozoic had reached a thickness of between
thirty and forty thousand feet.
Both by sedimentation and by subsidence the trough had now become
a belt of weakness in the crust of the earth. Here the crust was
now made of layers to the depth of six or seven miles. In
comparison with the massive crystalline rocks of Appalachia on the
east, the layered rock of the trough was weak to resist lateral
pressure, as a ream of sheets of paper is weak when compared with
a solid board of the same thickness. It was weaker also than the
region to the west, since there the sediments were much thinner.
Besides, by the long-continued depression the strata of the trough
had been bent from the flat-lying attitude in which they were laid
to one in which they were less able to resist a horizontal thrust.
There now occurred one of the critical stages in the history of
the planet, when the crust crumples under its own weight and
shrinks down upon a nucleus which is diminishing in volume and no
longer able to support it. Under slow but resistless pressure the
strata of the Appalachian trough were thrust against the rigid
land, and slowly, steadily bent into long folds whose axes ran
northeast-southwest parallel to the ancient coast line. It was on
the eastern side next the buttress of the land that the
deformation was the greatest, and the folds most steep and close.
In central Pennsylvania and West Virginia the folds were for the
most part open. South of these states the folds were more closely
appressed, the strata were much broken, and the great thrust
faults were formed which have been described already. In eastern
Pennsylvania seams of bituminous coal were altered to anthracite,
while outside the region of strong deformation, as in western
Pennyslvania, they remained unchanged. An important factor in the
deformation was the massive limestones of the Cambrian-Ordovician.
Because of these thick, resistant beds the rocks were bent into
wide folds and sheared in places with great thrust faults. Had the
strata been weak shales, an equal pressure would have crushed and
mashed them.
Although the great earth folds were slowly raised, and no doubt
eroded in their rising, they formed in all probability a range of
lofty mountains, with a width of from fifty to a hundred and
twenty-five miles, which stretched from New York to central
Alabama.
From their bases lowlands extended westward to beyond the Missouri
River. At the same time ranges were upridged out of thick
Paleozoic sediments both in the Bay of Fundy region and in the
Indian Territory. The eastern portion of the North American
continent was now well-nigh complete.
The date of the Appalachian deformation is told in the usual way.
The Carboniferous strata, nearly two miles thick, are all infolded
in the Appalachian ridges, while the next deposits found in this
region--those of the later portion of the first period (the Trias)
of the succeeding era--rest unconformably on the worn edges of the
Appalachian folded strata. The deformation therefore took place
about the close of the Paleozoic. It seems to have begun in the
Permian, in, eastern Pennsylvania,--for here the Permian strata
are wanting,--and to have continued into the Trias, whose earlier
formations are absent over all the area.
With this wide uplift the subsidence of the sea floor which had so
long been general in eastern North America came to an end.
Deposition now gave place to erosion. The sedimentary record of
the Paleozoic was closed, and after an unknown lapse of time, here
unrecorded, the annals of the succeeding era were written under
changed conditions.
In western North America the closing stages of the Paleozoic were
marked by important oscillations. The Great Basin, which had long
been a mediterranean sea, was converted into land over western
Utah and eastern Nevada, while the waves of the Pacific rolled
across California and western Nevada.
The absence of tuffs and lavas among the Carboniferous strata of
North America shows that here volcanic action was singularly
wanting during the entire period. Even the Appalachian deformation
was not accompanied by any volcanic outbursts.
LIFE OF THE CARBONIFEROUS
PLANTS. The gloomy forests and dense undergrowths of the
Carboniferous jungles would appear unfamiliar to us could we see
them as they grew, and even a botanist would find many of their
forms perplexing and hard to classify. None of our modern trees
would meet the eye. Plants with conspicuous flowers of fragrance
and beauty were yet to come. Even mosses and grasses were still
absent.
Tree ferns lifted their crowns of feathery fronds high in air on
trunks of woody tissue; and lowly herbaceous ferns, some belonging
to existing families, carpeted the ground. Many of the fernlike
forms, however, have distinct affinities with the cycads, of which
they may be the ancestors, and some bear seeds and must be classed
as gymnosperms.
Dense thickets, like cane or bamboo brakes, were composed of thick
clumps of CALAMITES, whose slender, jointed stems shot up to a
height of forty feet, and at the joints bore slender branches set
with whorls of leaves. These were close allies of the Equiseta or
"horsetails," of the present; but they bore characteristics of
higher classes in the woody structures of their stems.
There were also vast monotonous forests, composed chiefly of trees
belonging to the lycopods, and whose nearest relatives to-day are
the little club mosses of our eastern woods. Two families of
lycopods deserve special mention,--the Lepidodendrons and the
Sigillaria.
The LEPIDODENDRON, or "scale tree," was a gigantic club moss fifty
and seventy-five feet high, spreading toward the top into stout
branches, at whose ends were borne cone-shaped spore cases. The
younger parts of the tree were clothed with stiff needle-shaped
leaves, but elsewhere the trunk and branches were marked with
scalelike scars, left by the fallen leaves, and arranged in spiral
rows.
The SIGILLARIA, or "seal tree," was similar to the Lepidodendron,
but its fluted trunk divided into even fewer branches, and was
dotted with vertical rows of leaf scars, like the impressions of a
seal.
Both Lepidodendron and Sigillaria were anchored by means of great
cablelike underground stems, which ran to long distances through
the marshy ground. The trunks of both trees had a thick woody
rind, inclosing loose cellular tissue and a pith. Their hollow
stumps, filled with sand and mud, are common in the Coal Measures,
and in them one sometimes finds leaves and stems, land shells, and
the bones of little reptiles of the time which made their home
there.
It is important to note that some of these gigantic lycopods,
which are classed with the CRYPTOGAMS, or flowerless plants, had
pith and medullary rays dividing their cylinders into woody
wedges. These characters connect them with the PHANEROGAMS, or
flowering plants. Like so many of the organisms of the remote
past, they were connecting types from which groups now widely
separated have diverged.
Gymnosperms, akin to the cycads, were also present in the
Carboniferous forests. Such were the different species of
CORDAITES, trees pyramidal in shape, with strap-shaped leaves and
nutlike fruit. Other gymnosperms were related to the yews, and it
was by these that many of the fossil nuts found in the Coal
Measures were borne. It is thought by some that the gymnosperms
had their station on the drier plains and higher lands.
The Carboniferous jungles extended over parts of Europe and of
Asia, as well as eastern North America, and reached from the
equator to within nine degrees of the north pole. Even in these
widely separated regions the genera and species of coal plants are
close akin and often identical.
INVERTEBRATES. Among the echinoderms, crinoids are now exceedingly
abundant, sea urchins are more plentiful, and sea cucumbers are
found now for the first time. Trilobites are rapidly declining,
and pass away forever with the close of the period. Eurypterids
are common; stinging scorpions are abundant; and here occur the
first-known spiders.
We have seen that the arthropods were the first of all animals to
conquer the realm of the air, the earliest insects appearing in
the Ordovician. Insects had now become exceedingly abundant, and
the Carboniferous forests swarmed with the ancestral types of
dragon flies,--some with a spread of wing of more than two feet,--
May flies, crickets, and locusts. Cockroaches infested the swamps,
and one hundred and thirty-three species of this ancient order
have been discovered in the Carboniferous of North America. The
higher flower-loving insects are still absent; the reign of the
flowering plants has not yet begun. The Paleozoic insects were
generalized types connecting the present orders. Their fore wings
were still membranous and delicately veined, and used in flying;
they had not yet become thick, and useful only as wing covers, as
in many of their descendants.
FISHES still held to the Devonian types, with the exception that
the strange ostracoderms now had perished.
AMPHIBIANS. The vertebrates had now followed the arthropods and
the mollusks upon the land, and had evolved a higher type adapted
to the new environment. Amphibians--the class to which frogs and
salamanders belong--now appear, with lungs for breathing air and
with limbs for locomotion on the land. Most of the Carboniferous
amphibians were shaped like the salamander, with weak limbs
adapted more for crawling than for carrying the body well above
the ground. Some legless, degenerate forms were snakelike in
shape.
The earliest amphibians differ from those of to-day in a number of
respects. They were connecting types linking together fishes, from
which they were descended, with reptiles, of which they were the
ancestors. They retained the evidence of their close relationship
with the Devonian fishes in their cold blood, their gills and
aquatic habit during their larval stage, their teeth with dentine
infolded like those of the Devonian ganoids but still more
intricately, and their biconcave vertebrae which never completely
ossified. These, the highest vertebrates of the time, had not yet
advanced beyond the embryonic stage of the more or less
cartilaginous skeleton and the persistent notochord.
On the other hand, the skull of the Carboniferous amphibians was
made of close-set bony plates, like the skull of the reptile,
rather than like that of the frog, with its open spaces (Figs. 313
and 314). Unlike modern amphibians, with their slimy skin, the
Carboniferous amphibians wore an armor of bony scales over the
ventral surface and sometimes over the back as well.
It is interesting to notice from the footprints and skeletons of
these earliest-known vertebrates of the land what was the
primitive number of digits. The Carboniferous amphibians had five-
toed feet, the primitive type of foot, from which their
descendants of higher orders, with a smaller number of digits,
have diverged.
The Carboniferous was the age of lycopods and amphibians, as the
Devonian had been the age of rhizocarps and fishes.
LIFE OF THE PERMIAN. The close of the Paleozoic was, as we have
seen, a time of marked physical changes. The upridging of the
Appalachians had begun and a wide continental uplift--proved by
the absence of Permian deposits over large areas where
sedimentation had gone on before--opened new lands for settlement
to hordes of air-breathing animals. Changes of climate compelled
extensive migrations, and the fauna of different regions were thus
brought into conflict. The Permian was a time of pronounced
changes in plant and animal life, and a transitional period
between two great eras. The somber forests of the earlier
Carboniferous, with their gigantic club mosses, were now replaced
by forests of cycads, tree ferns, and conifers. Even in the lower
Permian the Lepidodendron and Sigillaria were very rare, and
before the end of the epoch they and the Calamites also had become
extinct. Gradually the antique types of the Paleozoic fauna died
out, and in the Permian rocks are found the last survivors of the
cystoid, the trilobite, and the eurypterid, and of many long-lived
families of brachiopods, mollusks, and other invertebrates. The
venerable Orthoceras and the Goniatite linger on through the epoch
and into the first period of the succeeding era. Forerunners of
the great ammonite family of cephalopod mollusks now appear. The
antique forms of the earlier Carboniferous amphibians continue,
but with many new genera and a marked increase in size.
A long forward step had now been taken in the evolution of the
vertebrates. A new and higher type, the reptiles, had appeared,
and in such numbers and variety are they found in the Permian
strata that their advent may well have occurred in a still earlier
epoch. It will be most convenient to describe the Permian reptiles
along with their descendants of the Mesozoic.
CHAPTER XX
THE MESOZOIC
With the close of the Permian the world of animal and vegetable
life had so changed that the line is drawn here which marks the
end of the old order and the beginning of the new and separates
the Paleozoic from the succeeding era,--the Mesozoic, the Middle
Age of geological history. Although the Mesozoic era is shorter
than the Paleozoic, as measured by the thickness of their strata,
yet its duration must be reckoned in millions of years. Its
predominant life features are the culmination and the beginning of
the decline of reptiles, amphibians, cephalopod mollusks, and
cycads, and the advent of marsupial mammals, birds, teleost
fishes, and angiospermous plants. The leading events of the long
ages of the era we can sketch only in the most summary way.
The Mesozoic comprises three systems,--the TRIASSIC, named from
its threefold division in Germany; the JURASSIC, which is well
displayed in the Jura Mountains; and the CRETACEOUS, which
contains the extensive chalk (Latin, creta) deposits of Europe.
In eastern North America the Mesozoic rocks are much less
important than the Paleozoic, for much of this portion of the
continent was land during the Mesozoic era, and the area of the
Mesozoic rocks is small. In western North America, on the other
hand, the strata of the Mesozoic--and of the Cenozoic also--are
widely spread. The Paleozoic rocks are buried quite generally from
view except where the mountain makings and continental uplifts of
the Mesozoic and Cenozoic have allowed profound erosion to bring
them to light, as in deep canyons and about mountain axes. The
record of many of the most important events in the development of
the continent during the Mesozoic and Cenozoic eras is found in
the rocks of our western states.
THE TRIASSIC AND JURASSIC
EASTERN NORTH AMERICA. The sedimentary record interrupted by the
Appalachian deformation was not renewed in eastern North America
until late in the Triassic. Hence during this long interval the
land stood high, the coast was farther out than now, and over our
Atlantic states geological time was recorded chiefly in erosion
forms of hill and plain which have long since vanished. The area
of the later Triassic rocks of this region, which take up again
the geological record, is seen in the map of Figure 260. They lie
on the upturned and eroded edges of the older rocks and occupy
long troughs running for the most part parallel to the Atlantic
coast. Evidently subsidence was in progress where these rocks were
deposited. The eastern border of Appalachia was now depressed. The
oldland was warping, and long belts of country lying parallel to
the shore subsided, forming troughs in which thousands of feet of
sediment now gathered.
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