THE CARBONIFEROUS
The Carboniferous system is so named from the large amount of coal
which it contains. Other systems, from the Devonian on, are coal
bearing also, but none so richly and to so wide an extent. Never
before or since have the peculiar conditions been so favorable for the
formation of extensive coal deposits.
With few exceptions the Carboniferous strata rest on those of the
Devonian without any marked unconformity; the one period passed
quietly into the other, with no great physical disturbances.
The Carboniferous includes three distinct series. The lower is called
the _Mississippian_, from the outcrop of its formations along the
Mississippi River in central and southern Illinois and the adjacent
portions of Iowa and Missouri. The middle series is called the
_Pennsylvanian_ (or Coal Measures), from its wide occurrence over
Pennsylvania. The upper series is named the _Permian_, from the
province of Perm in Russia.
=The Mississippian series.= In the interior the Mississippian is
composed chiefly of limestones, with some shales, which tell of a
clear, warm, epicontinental sea swarming with crinoids, corals, and
shells, and occasionally clouded with silt from the land.
In the eastern region, New York had been added by uplift to the
Appalachian land which now was united to the northern area. From
eastern Pennsylvania southward there were laid in a subsiding trough,
first, thick sandstones (the Pocono sandstone), and later still
heavier shales,--the two together reaching the thickness of four
thousand feet and more. We infer a renewed uplift of Appalachia
similar to that of the later epochs of the Devonian, but as much less
in amount as the volume of sediments is smaller.
The Pennsylvanian Series
The Mississippian was brought to an end by a quiet oscillation which
lifted large areas slightly above the sea, and the Pennsylvanian began
with a movement in the opposite direction. The sea encroached on the
new land, and spread far and wide a great basal conglomerate and
coarse sandstones. On this ancient beach deposit a group of strata
rests which we must now interpret. They consist of alternating shales
and sandstones, with here and there a bed of limestone and an
occasional seam of coal. A stratum of fire clay commonly underlies a
coal seam, and there occur also beds of iron ore. We give a typical
section of a very small portion of the series at a locality in
Pennsylvania. Although some of the minor changes are omitted, the
section shows the rapid alternation of the strata:
9 Sandstone and shale ..... 25
8 Limestone ..... 18
7 Sandstone ..... 10
6 Coal ..... 1-6
5 Shale ..... 0-2
4 Sandstone ..... 40
3 Limestone ..... 10
2 Coal ..... 5-12
1 Fire clay ..... 3
This section shows more coal than is usual; on the whole, coal seams
do not take up more than one foot in fifty of the Coal Measures. They
vary also in thickness more than is seen in the section, some
exceptional seams reaching the thickness of fifty feet.
=How coal was made.= 1. Coal is of vegetable origin. Examined under
the microscope even anthracite, or hard coal, is seen to contain
carbonized vegetal tissues. There are also all gradations connecting
the hardest anthracite--through semibituminous coal, bituminous or
soft coal, lignite (an imperfect coal in which sometimes woody fibers
may be seen little changed)--with peat and decaying vegetable tissues.
Coal is compressed and mineralized vegetal matter. Its varieties
depend on the perfection to which the peculiar change called
bituminization has been carried, and also, as shown in the table
below, on the degree to which the volatile substances and water have
escaped, and on the per cent of carbon remaining.
Peat Bituminous
Dismal Lignite Coal Anthracite
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.
[Illustration: Fig. 303. Fossil Tree Stumps of a Carboniferous
Forest, Scotland]
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 fresh-water
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.
[Illustration: Fig. 304. Ideal Landscape of the Pennsylvanian
Epoch]
=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 Pennsylvania, 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.
[Illustration: Fig. 305. Carboniferous Ferns]
[Illustration: Fig. 306. Calamites]
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.
[Illustration: Fig. 307. Lepidodendron]
[Illustration: Fig. 308. 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.
[Illustration: Fig. 309. Carboniferous Brachiopods
_A_, Productus; _B_, Spirifer, the right-hand figure showing the
interior with the calcareous spires for the support of the arms]
=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 developed 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.
[Illustration: Fig. 310. A Carboniferous Dragon Fly
One tenth natural size]
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 vertebræ 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.
[Illustration: Fig. 311. A Carboniferous Amphibian]
[Illustration: Fig. 312. Transverse Section of
Segment of Tooth of Carboniferous Amphibian]
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.
[Illustration: Fig. 313. Skull of a Permian Amphibian from Texas]
[Illustration: Fig. 314. Skull of a Frog]
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.