WHEN COAL WAS FORMED.
It becomes of interest now to examine briefly into the causes and
process of the transformation from vegetable substance into coal, to
note the character of the vegetation which went to make up the coal
beds, and to glance at the animal life of the period.
As has already been said, the plants of the Carboniferous age were
exceedingly abundant and luxuriant. They grew up richly from the clayey
soil, and formed dense jungles in the vast marshes which covered so
large an area of the earth’s surface. Ferns, mosses, and tufts of
surface vegetation, and the leaves, branches, and trunks of trees fell
and decayed on the place where they grew, only to make the soil more
fertile and the next growth richer and more luxuriant. Year after
year, century after century, this process of growth and decay went on,
until the beds of vegetable matter thus deposited had reached a great
thickness. But condensation was still in progress in the earth’s body,
and in consequence of it her crust, of necessity, at times contracted
and fell. When it did so the land sank throughout vast areas, these
beds of incipient coal went down, and over the great marshes the waters
swept again, bringing drift of vegetation from higher levels to add
to that already buried. Then over these deposits of vegetable matter
the sand and mud and gravel were laid up anew, and the clayey soil
from which the next rich growth should spring was spread out upon the
surface. This process was repeated again and again, as often, indeed,
as we find seams of coal in any coal bed. Thus the final condition for
the formation of coal was met, the exclusion of atmospheric air from
this mass of decaying vegetation was complete, and under the water
of the ocean, under the sand and silt of the shore, under the new
deposits of succeeding ages, the transformation went on, the wood of
the Carboniferous era became the coal of to-day, while above and below
it the sand and clay were hardened into rock and shale.
The remarkable features of the vegetation of the coal era were the
size and abundance of its plants. Trees of that time whose trunks
were from one to three feet in diameter, and which grew to a height
of from forty to one hundred feet, are represented in our day by mere
stems a fraction of an inch in diameter and but one or two feet high.
A comparison of quantity would show differences as great as does the
comparison of size.
But at that time all the conditions were favorable for the rapid and
enormous growth of vegetation. The air was laden with carbon, which
is the principal food for plants; so laden, indeed, that man, who is
eminently an oxygen-breathing animal, could not have lived in it. The
great humidity of the atmosphere was another element favorable to
growth. Vegetation never lacked for an abundance of moisture either at
root or leaf. Then, too, the climate was universally warm. Over the
entire surface of the earth the heat was greater than it is to-day at
the torrid zone. It must be remembered that the internal fires of the
globe have been constantly cooling and receding, and that the earth, in
the Carboniferous age, was subjected to the greater power of a larger
sun than shines upon us to-day.
With all these circumstances in its favor, warmth, moisture, and an
atmosphere charged heavily with carbon, vegetation could not help but
flourish. That it did flourish amazingly is abundantly shown by its
fossil remains. The impressions of more than five hundred different
species of plants that grew in the Carboniferous era have been found
in the coal measures. There are few of them that bear any direct
analogy to existing species, and these few have their counterparts
only in the torrid zone. The most abundant of the plants of the coal
era were the ferns. Their fossil remains are found in great profusion
and variety in most of the rocks of the coal-bearing strata. There was
also the plant known as the tree fern, which attained a height of
twenty or thirty feet and carried a single tuft of leaves radiating
from its top. Probably the species next in abundance, as it certainly
is next in importance, to the ferns is that of the Lepidodendrids. It
doubtless contributed the greatest proportion of woody material to the
composition of coal. The plants of this species were forest trees,
but are supposed to have been analogous to the low club mosses of the
present. Fossil trunks of Lepidodendrids have been found measuring from
one hundred to one hundred and thirty feet in length, and from six to
ten feet in diameter.
Similar in appearance to the Lepidodendrids were the Sigillariæ, which
were also very abundant. The Conifers were of quite a different species
from those already named, and probably grew on higher ground. They were
somewhat analogous to the modern pine.
The Calamites belonged to the horsetail family. They grew up with long,
reed-like, articulated stems to a height of twenty feet or more, and
with a diameter of ten or twelve inches. They stood close together in
the muddy ground, forming an almost impenetrable thicket, and probably
made up a very large percentage of the vegetation which was transformed
into coal.
One of the most abundant species of plants of the coal era is that of
Stigmaria. Stout stems, from two to four inches in diameter, branched
downward from a short trunk, and then grew out in long root-like
processes, floating in the water or trailing on the mud to distances of
twenty or thirty feet. These are the roots with which the under clay of
every coal seam is usually filled.
The plants which have been described, together with their kindred
species, formed the largest and most important part of the vegetation
of the Carboniferous age. But of the hundreds of varieties which then
abounded, the greater portion reached their highest stage of perfection
in the coal era, and became extinct before the close of Paleozoic
time. Other types were lost during Mesozoic time, and to-day there is
scarcely a counterpart in existence of any of the multitude of forms of
plant life that grew and flourished in that far-off age of the world.
The animal life of the Carboniferous era was confined almost entirely
to the water. The dry land had not yet begun to produce in abundance
the higher forms of living things. There were spiders there, however,
and scorpions, and centipedes, and even cockroaches. There were also
land snails, beetles, locusts, and mayflies. Reptiles, with clumsy
feet and dragging tails, prowled about on the wet sands of the shore,
leaving footprints that were never effaced by time or the elements, and
are found to-day in the layers of the rocks, almost as perfect as when
they were formed, millions of years ago. But the waters teemed with
animal life. On the bottom of the shallow seas lay shells and corals
in such abundance and variety that from the deposits of their remains
great beds of limestone have been formed. Broken into minute fragments
by the action of the waves and washed up by the sea during periods of
submergence, they were spread over the beds of carboniferous deposits,
and became the rock strata through which the drills and shafts of
to-day are sunk to reach the veins of mineral coal.
Fishes were numerous. Some of them, belonging to species allied to the
modern shark, were of great size, with huge fin spines fully eighteen
inches in length. These spines have been found as fossils, as have also
the scales, teeth, and bones. Complete skeletons of smaller fishes of
the ganoid order were preserved in the rock as it hardened, and now
form fossil specimens which are unequaled in beauty and perfection.
Besides the fishes, there were the swimming reptiles; amphibian
monsters, allied to the ichthyosaurs and plesiosaurs which were so
abundant during the Reptilian age that followed. These animals are
known as enaliosaurs. They attained great size, being from twenty-five
to fifty feet in length; they had air-breathing apparatus, and
propelled themselves through the water with paddles like the paddles
of whales. Their enormous jaws were lined with rows of sharp, pointed
teeth, and their food was fish, shell-fish, and any other kind of
animal life that came within their reach. They devoured even their own
species. Living mostly in the open seas or fresh-water lagoons, they
sometimes chased their prey far up the rivers, and sometimes basked in
the sunshine on the sands of the shore. Frightful in aspect, fierce,
and voracious, they were the terror and the tyrants of the seas.
Such were the animals, such were the plants, that lived and died, that
flourished and decayed, in the age when coal was being formed and
fashioned and hidden away in the crust of the earth. That the fauna and
flora of to-day have few prototypes among them should be little cause
for regret. There was, indeed, hardly a feature in the landscape of the
coal era that would have had a familiar look to an inhabitant of the
world in its present age. In place of the hills and valleys as we have
them now, there were great plains sloping imperceptibly to the borders
of the sea. There were vast marshes, shallow fresh-water lakes, and
broad and sluggish rivers. Save by isolated peaks the Rocky Mountains
had not yet been uplifted from the face of the deep, and the great West
of to-day was a waste of waters. In the wide forests no bird’s song was
ever heard, no flashing of a wing was ever seen, no serpent trailed
its length upon the ground, no wild beast searched the woods for prey.
The spider spun his web in silence from the dew-wet twigs, the locust
hopped drowsily from leaf to leaf, the mayfly floated lightly in the
heavy air, the slow-paced snail left his damp track on the surfaces of
the rocks, and the beetles, lifting the hard coverings from their gauzy
wings, flew aimlessly from place to place. In seas and lakes and swampy
pools strange fishes swam, up from the salt waters odd reptiles crawled
to sun themselves upon the sandy shore or make their way through
the dense jungles of the swamps, and out where the ocean waves were
dashing, fierce monsters of the sea darted on their prey, or churned
the water into foam in savage fights with each other.
But in all the world there were no flowers. Stems grew to be trunks,
branches were sent out, leaves formed and fell, the land was robed and
wrapped in the richest, most luxuriant foliage, yet the few buds that
tried to blossom were scentless and hidden, and earth was still void of
the beauty and the fragrance of the flowers.