HOW THE COAL BEDS WERE LAID DOWN
In following the history of the rocks, we shall have presently to
speak of that period which embraces the strata which contain coal.
The geologists who lived in the early part of last century--William
Smith and others--noticed that beneath the coal-bearing strata
there lay a considerable thickness of red sandy beds containing the
remains of fresh-water fishes, shells, and plants, while above the
coaly strata they found another thick mass of red sandstone. To the
lower and older red rocks they consequently gave the name of Old Red
Sandstone, and to the upper and newer ones the name of the New Red
Sandstone. The Old Red Sandstone formation, therefore, lies between
the Silurian rocks below and the rocks of the coal period above. But
in Devonshire we find a considerable thickness of shales, slates, and
limestones containing _marine_ fossils, and these also lie between
these two formations, and must therefore be somewhere about the
same period, or geological age, as the Old Red Sandstone. The name
"Devonian" has therefore been given to these shales, slates, and
limestones, which were evidently being deposited in an open sea at
or about the same time at which the Old Red Sandstone strata were
being laid down on the floors of inland fresh-water lakes.
In the west of England the Old Red Sandstone stretches from Hereford
and Monmouth into the neighbouring Welsh counties of Brecknock
and Glamorgan. It is here at its greatest thickness of nearly two
miles. The lower part consists of red and yellow sandstones, marls,
and shales, with a certain kind of limestone concrete. The red
colour is due to iron, and wherever this is abundant fossils are
scarce, though remains of fishes have been found in it. Scotland
is the classic ground of the Old Red Sandstone, for it was here
that Hugh Miller, when a working mason at Cromarty, first collected
its wonderful fossil fishes. Hugh Miller's discovery is one of the
romances of geological annals. "Let any one picture to himself,"
wrote the late Mr. Bristow, "the surprise he would feel should he, on
taking his first lesson in geology, and on first breaking a stone--a
pebble, for instance, exhibiting every external sign of a water-worn
surface--find, to appropriate Archdeacon Paley's illustration, a
watch, or any other delicate piece of mechanism, in its centre.
Now this, many years ago, is exactly the kind of surprise that
Hugh Miller experienced in the sandstone quarry opened in a lofty
wall of cliff overhanging the northern shore of the Moray Frith.
He had picked up a nodular mass of blue Lias limestone, which he
laid open by a stroke of the hammer, when, behold! an exquisitely
shaped Ammonite was displayed before him. It is no surprising that
henceforth the half mason, half sailor, and poet, became a geologist.
He sought for information, and found it; he found that the rocks
among which he laboured swarmed with the relics of a former age. He
pursued his investigations, and found, while working in this zone
of strata all around the coast, that a certain class of fossils
abounded; but that in a higher zone these familiar forms disappeared,
and others made their appearance.
"He read and learned that in other lands--lands of more recent
formation--strange forms of animal life had been discovered; forms
which in their turn had disappeared, to be succeeded by others, more
in accordance with beings now living. He came to know that he was
surrounded, in his native mountains, by the sedimentary deposits of
other ages; he became alive to the fact that these grand mountain
ranges had been built up grain by grain in the bed of the ocean, and
the mountains had been subsequently raised to their present level
by the upheaval of one part of its bed, or by the subsidence of
another...." _The Old Red Sandstone_, a book which was the result of
Hugh Miller's researches, is a geological classic.
There are three other regions in England and Scotland where the Old
Red Sandstone is conspicuous, and all of them were probably old
fresh-water lakes of great extent in which sands accumulated. Sir
Archibald Geikie has named them the Welsh Lake, Lake Cheviot, Lake
Caledonia, Lake Arcadie, Lake Lorne. There are similar sandy deposits
in Russia, in North America, near the Catskill Mountains, and in many
parts of Canada; and there is little doubt that in all these places
there were great lakes which gradually became the depositories of
rivers and developed a life of their own.
The most remarkable fossils of these deposits were the fishes. The
fishes began to appear in the later Silurian: they are strikingly
abundant in Devonian times. The most remarkable of them are fishes
which are only just like fishes after having been developed out of,
or perhaps descending from, some other form of life. These fishes are
now called the _Ostracoderm_ group, and they bear strange resemblance
to some of the trilobites and the king crabs of previous eras. The
_Pteraspis_ is one of the earliest of these strange creatures, and
its "fins," very much developed, were used as oars. Perhaps the most
curious of all these strange creatures were the _Pterichthyds_ or
winged fish; though it is not at all likely that the appendages we
call wings were used for aerial flight. These fishes were all small;
their forms were clumsy and their powers of moving about small. They
had poor mouths and eyes, and they probably ploughed the soft bottoms
of the sluggish waters, above which little besides their peculiarly
placed eyes and the backs of their plated bucklers were habitually
exposed. Another strange class of fish-like creatures was represented
by a little creature which was found in Scotland and is sometimes
supposed to be the ancestor of the lamprey.
Besides the fresh-water fishes there were some which dwelt in the
sea; but in the Devonian era the fresh-water fishes were far more
numerous. We cannot mention them all. The fish called _Coccosteus_
and its allies had great bony plates of considerable thickness on
its head and shoulders (some fine examples are to be seen at the
Natural History Museum, Cromwell Road, London), but its tail and
middle body were left unprotected. The sharks of to-day had their
representatives among the Devonian fishes. Sharks have throughout
geological time nearly always been sea-dwellers, though they still
occasionally live in fresh water, as in Lake Baikal in Siberia and
Lake Nicaragua. It seems clear, however, that in the Devonian period
they lived in the open sea. But their remains are found in the Old
Red Sandstone, and therefore it is likely that they lived in fresh
and brackish waters also. In the same strata as these remarkable
fishes there are found some large and peculiar crustaceans, something
like our modern king crabs, but reaching the enormous length of six
feet. There have also been mussels found and a few water plants, but
not many.
In the Devonian relics the land vegetation has for the first time
been fairly well preserved. The huge club mosses made good their
tenure on the land; and along the flats and low-lying lands by the
rivers there were dense brakes of reedy calamites and masses of true
ferns. The club mosses and the calamites diminished from their giant
size eventually, but the ferns went on increasing, and ancestral
types of the pines and the yews began to appear. The vegetation of
Devonian times was sombre; there could have been no flowers, and the
insects were not of the kind that speed from bloom to bloom. Insects
there were, gigantic dragon-flies and insects akin to the many flies
that haunt the water; but the myriad buzz of insect life as we know
it in field and forest was not yet heard. It is rather an interesting
fact that unmistakable evidence has been collected of the existence
in Devonian times of those smallest of living things, the bacteria.
Of the general distribution of the land we cannot speak with great
certainty. The violent disturbances of Silurian times seem to have
ceased, but movements of the land did not cease. Great parts of
England were rising from the water, and stretching out above the
waves to Belgium and Northern France. There was no German Ocean and
no St. George's Channel at the end of the period; and Scotland,
also rising above the waves, was accumulating deposits of volcanic
ash and lava. While, however, the British Isles and great parts of
Belgium, Denmark, Scandinavia, and Western Russia, and smaller areas
in mid-France, mid-Germany, and the Balkans were rising the rest of
Europe was submerged beneath the waters. In the United States there
were similar risings and sinkings of the land, but, on the whole, the
course of geological history seems to have been more peaceful across
the Atlantic. In Europe, as in America, there do not seem to have
been notable changes at the end of the Devonian, though there was
some alteration in level in Russia, Bohemia, and Great Britain. The
rolling waste of waters south of the Bristol Channel began to deepen.
The continental area in which the Old Red Sandstone lakes lay (a kind
of far Western Europe without a Russia) began now to sink in its
turn. All of the British Isles, except a very thin slice just cut
across the Midlands from North Wales to Norfolk, was sunk beneath
the sea. The lakes disappeared, and above their deposits, as above
the rest of England and nearly all Europe except Scandinavia and
patches of Spain, Italy, and the Balkans, a deep ocean rolled, and
for many thousands of years deposited a grey ooze of limestone. This
limestone is called the Carboniferous or Mountain Limestone. But as
time went on this old sea floor began to be slowly raised, and in
the shallower waters a great quantity of coarse sand and stones and
conglomerate--the Millstone Grit, as it is called--was deposited.
Limestone denotes clear seas; but the borders of clear seas are often
the sites of accumulation of land rocks, and the clear waters of the
early Carboniferous sea which stretched from Ireland to the north of
Europe were bordered by shores along which mud and shale, gravel and
sand were deposited.
The end of this period was marked in Europe by great disturbances
of the earth's crust--though perhaps these disturbances, as we have
shown in a previous chapter, were not sudden or violent, but were
slow upheavals, lasting hundreds of thousands of years. It was at
this time that a great system of mountains, sometimes referred to
as the Palæozoic Alps, began to rise. This system of mountains
crossed the central part of Europe from the Western Islands to the
Sudetes Mountains in the east. Their remnants are seen in the Vosges
Mountains, the Hartz Mountains, and the Black Forest at the present
time; and the development of the Ural Mountains was contemporaneous
with them. During this time a mild climate spread all over Europe,
and as far north as Spitzbergen the waters were warm enough to
support coral reefs and plants which we associate with the seas of
genial latitudes. In time the Carboniferous sea became quite filled
up; and its floor was raised up to or a little above the waters. Then
in great swamps, marshes, and low lands, the burgeoning vegetable
life of the northern hemisphere entered on its long-deferred reign.
It was then that the coal which we burn in our grates to-day was laid
down. Let us consider the circumstances in which coal is to be found.
The coal formations, as we know them, are found in the same state,
and evidently laid down in the same era, from the Equator up to
Melville Island in the Arctic regions, where in our day it is always
freezing. They stretch from Nova Zembla to the middle of China; and
they are much the same in New Zealand and New South Wales. Therefore
the first conclusion we draw was that nearly all over the globe the
climate was the same--hot, close, moist, muggy. Whatever the climate
was the growth of vegetation was tremendous.
We shall have presently to say a little more about the vegetation;
but for the present we need only say that it was very different
from the vegetation with which most of us are familiar. Imagine a
hot, damp atmosphere, a kind of perpetual warm fog through which
the rays of the sun struggled with difficulty, and where rain fell
on most days of the year--a perpetual steaming hothouse. There
was little variety in the appearance of the vast forest swamps.
It certainly possessed, wrote Louis Figuier, the advantage of
size and rapid growth; but how poor it was in species--how uniform
in appearance! No flowers yet adorned the foliage or varied the
tints of the forests. Eternal verdure clothed the branches of the
Ferns, the Lycopods (club mosses), and Equiseta, which composed to
a great extent the vegetation of the age. The forests presented an
innumerable collection of individuals, but very few species, and all
belonging to the lower types of vegetation. No fruit appeared fit for
nourishment; none would seem to have been on the branches. Suffice
it to say that few land animals seem to have existed yet; while the
vegetable kingdom occupied the land, which at a later period was more
thickly inhabited by air-breathing animals. A few winged insects gave
animation to the air while exhibiting their variegated colours; and
many mollusca (such as land-snails) lived at the same time.
Ultimately all this richness of vegetation became by decay, by
compression, by submergence, perhaps by being buried under earthquake
movements and volcanic outbreaks, converted into coal; and we
may now ask how long did this process take. A vigorous growth of
vegetation has been estimated to yield annually about one ton of
dried vegetable matter per acre, or 640 tons to the square mile. If
this annual growth of vegetable matter were all preserved for 1000
years, and compressed till it was as dense and heavy as coal, it
would form a layer about seven inches thick. But a large portion of
the vegetable matter even in peat bogs escapes as gas in the making
of coal. Four-fifths of it escapes in this way. If this be true the
seven-inch layer would be reduced to less than one and a half inches,
and a layer a foot in thickness would require between 8000 and 9000
years. The aggregate thickness of coal is frequently as much as 100
feet (when all the thicknesses of the seams are added together), and
sometimes as much as 250 feet. At the foregoing rate of accumulation
periods ranging from 1,000,000 to 2,500,000 years would be needed
for the accumulation of such thicknesses of coal. It must be borne
in mind that much depends on the rate of growth of Carboniferous
vegetation, which may have been, and probably was, much more rapid
than any we know outside tropical forests. On the other hand, we
have been speaking of the aggregate thickness of the coal beds only.
The greater part of the coal-bearing strata consists of shale and
sandstone with layers or seams of coal like streaky bacon. Of the
shale and sandstone there are thousands of feet, even where the
sediments are fine and their accumulations therefore probably slow.
For, as we have said, this was a period of great change, in which
the forests were always sinking and rising again, being submerged by
lakes, being covered by the sea, and again emerging as islands, to be
overrun by vegetation.
As sinks some sylvan scene in all its pride
Changed to lagoons of overflowing tide,
Assiduous labours Land to win again
Her leafy breadths, invaded by the main.
Down bring the rivers to the flooded shore
Cargoes of shale and silt that slow restore
The sunken glebes, till they again can hold
Thick ferny brakes, and forests as of old.
(H. R. Knipe.)
It would hardly seem unreasonable to suppose that these depositions,
and the changes that brought them about, might have occupied as
much time as the formation of the coal beds. This would double the
figures, and make this period last something between two million and
five million years.
In the coal as we know it are the remains of great forest trees;
gigantic tree-ferns for the most part, and of many small plants
forming a close thick sod, partially buried in whole countries of
marsh land.
Every one knows those marsh plants, which bear the vulgar name of
"mare's-tail." These humble plants were represented during the coal
period by trees from twenty to thirty feet high and four to six
inches in diameter. Their trunks have been preserved to us: they bear
the name of _Calamites_.
The _Lycopods_ of our age are humble plants, scarcely a yard in
height, and most commonly creepers; but those of the ancient world
were trees of eighty or ninety feet in height. Their leaves were
sometimes twenty inches long, and their trunks a yard in diameter.
Such are the dimensions of some specimens which have been found.
Another Lycopod of this period attained dimensions still more
colossal. The _Sigillarias_ sometimes exceeded 100 feet in height.
Herbaceous Ferns were also exceedingly abundant and grew beneath
the shade of these gigantic trees. It was the combination of these
lofty trees with the undergrowth of smaller vegetation which formed
the forests of the Carboniferous period. "What could be more
surprising," exclaims Figuier, "than the aspect of this exuberant
vegetation, these immense trees, these elegant arborescent ferns
with airy foliage, fine cut, like delicate lace. Nothing at the
present day can convey to us an idea of the prodigious and immense
extent of never-changing verdure which clothed the earth, from pole
to pole, under the high temperature which everywhere prevailed over
the whole terrestrial globe. In the depths of these inextricable
forests parasitic plants were suspended from the trunks of the great
trees, in tufts or garlands, like the wild vines of our tropical
forests. They were nearly all pretty, fern-like plants, they attached
themselves to the stems of the great trees, like the orchids of our
times." The margin of the waters would also be covered with various
plants with light and whorled leaves, belonging, perhaps, to the
Dicotyledons. Before leaving the subject of the plants of the coal
measures, we should perhaps mention as one of the most interesting
discoveries of the present generation that whereas the links between
the fern-like trees of those days and the cycads, or early group of
seed-bearing plants, were for long missing, they have been found by
the researches of Professor F. W. Oliver, F.R.S., who has identified
in the _Lyginodendron_ a seed-bearing fern from the coal measures.
We must now turn to the less interesting but not less important topic
of the animal life of the Carboniferous period. At the beginning of
the period when only a small portion of the British Isles was above
the waters, and an ocean rolled from Ireland to China, the life of
which the most important relics were left was that of the sea. In
the early Carboniferous seas the rhizopods, some small as dust, laid
down with their tiny shells the foundations of mountains yet to be;
the "sea lilies" were at the height of their pride, occupying vast
areas in the flowing tide; forms like the present-day nautilus began
to appear, and the "lamp-shells" attained their greatest size. The
trilobites, hitherto the most conspicuous and noticeable animals of
the earth's childhood, were beginning to die out, vanquished in the
struggle for life by more adaptable forms, and the big sea scorpions
were waning fast. The king crabs and the water fleas still throve,
and the fishes, though most of them not very large, were growing
larger, some of them taking the appearance of the dog-fish, some of
the ray, some of the shark; and, what is more important than the
fact of size, the fishes were growing speedier and more capable of
attacking weaker creatures.
In the course of these ages the sea invaded the land; and shores
where land-snails and millipedes and centipedes, beetles and
scorpions, spiders and cockroaches had found a home became entirely
changed, not only in their appearance and character, but in the
type which subsisted on them. It is possible (for something of the
kind has been noticed in our own days in the West Indies, where
a sea-crab species is showing signs of becoming a land animal)
that some of the forms of water animals became used to living in
shallower and shallower water as the generations went on till they
became partly land and partly water animals--amphibians, as they
are called. Thus small newt-like beings, moving clumsily through
the swamps, made their appearance, and others with stronger limbs
pushed onwards through the forest. Others in form resembling snakes
crept through the mud and lived among the swamps by the side of the
sea. Not much is known of the food and life-habits of any of these
amphibians. From their teeth we may perhaps judge that they lived on
fish, crustaceans, insects, and on one another, and their predatory
life sometimes led them to climb trees in search of food. What,
however, is most important about the amphibian is that they were the
pioneers of the march of those creatures which had backbones--the
vertebrates--from the sea to the land.