<h2 class="nobreak" id="CHAPTER_XVIII">CHAPTER XVIII</h2>
<h3>HOW THE COAL BEDS WERE LAID DOWN</h3></div>
<p>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 <span class="smcap">Old Red Sandstone</span>,
and to the upper and newer ones the name of the
<span class="smcap">New Red Sandstone</span>. 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 <i>marine</i> 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
<span class="pagenum" id="Page_213">-213-</span>
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.</p>
<p>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
<span class="pagenum" id="Page_214">-214-</span>
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.</p>
<p>"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...."
<i>The Old Red Sandstone</i>, a book which was the result
of Hugh Miller's researches, is a geological classic.</p>
<p>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
<span class="pagenum" id="Page_215">-215-</span>
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.</p>
<p>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 <i>Ostracoderm</i> group, and they bear
strange resemblance to some of the trilobites and the
king crabs of previous eras. The <i>Pteraspis</i> 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 <i>Pterichthyds</i>
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.</p>
<p>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 <i>Coccosteus</i> and its allies had great bony
<span class="pagenum" id="Page_216">-216-</span>
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.</p>
<p>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
<span class="pagenum" id="Page_217">-217-</span>
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.</p>
<p>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.</p>
<p>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
<span class="pagenum" id="Page_218">-218-</span>
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.</p>
<p>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
<span class="pagenum" id="Page_219">-219-</span>
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.</p>
<p>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
<span class="pagenum" id="Page_220">-220-</span>
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.</p>
<p>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.
<span class="pagenum" id="Page_221">-221-</span>
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.</p>
<div class="poetry-container">
<div class="poetry">
<div class="stanza">
<div class="verse indent0">As sinks some sylvan scene in all its pride</div>
<div class="verse indent0">Changed to lagoons of overflowing tide,</div>
<div class="verse indent0">Assiduous labours Land to win again</div>
<div class="verse indent0">Her leafy breadths, invaded by the main.</div>
<div class="verse indent0">Down bring the rivers to the flooded shore</div>
<div class="verse indent0">Cargoes of shale and silt that slow restore</div>
<div class="verse indent0">The sunken glebes, till they again can hold</div>
<div class="verse indent0">Thick ferny brakes, and forests as of old.</div>
</div></div>
</div>
<p>(<span class="smcap">H. R. Knipe.</span>)</p>
<p><span class="pagenum" id="Page_222">-222-</span></p>
<p>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.</p>
<p>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.</p>
<p>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 <i>Calamites</i>.</p>
<p>The <i>Lycopods</i> 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 <i>Sigillarias</i> 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
<span class="pagenum" id="Page_223">-223-</span>
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, <span class="allsmcap">F.R.S.</span>, who has
identified in the <i>Lyginodendron</i> a seed-bearing fern from
the coal measures.</p>
<p>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
<span class="pagenum" id="Page_224">-224-</span>
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.</p>
<p>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,
<span class="pagenum" id="Page_225">-225-</span>
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.</p>
<hr class="chap x-ebookmaker-drop" />
<p><span class="pagenum" id="Page_226">-226-</span></p>
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