CHAPTER XVIII
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.