[L]Verril has suggested that the Tabulata may be divided into two groups, one referable to Actinoids, the other to Hydroids.
[L]Verril has suggested that the Tabulata may be divided into two groups, one referable to Actinoids, the other to Hydroids.
If the Devonian witnessed the culmination of the Palæozoic corals, its later stages saw the final decadence of the great dynasty of the Trilobites. Of these creatures there are in the Devonian some large and ornate species, remarkable for their spines and tubercles; as if in this, the latter day of their dominion, they had fallen into habits of luxurious decoration unknown to their sterner predecessors, and at the same time had found it necessary to surround their now disputed privileges with new safeguards of defensive armour. Not improbably the decadence of the Trilobites may have been connected with the introduction of the numerous and formidable fishes of the period.
But while the venerable race of the Trilobites was preparing to fight its last and unsuccessful battle, another and scarcely less ancient tribe of crustaceans, the Eurypterids, already strong in the Silurian, was armed with new and formidable powers. ThePterygotus anglicus, which should have been namedscoticus, since its head-quarters are in Scotland, was in point of size the greatest of known crustaceans, recent or fossil. According to Mr. Henry Woodward, who has published an admirable description and figures of the creature in the PalæontographicalSociety’s Memoirs, it must have been six feet in length, and nearly two feet in breadth. Its antennæ were, unlike the harmless feelers of modern Crustacea, armed with powerful claws. Two great eyes stood in the front of the head, and two smaller ones on the top. It had four pairs of great serrated jaws, the largest as wide as a man’s hand. At the sides were a pair of powerful paddles, capable of urging it swiftly through the water as it pursued its prey; and when attacked by any predaceous fish, it could strike the water with its broad tail, terminated by a great flat “telson,” and retreat backward with the rapidity of an arrow. Woodward says it must have been the “shark of the Devonian seas;” rather, it was the great champion of the more ancient family of the lobsters, set to arrest, if possible, the encroachments of the coming sharks.
The Trilobites and Eurypterids constitute a hard case for the derivationists. Unlike those Melchisedeks, the fishes of the Silurian, which are without father or mother, the Devonian crustaceans may boast of their descent, but they have no descendants. No distinct link connects them with any modern crustaceans except the Limuli, or horse-shoe crabs; and here the connection is most puzzling, for while there seems some intelligible resemblance between the adult Eurypterids and the horse-shoe, or king-crabs, the latter, in their younger state, rather resemble Trilobites, as Dr. Packard has recently shown. Thus the two great tribes of Eurypterids and Trilobiteshave united in the small modern group of king-crabs, while on the other hand, there are points of resemblance, as already stated, between Trilobites and Isopods, and the king-crabs had already begun to exist, since one species is now known in the Upper Silurian. So puzzling are these various relationships, that one naturalist of the derivationist school has recently attempted to solve the difficulty by suggesting that the Trilobites are allied to the spiders! Thus nature sports with our theories, showing us in some cases, as in the corals and fishes, partnerships split up into individuals, and in others distinct lines of being converging and becoming lost in one slender thread. Barrande, the great palæontologist of Bohemia, has recently, in an elaborate memoir on the Trilobites, traced these and other points through all their structures and their whole succession in geological time thereby elaborating a most powerful inductive argument against the theory of evolution, and concluding that, so far from the history of these creatures favouring such a theory, it seems as if expressly contrived to exclude its possibility.
But, while the gigantic Eurypterids and ornate Trilobites of the Devonian were rapidly approaching their end, a few despised little crustaceans,—represented by theAmphipeltisof New Brunswick andKampecarisof Scotland,—were obscurely laying the foundation of a new line of beings, that of the Stomapods, destined to culminate in the Squillas and their allies, which, however different in structure, arepractically the Eurypterids of the modern ocean. So change the dynasties of men and animals.
"Thou takest away their breath, they die,They return to their dust;Thou sendest forth Thy Spirit,They are created;Thou renewest the form of the earth."
The reign of fishes began in the Upper Silurian, for in the rocks of this age, more especially in England, several species have been found. They occur, however, only in the newer beds of this formation, and are not of large size, nor very abundant. It is to be observed that, in so far as the fragments discovered can be interpreted, they indicate the existence already of two distinct types of fishes, the Ganoids, or gar-fishes, protected with bony plates and scales, and the Placoids, or shark-like fishes; and that in the existing world these fishes are regarded as occupying a high place in their class. Further, these two groups of fishes are those which throughout a large portion of geological time continue to prevail to the exclusion of other types, the ordinary bony fishes having been introduced only in comparatively recent periods. With the Devonian, however, there comes a vast increase to the finny armies; and so characteristic are these that the Devonian has been called the age of fishespar excellence, and we must try, with the help of our illustration, to paint these old inhabitants of the waters as distinctly as we can. Among themost ancient and curious of these fishes are those singular forms covered with broad plates, of which thePteraspisof the Upper Silurian is the herald, and which are represented in the Lower Devonian by several distinct genera. Of these, one of the most curious is theCephalaspis, or buckler-head, distinguished by its broad flat head, rounded in front and prolonged at the sides into two great spines, which project far beyond the sides of the comparatively slender body. This fish, it may be mentioned, is the type of a family highly characteristic of the Lower Devonian, as well as of the Upper Silurian, and all of which are provided with large plate-like cephalic coverings, sometimes with a long snout in front, and, in so far as is known, a comparatively weak body and tail. They were all probably ground-living creatures, feeding on worms and shell-fishes, and “rooting” for these in the mud, or burrowing therein for their safety. In these respects they have a most curious analogy to the Trilobites, which in habits they must have greatly resembled, though belonging by their structure to an entirely different and much higher class. So close is this resemblance, that their head-shields used to be mistaken for those of Trilobites. The case is one of those curious analogies which often occur in nature, and which must always be distinguished from the true affinities which rest on structural resemblances. Another group of small fishes, likewise cuirassed in bony armour of plates, may be represented by thePterichthys, with its two strong bony fins at the sides, which may have served for swimming, but probably also for defence, and for creeping on or shovelling up the mud at the bottom of the sea. But, besides the Ganoids which were armed in plated cuirasses, there were others, active and voracious, clad in shining enamelled scales, like the bony pikes of the American rivers and thePolypterusof the Nile. Some of these, like theDiplacanthus, or “double-spine” were of small size, and chiefly remarkable for their sharp defensive bony spines. Others, likeHoloptychius(wrinkled-scale) andOsteolepis(bone-scale), were strongly built, and sometimes of great size. One Russian species ofAsterolepis(star-scale) is supposed to have been twenty feet in length, and furnished with strong and trenchant teeth in two rows. These great fishes afford a good reason for the spines and armour-plates of the contemporary trilobites and smaller fishes. Just as man has been endeavouring to invent armour impenetrable to shot, for soldiers and for ships, and, on the other hand, shot and shells that can penetrate any armoury so nature has always presented the spectacle of the most perfect defensive apparatus matched with the most perfect weapons for destruction. In the class of fishes, no age of the world is more eminent in these respects than the Devonian.[M]In additionto these fishes, there were others, represented principally by their strong bony spines, which must have been allied to some of the families of modern sharks, most of them, however, probably to that comparatively harmless tribe which, furnished with flat teeth, prey upon shell-fishes. There are other fishes difficult to place in our systems of classification; and among these an eminent example is the hugeDinichthysof Newberry, from the Hamilton group of Ohio. The head of this creature is more than three feet long and eighteen inches broad, with the bones extraordinarily strong and massive. In the upper jaw, in addition to strong teeth, there were in front two huge sabre-shaped tusks or incisors, each nearly a foot long; and corresponding to these in the massive lower jaw were two closely joined conical tusks, fitting between those of the upper jaw. No other fish presents so frightful an apparatus for destruction; and if, as is probable, this was attached to a powerful body, perhaps thirty feet in length, and capable of rapid motion through the water, we cannot imagine any creature so strong or so well armed as to cope with the mightyDinichthys.
[M]Many of these were discovered and successfully displayed and described by Hugh Miller, and are graphically portrayed in his celebrated work on the “Old Red Sandstone,” published in 1841.
[M]Many of these were discovered and successfully displayed and described by Hugh Miller, and are graphically portrayed in his celebrated work on the “Old Red Sandstone,” published in 1841.
The difference between the fishes of the Devonian and those of the modern seas is well marked by the fact that, while the ordinary bony fishes now amount to probably 9,000 species, and the ganoid fishes to less than thirty, the finny tribes of the Devonian are predominantly ganoids, and none of the ordinary type are known. To what is this related, with referenceto conditions of existence? Two explanations, different yet mutually connected, may be suggested. One is that armour was especially useful in the Devonian as a means of defence from the larger predaceous species, and the gigantic crustaceans of the period. That this was the case may be inferred from the conditions of existence of some modern ganoids. The common bony pike of Canada (Lepidosteus), frequenting shallow and stagnant waters, seems to be especially exposed to injury from its enemies. Consequently, while it is rare to find an ordinary fish showing any traces of wounds, a large proportion of the specimens of the bony pike which I have examined have scars on their scales, indicating injuries which they have experienced, and which possibly, to fishes not so well armed, might have proved fatal. Again, in the modern Amia, or mud-fish, in the bony pike andPolypterus, there is an extremely large air-bladder, amply supplied with blood-vessels, and even divided into cells or chambers, and communicating with the mouth by an “air-duct.” This organ is unquestionably in function a lung, and enables the animal to dispense in some degree with the use of its gills, which of course depend for their supply of vital air on the small quantity of oxygen dissolved in the water. Hence, by the power of partially breathing air, these fishes can live in stagnant and badly aerated waters, where other fishes would perish. In the case of theAmia, the grunting noises which it utters, its habit of frequenting the muddy creeks of swamps,and its possession of gill-cleaners, correspond with this view. It is possible that the Devonian fishes possessed this semi-reptilian respiration; and if so, they would be better adapted than other fishes to live in water contaminated with organic matter in a state of decay, or in waters rich in carbonic acid or deficient in oxygen. Possibly the palæozoic waters, as well as the palæozoic atmosphere, were less rich in pure oxygen than those of the present world; and it is certain that, in many of the beds in which the smaller Devonian fishes abound, there was so much decaying vegetable matter as to make it probable that the water was unfit for the ordinary fishes. Thus, though at first sight the possession of external armour and means to respire air, in the case of these peculiar fishes, may seem to have no direct connection with each other, their obvious correlation in some modern ganoids may have had its parallel on a more extensive scale among their ancient relatives. Just as the modern gar-fish, by virtue of its lungs, can live in stagnant shallows and hunt frogs, but on that account needs strong armour to defend it against the foes that assail it in such places; so in the Devonian the capacity to inhabit unaërated water and defensive plates and scales may have been alike necessary, especially to the feebler tribes of fishes. We shall find that in the succeeding carboniferous period there is equally good evidence of this.
We have reserved little space for the Devonian plants and insects; but we may notice both in a walkthrough a Devonian forest, in which we may include the vegetation of the several subordinate periods into which this great era was divisible. The Devonian woods were probably, like those of the succeeding carboniferous period, dense and dark, composed of but few species of plants, and these somewhat monotonous in appearance, and spreading out into broad swampy jungles, encroaching on the shallow bays and estuaries. Landing on one of these flats, we may first cast our eyes over a wide expanse, covered with what at a distance we might regard as reeds or rushes. But on a near approach they appear very different; rising in slender, graceful stems, they fork again and again, and their thin branches are sparsely covered with minute needle-like leaves, while the young shoots curl over in graceful tresses, and the older are covered with little oval fruits, or spore-cases; for these plants are cryptogamous, or flowerless. This singular vegetation stretches for miles along the muddy flats, and rises to a height of two or three feet from a knotted mass of cylindrical roots or root-stocks, twining like snakes through and over the soil. This plant may, according as we are influenced by its fruit or structure, be regarded as allied to the modern club-mosses or the modern pill-worts. It isPsilophyton, in every country one of the most characteristic plants of the period, though, when imperfectly preserved, often relegated by careless and unskilled observers to the all-engulfing group of fucoids. A little further inland we see a grove of graceful trees, forking likePsilophyton,but of grander dimensions, and with the branches covered with linear leaves, and sometimes terminated by cones. These areLepidodendra, giganticclub-mosses, which were developed to still greater dimensions in the coal period. Near these we may see a still more curious tree, more erect in its growth, with rounded and somewhat rigid leaves and cones of different form, and with huge cable-like roots, penetrating the mud, and pitted with the marks of long rootlets. This isCyclostigma, a plant near to theLepidodendron, but distinct, and peculiar to the Devonian. Some of its species attain to the dimensions of considerable trees; others are small and shrubby. Another small tree, somewhat like the others, but with very long shaggy leaves, and its bark curiously marked with regular diamond-shaped scars, is theLeptophleum. All these plants are probably allied to our modern club-mosses, which are, however, also represented by some low and creeping species cleaving to the ground. A little further, and we reach a dense clump ofSigillariæ, with tall sparsely forking stems, and ribbed with ridges holding rows of leaf-scars a group of plants which we shall have further occasion to notice in the coal formation; and here is an extensive jungle ofCalamites, gigantic and overgrown mares'-tails, allies of the modern equisetums.
Fig. 12.—VEGETATION OF THE DEVONIAN.To the left areCalamites; next to these,Leptophleum; in the centre areLepidodendron,Sigillaria, and a Pine. Below arePsilophyton,Cordaites, Ferns, andAsterophyllites.
Fig. 12.—VEGETATION OF THE DEVONIAN.
To the left areCalamites; next to these,Leptophleum; in the centre areLepidodendron,Sigillaria, and a Pine. Below arePsilophyton,Cordaites, Ferns, andAsterophyllites.
Amidst these trees, every open glade is filled with delicate ferns of marvellous grace and beauty; and here and there a tree-fern rears its head, crowned with its spreading and graceful leaves, and its trunk clad with a shaggy mass of aërial roots—an old botanical device, used in these ancient times, as wellas now, to strengthen and protect the stems of trees not fitted for lateral expansion. Beyond this mass of vegetation, and rising on the slopes of the distant hills, we see great trees that look like pines. We cannot approach them more nearly; but here on the margin of a creek we see some drift-trunks, that have doubtless been carried down by a land flood. One of them is certainly a pine, in form and structure of its wood very like those now living in the southern hemisphere; it is aDadoxylon. Another is different, its sides rough and gnarled, and marked with huge irregular ridges; its wood loose, porous, and stringy, more like the bark of modern pines, yet having rings of growth and a true bark of its own, and sending forth large branches and roots. It is the strange and mysteriousPrototaxites, one of the wonders of the Devonian land, and whose leaves and fruits would be worth their weight in gold in our museums, could we only procure them. A solitary fragment further indicates that in the yet unpenetrated solitudes of the Devonian forests there may be other trees more like our ordinary familiar friends of the modern woods; but of these we know as yet but little. What inhabitants have these forests? All that we yet know are a few large insects, relatives of our modern May-flies, flitting with broad veined wings over the stagnant waters in which their worm-like larvæ dwell, and one species at least assuming one of the properties of the grasshopper tribe, and enlivening the otherwise silent groves with a cricket-like chirp, the oldestmusic of living things that geology as yet reveals to us; and this, not by the hearing of the sound itself, but by the poor remains of the instrument attached to a remnant of a wing from the Devonian shales of New Brunswick.
A remarkable illustration of the abundance of certain plants in the Devonian, and also of the slow and gradual accumulation of some of its beds, is furnished by layers of fossil spore-cases, or the minute sacs which contain the microscopic germs of club-mosses and similar plants. In the American forests, in spring, the yellow pollen-grains of spruces and pines sometimes drift away in such quantities in the breeze that they fall in dense showers, popularly called showers of sulphur; and this vegetable sulphur, falling in lakes and ponds, is drifted to the shore in great sheets and swathes. The same thing appears to have occurred in the Devonian, not with the pollen of flowering plants, but with the similar light spores and spore-cases of species of Lepidodendron and allied trees. In a bed of shale, at Kettle Point, Lake Huron, from 12 to 14 feet thick, not only are the surfaces of the beds dotted over with minute round spore-cases, but, on making a section for the microscope, the substance of each layer is seen to be filled with them; and still more minute bodies, probably the escaped spores, are seen to fill up their interstices. The quantity of these minute bodies is so great that the shale is combustible, and burns with much flame. A bed of this nature must have been formed inshallow and still water, on the margin of an extensive jungle or forest; and as the spore-cases are similar to those of the Lepidodendra of the coal-measures, the trees were probably of this kind. Year after year, as the spores became ripe, they were wafted away, and fell in vast quantities into the water, to be mixed with the fine mud there accumulating. When we come to the coal period, we shall see that such beds of spore-cases occur there also, and that they have even been supposed to be mainly instrumental in the accumulation of certain beds of coal. Their importance in this respect may have been exaggerated, but the fact of their occurrence in immense quantities in certain coals and shales is indisputable.
This is but a slender sketch of the Devonian forests: but we shall find many of the same forms of plants in the carboniferous period which succeeds. With one thought we may close. We are prone to ask for reasons and uses for things, but sometimes we cannot be satisfied. Of what use were the Devonian forests? They did not, like those of the coal formation, accumulate rich beds of coal for the use of man. Except possibly a few insects, we know no animals that subsisted on their produce, nor was there any rational being to admire their beauty. Their use, except as helping us in these last days to complete the order of the vegetable kingdom as it has existed in geological time, is a mystery. We can but fall back on that ascription of praise to Him “who liveth for ever and ever,” on the part of the heavenlyelders who cast down their crowns before the throne and say, “Thou art worthy, Lord, to receive the glory, and the honour, and the might; because Thou didst create all things, and by reason ofThy willthey are and were created.”
CHAPTER VI.
THE CARBONIFEROUS AGE.
Thatage of the world’s history which, from its richness in accumulations of vegetable matter destined to be converted into coal, has been named the Carboniferous, is in relation to living beings the most complete and noble of the Palæozoic periods. In it those varied arrangements of land and water which had been increasing in perfection in the previous periods, attained to their highest development. In it the forms of animal and plant life that had been becoming more numerous and varied from the Eozoic onward, culminated. The Permian which succeeded was but the decadence of the Carboniferous, preparatory to the introduction of a new order of things. Thus the Carboniferous was to the previous periods what the Modern is to the preceding Tertiary and Mesozoic ages the summation and completion of them all, and the embodiment of their highest excellence. If the world’s history had closed with the Carboniferous, a naturalist, knowing nothing further, would have been obliged to admit that it had already fulfilled all the promise of its earlier years. It is important to remember this, since we shall find ourselves entering on an entirely new scene in the Mesozoicperiod, and since this character of the Carboniferous, as well as its varied conditions and products, may excuse us for dwelling on it a little longer than on the others, On the other hand, the immense economic importance of the coal formation, and the interesting points connected with it, have made the Carboniferous more familiar to general readers than most other geological periods, so that we may select points less common and well-known for illustration. Popular expositions of geology are, however, generally so one-sided and so distorted by the prevalent straining after effect, that the true aspect of this age is perhaps not much better known than that of others less frequently described.
Let us first consider the Carboniferous geography of the northern hemisphere; and in doing so we may begin with a fact concerning the preceding age. One of the most remarkable features of the Newer Devonian is the immense quantity of red rocks, particularly red sandstones, contained in it. Ked sandstones, it is true, occur in older formations, but comparatively rarely; their great head-quarters, both in Europe and America, in so far as the Palæozoic is concerned, are in the Upper Devonian. Now red sandstone is an infallible mark of rapid deposition, and therefore of active physical change. If we examine the grains of sand in a red sandstone, we shall find that they are stained or coated, externally, with the peroxide of iron, or iron rust; and that this coating, with perhaps a portion of the same substance in the interveningcement, is the cause of the colour. In finer sandstones and red clays the same condition exists, though less distinctly perceptible. Conrrequently, if red sands and clays are long abraded or scoured in water, or are subjected to any chemical agent capable of dissolving the iron, they cease to be red, and resume their natural grey or white colour. Now in nature, in addition to mechanical abrasion, there is a chemical cause most potent in bleaching red rocks, namely, the presence of vegetable or animal matter in a state of decay. Without entering into chemical details, we may content ourselves with the fact that organic matter decaying in contact with peroxide of iron tends to take oxygen from it, and then to dissolve it in the state of protoxide, while the oxygen set free aids the decay. Carrying this fact with us, we may next affirm that iron is so plentiful in the crust of the earth that nearly all sands and clays when first produced from the weathering of rocks are stained with it, and that when this weathering takes place in the air, the iron is always in the state of peroxide. More especially does this apply to the greater number of igneous or volcanic rocks, which nearly always weather brown or red. Now premising that the original condition of sediment is that of being reddened with iron, and that it may lose this by abrasion, or by the action of organic matter, it follows that when sand has been produced by decay of rocks in the air, and when it is rapidly washed into the sea and deposited there, red beds will result.For instance, in the Bay of Fundy, whose rapid tides cut away the red rocks of its shores and deposit their materials quickly, red mud and sand constitute the modern deposit. On the other hand, when the red Band and mud are long washed about, their red matter may disappear; and when the deposition is slow and accompanied with the presence of organic matter, the red colour is not only removed, but is replaced by the dark tints due to carbon. Thus, in the Gulf of St. Lawrence, where red rocks similar to those of the Bay of Fundy are being more slowly wasted, and deposited in the presence of sea-weeds and other vegetable substances, the resulting sands and clays are white and grey or blackened in colour. An intermediate condition is sometimes observed, in which red beds are stained with grey spots and lines, where sea-weeds or land-plants have rested on them. I have specimens of Devonian red shale with the forms of fern leaves, the substance of which has entirely perished, traced most delicately upon them in greenish marks.
It follows from these facts that extensive and thick deposits of red beds evidence sub-aërial decay of rocks, followed by comparatively rapid deposition in water, and that such red rocks will usually contain few fossils, not only because of their rapid deposition, but because the few organic fragments deposited with them will probably have been destroyed by the chemical action of the superabundant oxide of iron, which, so to speak, “iron-moulds” them, just as stains of iron eat holes out of linen. Now when SirRoderick Murchison tells us of 10,000 feet in thickness of red iron-stained rocks in the old red sandstone of England, we can see in this the evidence of rapid aqueous deposition, going on for a very long time, and baring vast areas of former land surface. Consequently we have proof of changes of level and immense and rapid denudation—a conclusion further confirmed by the apparent unconformity of different members of the series to each other in some parts of the British Islands, the lower beds having been tilted up before the newer were deposited. Such was the state of affairs very generally at the close of the Devonian, and it appears to have been accompanied with some degree of subsidence of the land, succeeded by re-elevation at the beginning of the Carboniferous, when many and perhaps large islands and chains of islands were raised out of the sea, along whose margins there were extensive volcanic eruptions, evidenced by the dykes of trap traversing the Devonian, and the beds of old lava interstratified in the lower part of the Carboniferous, where also the occurrence of thick beds of conglomerate or pebble-rock indicates the tempestuous action of the sea.
But a careful study of the Lower Carboniferous beds, where their margins rest upon the islands of older rocks, shows great varieties in these old shores. In some places there were shingly beaches; in others, extensive sand-banks; in others, swampy flats clothed with vegetation, and sometimes bearing peaty beds, still preserved as small seams of coal. The bays andcreeks swarmed with, fishes. A few sluggish reptiles crept along the muddy or sandy shores, and out sea-ward were great banks and reefs of coral and shells in the clear blue sea. The whole aspect of nature, taken in a general view, in the Older Carboniferous period, must have much resembled that at present seen among the islands of the southern hemisphere. And the plants and animals, though different, were more like those of the modern South Pacific than any others now living.
As the age wore on, the continents were slowly lifted out of the water, and the great continental plateaus were changed from coral seas into swampy flats or low uplands, studded in many places with shallow lakes, and penetrated with numerous creeks and sluggish streams. In the eastern continent these land surfaces prevailed extensively, more especially in the west; and in America they spread both eastward and westward from the Appalachian ridge, until only a long north and south Mediterranean, running parallel to the Rocky Mountains, remained of the former wide internal ocean. On this new and low land, comparable with the “Sylvas” of the South American continent, flourished the wondrous vegetation of the Coal period, and were introduced the new land animals, whose presence distinguishes the close of the Palæozoic.
After a vast lapse of time, in which only slow and gradual subsidence occurred, a more rapid settlement of the continental areas brought the greater part ofthe once fertile plains of the coal formation again under the waters; and shifting sand-banks and muddy tides engulfed and buried the remains of the old forests, and heaped on them a mass of sediment, which, like the weights of a botanical press, flattened and compressed the vegetabledébrispreserved in the leaves of the coal formation strata. Then came on that strange and terrible Permian period, which, like the more modern boulder-formation, marked the death of one age and the birth of another.
The succession just sketched is the normal one; but the terms in which it has been described show that it cannot be universal. There are many places in which the whole thickness of the Carboniferous is filled with fossils of the land, and of estuaries and creeks. There are places, on the other hand, where the deep sea appears to have continued during the whole period. In America this is seen on the grandest scale in the absence of the marine members along the western slopes of the Appalachians, and the almost exclusive prevalence of marine beds in the far west, where the great Carboniferous Mediterranean of America spread itself, and continued uninterruptedly into the succeeding Permian period.
In our survey of the Carboniferous age, though there are peculiarities in the life of its older, middle, and newer divisions, we may take the great coal measures of the middle portion as the type of the land life of the period, and the great limestones of the lower portion as that of the marine life; and asthe former is in this period by far the most important, we may begin with it. Before doing so, however, to prevent misapprehension, it is necessary to remind the reader that the Flora of the Middle Coal Period is but one of a succession of related floras that reach from the Upper Silurian to the Permian. The meagre flora of club-mosses and their allies in the Upper Silurian and Lower Devonian was succeeded by a comparatively rich and varied assemblage of plants in the Middle Devonian. The Upper Devonian was a period of decadence, and in the Lower Carboniferous we have another feeble beginning, presenting features somewhat different from those of the Upper Devonian. This was the time of the Culm of Germany, the Tweedian formation of the North of England and South of Scotland, and the Lower Coal formation of Nova Scotia. It was a period eminently rich in Lepidodendra. It was followed by the magnificent flora of the Middle Coal formation, and then there was a time of decadence in the Upper Coal formation and only a slight revival in the Permian.
In the present condition of our civilization, coal is the most important product which the bowels of the earth afford to man. And though there are productive beds of coal in most of the later geological formations, down to the peats of the modern period, which are only unconsolidated coals, yet the coal of the Carboniferous age is the earliest valuable coal in point of time, and by far the most important in point of quantity. Mineral coal may be defined to be vegetable matter which has been buried in the strata of the earth’s crust, and there subjected to certain chemical and mechanical changes. The proof of its vegetable origin will grow upon us as we proceed. The chemical changes which it has undergone are not very material. Wood or bark, taken as an example of ordinary vegetable matter, consists of carbon or charcoal, with the gases hydrogen and oxygen. Coal has merely parted with a portion of these ingredients in the course of a slow and imperfect putrefaction, so that it comes to have much less oxygen and considerably less hydrogen than wood, and it has been blackened by the disengagement of a quantity of free carbon. The more bituminous flaming coals have a larger amount of residual hydrogen. In the anthracite coals the process of carbonisation has proceeded further, and little remains but charcoal in a dense and compact form. In cannel coals, and in certain bituminous shales, on the contrary, the process seems to have taken place entirely under water, by which putrefaction has been modified, so that a larger proportion than usual of hydrogen has been retained. The mechanical change which the coal has experienced consists in the flattening and hardening effect of the immense pressure of thousands of feet of superincumbent rock, which has crashed together the cell-walls of the vegetable matter, and reduced what was originally a pulpy mass of cellular tissue to the condition of a hard laminated rock. To understand this, perhaps thasimplest way is to compare under the microscope a transverse section of recent pine-wood with a similar section of a pine trunk compressed into brown coal or jet. In the one the tissue appears as a series of meshes with thin woody walls and comparatively wide cavities for the transmission of the sap. In the other the walls of the cells have been forced into direct contact, and in some cases have altogether lost their separate forms, and have been consolidated into a perfectly compact structureless mass.
With regard to its mode of occurrence, coal is found in beds ranging in vertical thickness from less than an inch to more than thirty feet, and of wide horizontal extent. Many such beds usually occur in the thickness of the coal formation, or “coal measures,” as the miners call it, separated from each other by beds of sandstone and compressed clay or shale. Very often the coal occurs in groups of several beds, somewhat close to each other and separated from other groups by “barren measures” of considerable thickness. In examining a bed of coal, where it is exposed in a cutting or shore cliff, we nearly always find that the bed below it, or the “underclay,” as it is termed by miners, is a sort of fossil soil, filled with roots and rootlets. On this rests the coal, which, when we examine it closely, is found to consist of successive thin layers of hard coal of different qualities as to lustre and purity, and with intervening laminae of a dusty fibrous substance, like charcoal, called “mother coal” by miners, and sometimes mineralcharcoal. Thin partings of dark shale also occur, and these usually present marks and impressions of the stems and leaves of plants. Above the coal is its “roof” of hardened clay or sandstone, and this generally holds great quantities of remains of plants, and sometimes large stumps of trees with their bark converted into coal, and the hollow once occupied with wood filled with sandstone, while their roots spread over the surface of the coal. Such fossil forests of erect stumps are also found at various levels in the coal measures, resting directly on under-clays without any coals. A bed of coal would thus appear to be a fossil bog or swamp.
This much being premised about the general nature of the sooty blocks which fill our coal-scuttles, we may now transport ourselves into the forests and bogs of the coal formation, and make acquaintance with this old vegetation, while it still waved its foliage in the breeze and drank in the sunshine and showers. We are in the midst of one of those great low plains formed by the elevation of the former sea bed. The sun pours down its fervent rays upon us, and the atmosphere, being loaded with vapour, and probably more rich in carbonic acid than that of the present world, the heat is as it were accumulated and kept near the surface, producing a close and stifling atmosphere like that of a tropical swamp. This damp and oppressive air is, however, most favourable to the growth of the strange and grotesque trees which tower over our heads, andto the millions of delicate ferns and club-mosses, not unlike those of our modern woods, which carpet the ground. Around us for hundreds of miles spreads a dense and monotonous forest, with here and there open spaces occupied by ponds and sluggish streams, whose edges are bordered with immense savannahs of reed-like plants, springing from the wet and boggy soil. Everything bespeaks a rank exuberance of vegetable growth; and if we were to dig downward into the soil, we should find a thick bed of vegetable mould evidencing the prevalence of such conditions for ages. But the time will come when this immense flat will meet with the fate which in modern times befell a large district at the mouth of the Indus. Quietly, or with earthquake shocks, it will sink under the waters; fishes and mollusks will swarm where trees grew, beds of sand and mud will be deposited by the water, inclosing and preserving the remains of the vegetation, and in some places surrounding and imbedding the still erect trunks of trees. Many feet of such deposits may be formed, and our forest surface, with its rich bed of vegetable mould, has been covered up and is in process of transformation into coal; while in course of time the shallow waters being filled up with deposit, or a slight re-elevation occurring, a new forest exactly like the last will flourish on the same spot. Such changes would be far beyond the compass of the life even of a Methuselah; but had we lived in the Coal period, we might have seen all stages of thesaprocesses contemporaneously in different parts of either of the great continents.
But let us consider the actual forms of vegetation presented to us in the Coal period, as we can restore them from the fragments preserved to us in the beds of sandstone and shale, and as we would have seen them in our imaginary excursion through the Carboniferous forests. To do this we must first glance slightly at the great subdivisions of modern plants, which we may arrange in such a way as to give an easy means for comparison of the aspects of the vegetable kingdom in ancient and modern times. In doing this I shall avail myself of an extract from a previous publication of my own on this subject.
"The modern flora of the earth admits of a grand twofold division into thePhænogamous, or flowering and seed-bearing plants, and theCryptogamous, or flowerless and spore-bearing plants. In the former series, we have, first, those higher plants which start in life with two seed-leaves, and have stems with distinct bark, wood, and pith—theExogens; secondly, those similar plants which begin life with one seed-leaf only, and have no distinction of bark, wood, and pith, in the stem—theEndogens; and, thirdly, a peculiar group starting with two or several seed-leaves, and having a stem with bark, wood, and pith, but with very imperfect flowers, and wood of much simpler structure than either of the others—theGymnosperms. To the first of these groups or classesbelong most of the ordinary trees of temperate climates. To the second belong the palms and allied trees found in tropical climates. To the third belong the pines and cycads. In the second or Cryptogamous series we have also three classes,—(1.) TheAcrogens, or ferns and club-mosses, with stems having true vessels marked on the sides with cross-bars—the Scalariform vessels. (2.) TheAnophytes, or mosses and their allies, with stems and leaves, but no vessels. (3.) TheThallophytes, or lichens, fungi, sea-weeds, etc., without true stems and leaves.
"In the existing climates of the earth we find these classes of plants variously distributed as to relative numbers. In some, pines predominate. In others, palms and tree-ferns form a considerable part of the forest vegetation. In others, the ordinary exogenous trees predominate, almost to the exclusion of others. In some Arctic and Alpine regions, mosses and lichens prevail. In the Coal period we have found none of the higher Exogens, though one species is known in the Devonian, and only a few obscure indications of the presence of Endogens; but Gymnosperms abound, and are highly characteristic. On the other hand, we have no mosses or lichens, and very few algæ, but a great number of ferns and Lycopodiaceæ or club-mosses. Thus the coal formation period is botanically a meeting-place of the lower Phænogams and the higher Cryptogams, and presents many forms which,when imperfectly known, have puzzled botanists in regard to their position in one or other series. In the present world, the flora most akin to that of the Coal period is that of moist and warm islands in the southern hemisphere. It is not properly a tropical flora, nor is it the flora of a cold region, but rather indicative of a moist and equable climate. In accordance with this is the fact that the equable but not warm climate of the southern hemisphere at present (which is owing principally to its small extent of land) enables sub-tropical plants to extend into high latitudes. In the Coal period this uniformity was evidently still more marked, since we find similar plants extending from regions within the Arctic circle to others near to the tropics. Still we must bear in mind that we may often be mistaken in reasoning as to the temperature required by extinct species of plants differing from those now in existence. Further, we must not assume that the climatal conditions of the northern hemisphere were in the Coal period at all similar to those which now prevail. As Sir Charles Lyell has argued, a less amount of land in the higher latitudes would greatly modify climates, and there is every reason to believe that in the Coal period there was less land than now. It has been shown by Tyndall that a very small additional amount of carbonic acid in the atmosphere would, by obstructing the radiation of heat from the earth, produce almost the effect of a glass roof or conservatory, extending over thewhole world. There is much in the structure of the leaves of the coal plants, as well as in the vast amount of carbon which they accumulated in the form of coal, and the characteristics of the animal life of the period, to indicate, on independent grounds, that the Carboniferous atmosphere differed from that of the present world in this way, or in the presence of more carbonic acid a substance now existing in the very minute proportion of one-thousandth of the whole by weight, a quantity adapted to the present requirements of vegetable and animal life, but probably not to those of the Coal period."
Returning from this digression to the forests of the Coal period, we may first notice that which is the most conspicuous and abundant tree in the swampy levels—the Sigillaria or seal-tree, so called from the stamp-like marks left by the fall of its leaves—a plant which has caused much discussion as to its affinities. Some regard it as a gymnosperm, others as a cryptogam. Most probably we have under this name trees allied in part to both groups, and which, when better known, may bridge over the interval between them. These trees present tall pillar-like trunks, often ribbed vertically with raised bands, and marked with rows of scars left by the fallen leaves. They are sometimes branchless, or divide at top into a few thick limbs, covered with long rigid grass-like foliage. On their branches they bear long slender spikes of fruit, and we may conjecture that quantities of nut-like seeds scattered over the ground around their trunks aretheir produce. If we approach one of these trees closely, more especially a young specimen not yet furrowed by age, we are amazed to observe the accurate regularity and curious forms of the leaf-scars, and the regular ribbing, so very different from that of our ordinary forest trees. If we cut into its stem, we are still further astonished at its singular structure. Externally it has a firm and hard rind. Within this is a great thickness of soft cellular inner bark, traversed by large bundles of tough fibres. In the centre is a core or axis of woody matter very slender in proportion to the thickness of the trunk, and still further reduced in strength by a large cellular pith. Thus a great stem four or five feet in diameter is little else than a mass of cellular tissue, altogether unfit to form a mast or beam, but excellently adapted, when flattened and carbonised, to blaze upon our winter hearth as a flake of coal. The roots of these trees were perhaps more singular than their stems; spreading widely in the soft soil by regular bifurcation, they ran out in long snake-like cords, studded all over with thick cylindrical rootlets, which spread from them in every direction. They resembled in form, and probably in function, those cable-like root-stocks of the pond-lilies which run through the slime of lakes, but the structure of the rootlets was precisely that of those of some modern Cycads. It was long before these singular roots were known to belong to a tree. They were supposed to be the branches of some creeping aquatic plant, and botanists objected to the idea of their being roots; but at length their connection with Sigillaria was observed simultaneously by Mr. Binney, in Lancashire, and by Mr. Kichard Brown, in Cape Breton, and it has been confirmed by many subsequently observed facts. This connection, when once established, further explained the reason of the almost universal occurrence of Stigmaria, as these roots were called, under the coal beds; while trunks of the same plants were the most abundant fossils of their partings and roofs. The growth of successive generations of Sigillariæ was, in fact, found to be the principal cause of the accumulation of a bed of coal. Two species form the central figures in our illustration.