Chapter 5

Some features of the condition of the earth during the deposition of the carboniferous group, are made out with a clearness which must satisfy most minds. First we are told of a time when carbonate of lime was formed in vast abundance at the bottoms of profound seas, accompanied by an unusually large population of corals and encrinites; while in some parts of the earth there were patches of dry land, covered with a luxuriant vegetation. Next we have a comparatively brief period of volcanic disturbance, (when the conglomerate was formed.) Then the causes favourable to the so abundant production of limestone, and the large population of marine acrita, decline, and we find the masses of dry land increase in number and extent, and begin to bear an amount of forest vegetation, far exceeding that of the most sheltered tropical spots of the present surface. The climate, even in the latitude of Baffin’s Bay, was torrid, and perhaps the atmosphere contained a larger charge of carbonic acid gas (the material of vegetation) than it now does. The forests or thickets of the period, included no species of plants now known upon earth. They mainly consisted of gigantic shrubs, which are either not represented by any existing types, or are akin to kinds which are now only found in small and lowly forms. That these forests grew upon a Polynesia, or multitude of small islands, is considered probable, from similar vegetation being now found in such situations within the tropics. With regard to the circumstances under which the masses of vegetable matter were transformed into successive coal strata, geologists are divided. From examples seen at the present day, at the mouths of such rivers as the Mississippi, which traverse extensive sylvan regions, and from other circumstances to be adverted to, it is held likely by some that the vegetable matter, the rubbish of decayed forests, was carried by rivers into estuaries, and there accumulated in vast natural rafts, until it sunk to the bottom, where an overlayer of sand or mud would prepare it for becoming a stratum of coal. Others conceive that the vegetation first went into the condition of a peat moss, that a sink in the level then exposed it to be overrun by the sea, and covered with a layer of sand or mud; that a subsequent uprise made the mud dry land, and fitted it to bear a new forest, which afterwards, like its predecessor, became a bed of peat; that, in short, by repetitions of this process, the alternate layers of coal, sandstone, and shale, constituting the carboniferous group, were formed. It is favourable to this last view that marine fossils are scarcely found in the body of the coal itself, though abundant in the shale layers above and below it; also that in several places erect stems of trees are found with their roots still fixed in the shale beds, and crossing the sandstone beds at almost right angles, shewing that these, at least, had not been drifted from their original situations. On the other hand, it is not easy to admit such repeated risings and sinkings of surface as would be required, on this hypothesis, to form a series of coal strata. Perhaps we may most safely rest at present with the supposition that coal has been formed under both classes of circumstances, though in the latter only as an exception to the former.

Upwards of three hundred species of plants have been ascertained to exist in the coal formation; but it is not necessary to suppose that the whole contained in that system are now, or ever will be distinguished. Experiments shew that some great classes of plants become decomposed in water in a much less space of time than others, and it is remarkable that those which decompose soonest, are of the classes found most rare, or not at all, in the coal strata. It is consequently to be inferred that there may have been grasses and mosses at this era, and many species of trees, the remains of which had lost all trace of organic form before their substance sunk into the mass of which coal was formed. In speaking, therefore, of the vegetation of this period, we must bear in mind that it may have comprehended forms of which we have no memorial.

Supposing, nevertheless, that, in the main, the ascertained vegetation of the coal system is that which grew at the time of its formation, it is interesting to find that the terrestrial botany of our globe begins with classes of comparatively simple forms and structure. In the ranks of the vegetable kingdom, the lowest place is taken by plants of cellular tissue, and which have no flowers, (_cryptogamia_,) as lichens, mosses, fungi, ferns, sea-weeds. Above these stand plants of vascular tissue, and bearing flowers, in which again there are two great subdivisions; first, plants having one seed-lobe, (_monocotyledons_,) and in which the new matter is added within, (_endogenous_,) of which the cane and palm are examples; second, plants having two seed-lobes, (_dicotyledons_,) and in which the new matter is added on the outside under the bark, (_exogenous_,) of which the pine, elm, oak, and most of the British forest-trees are examples; these subdivisions also ranking in the order in which they are here stated. Now it is clear that a predominance of these forms in succession marked the successive epochs developed by fossil geology; the simple abounding first, and the complex afterwards.

Two-thirds of the plants of the carboniferous era are of the cellular or cryptogamic kind, a proportion which would probably be much increased if we knew the whole Flora of that era. The ascertained dicotyledons, or higher-class plants, are comparatively few in this formation; but it will be found that they constantly increased as the globe grew older.

The master-form or type of the era was the _fern_, or breckan, of which about one hundred and thirty species have already been ascertained as entering into the composition of coal. {84a} The fern is a plant which thrives best in warm, shaded, and moist situations. In tropical countries, where these conditions abound, there are many more species than in temperate climes, and some of these are arborescent, or of a tree-like size and luxuriance. {84b} The ferns of the coal strata have been of this magnitude, and that without regard to the parts of the earth where they are found. In the coal of Baffin’s Bay, of Newcastle, and of the torrid zone alike, are the fossil ferns arborescent, shewing clearly that, in that era, the present tropical temperature, or one even higher, existed in very high latitudes.

In the swamps and ditches of England there grows a plant called the horse-tail (_equisetum_), having a succulent, erect, jointed stem, with slender leaves, and a scaly catkin at the top. A second large section of the plants of the carboniferous era were of this kind (_equisetaceæ_), but, like the fern, reaching the magnitudes of trees. While existing equiseta rarely exceed three feet in height, and the stems are generally under half an inch in diameter, their kindred, entombed in the coal beds, seem to have been generally fourteen or fifteen feet high, with stems from six inches to a foot in thickness. Arborescent plants of this family, like the arborescent ferns, now grow only in tropical countries, and their being found in the coal beds in all latitudes is consequently held as an additional proof, that at this era a warm climate was extended much farther to the north than at present. It is to be remarked that plants of this kind (forming two genera, the most abundant of which is the _calamites_) are only represented on the present surface by plants of the same _family_: the _species_ which flourished at this era gradually lessen in number as we advance upwards in the series of rocks, and disappear before we arrive at the tertiary formation.

The club-moss family (_lycopodiaceæ_) are other plants of the present surface, usually seen in a lowly and creeping form in temperate latitudes, but presenting species which rise to a greater magnitude within the tropics. Many specimens of this family are found in the coal beds; it is thought they have contributed more to the substance of the coal than any other family. But, like the ferns and equisetaceæ, they rise to a prodigious magnitude. The lepidodendra (so the fossil genus is called) have probably been from sixty-five to eighty feet in height, having at their base a diameter of about three feet, while their leaves measured twenty inches in length. In the forests of the coal era, the lepidodendra would enjoy the rank of firs in our forests, affording shade to the only less stately ferns and calamites. The internal structure of the stem, and the character of the seed-vessels, shew them to have been a link between single-lobed and double-lobed plants, a fact worthy of note, as it favours the idea that, in vegetable, as well as animal creation, a progress has been observed, in conformity with advancing conditions. It is also curious to find a missing link of so much importance in a genus of plants which has long ceased to have a living place upon earth.

The other leading plants of the coal era are without representatives on the present surface, and their characters are in general less clearly ascertained. Amongst the most remarkable are—the _sigillaria_, of which large stems are very abundant, shewing that the interior has been soft, and the exterior fluted with separate leaves inserted in vertical rows along the flutings—and the _stigmaria_, plants apparently calculated to flourish in marshes or pools, having a short, thick, fleshy stem, with a dome-shaped top, from which sprung branches of from twenty to thirty feet long. Amongst monocotyledons were some palms, (_flabellaria_ and _næggerathia_,) besides a few not distinctly assignable to any class.

The dicotyledons of the coal are comparatively few, though on the present surface they are the most numerous sub-class. Besides some of doubtful affinity, (_annularia_, _asterophyllites_, &c.,) there were a few of the pine family, which seem to have been the highest class of trees of this era, and are only as yet found in isolated cases, and in sandstone beds. The first discovered lay in the Craigleith quarry, near Edinburgh, and consisted of a stem about two feet thick, and forty-seven feet in length. Others have since been found, both in the same situation, and at Newcastle. Leaves and fruit being wanting, an ingenious mode of detecting the nature of these trees was hit upon by Mr. Witham of Lartington. Taking thin polished cross slices of the stem, and subjecting them to the microscope, he detected the structure of the wood to be that of a cone-bearing tree, by the presence of certain “reticulations” which distinguish that family, in addition to the usual radiating and concentric lines. That particular tree was concluded to be an araucaria, a species now found in Norfolk Island, in the South Sea, and in a few other remote situations. The coniferæ of this era form the dawn of dicotyledenous trees, of which they may be said to be the simplest type, and to which, it has already been noticed, the lepidodendra are a link from the monocotyledons. The concentric rings of the Craigleith and other coniferæ of this era have been mentioned. It is interesting to find in these a record of the changing seasons of those early ages, when as yet there were no human beings to observe time or tide. They are clearly traced; but it is observed that they are more slightly marked than is the case with their family at the present day, as if the changes of temperature had been within a narrower range.

Such was the vegetation of the carbonigenous era, composed of forms at the bottom of the botanical scale, flowerless, fruitless, but luxuriant and abundant beyond what the most favoured spots on earth can now shew. The rigidity of the leaves of its plants, and the absence of fleshy fruits and farinaceous seeds, unfitted it to afford nutriment to animals; and, monotonous in its forms, and destitute of brilliant colouring, its sward probably unenlivened by any of the smaller flowering herbs, its shades uncheered by the hum of insects, or the music of birds, it must have been but a sombre scene to a human visitant. But neither man nor any other animals were then in existence to look for such uses or such beauties in this vegetation. It was serving other and equally important ends, clearing (probably) the atmosphere of matter noxious to animal life, and storing up mineral masses which were in long subsequent ages to prove of the greatest service to the human race, even to the extent of favouring the progress of its civilization.

The animal remains of this era are not numerous, in comparison with those which go before, or those which come after. The mountain limestone, indeed, deposited at the commencement of it, abounds unusually in polypiaria and crinoidea; but when we ascend to the coal-beds themselves, the case is altered, and these marine remains altogether disappear. We have then only a limited variety of conchifers and shell mollusks, with fragments of a few species of fishes, and these are rarely or never found in the coal seams, but in the shales alternating with them. Some of the fishes are of a sauroid character, that is, partake of the nature of the lizard, a genus of the reptilia, a land class of animals, so that we may be said here to have the first approach to a kind of animals calculated to breathe the atmosphere. Such is the Megalichthys Hibbertii, found by Dr. Hibbert Ware, in a limestone bed of fresh-water origin, underneath the coal at Burdiehouse, near Edinburgh. Others of the same kind have been found in the coal measures in Yorkshire, and in the low coal shales at Manchester. This is no more than might be expected, as collections of fresh water now existed, and it is presumable that they would be peopled. The chief other fishes of the coal era are named palæothrissum, palæoniscus, diperdus.

Coal strata are nearly confined to the group termed the carboniferous formation. Thin beds are not unknown afterwards, but they occur only as a rare exception. It is therefore thought that the most important of the conditions which allowed of so abundant a terrestrial vegetation, had ceased about the time when this formation was closed. The high temperature was not one of the conditions which terminated, for there are evidences of it afterwards; but probably the superabundance of carbonic acid gas supposed to have existed during this era was expended before its close. There can be little doubt that the infusion of a large dose of this gas into the atmosphere at the present day would be attended by precisely the same circumstances as in the time of the carboniferous formation. Land animal life would not have a place on earth; vegetation would be enormous; and coal strata would be formed from the vast accumulations of woody matter, which would gather in every sea, near the mouths of great rivers. On the exhaustion of the superabundance of carbonic acid gas, the coal formation would cease, and the earth might again become a suitable theatre of being for land animals.

The termination of the carboniferous formation is marked by symptoms of volcanic violence, which some geologists have considered to denote the close of one system of things and the beginning of another. Coal beds generally lie in basins, as if following the curve of the bottom of seas. But there is no such basin which is not broken up into pieces, some of which have been tossed up on edge, others allowed to sink, causing the ends of strata to be in some instances many yards, and in a few several hundred feet, removed from the corresponding ends of neighbouring fragments. These are held to be results of volcanic movements below, the operation of which is further seen in numerous upbursts and intrusions of volcanic rock (trap). That these disturbances took place about the close of the formation, and not later, is shewn in the fact of the next higher group of strata being comparatively undisturbed. Other symptoms of this time of violence are seen in the beds of conglomerate which occur amongst the first strata above the coal. These, as usual, consist of fragments of the elder rocks, more or less worn from being tumbled about in agitated water, and laid down in a mud paste, afterwards hardened. Volcanic disturbances break up the rocks; the pieces are worn in seas; and a deposit of conglomerate is the consequence. Of porphyry, there are some such pieces in the conglomerate of Devonshire, three or four tons in weight. It is to be admitted for strict truth that, in some parts of Europe, the carboniferous formation is followed by superior deposits, without the appearance of such disturbances between their respective periods; but apparently this case belongs to the class of exceptions already noticed. {93} That disturbance was general, is supported by the further and important fact of the destruction of many forms of organic being previously flourishing, particularly of the vegetable kingdom.

ERA OF THE NEW RED SANDSTONE. TERRESTRIAL ZOOLOGY COMMENCES WITH REPTILES. FIRST TRACES OF BIRDS.

THE next volume of the rock series refers to an era distinguished by an event of no less importance than the commencement of land animals. The _New Red Sandstone System_ is subdivided into groups, some of which are wanting in some places; they are pretty fully developed in the north of England, in the following ascending order:—1. Lower red sandstone; 2. Magnesian limestone; 3. Red and white sandstones and conglomerate; 4. Variegated marls. Between the third and fourth there is, in Germany, another group, called the Muschelkalk, a word expressing a limestone full of shells.

The first group, containing the conglomerates already adverted to, seems to have been produced during the time of disturbance which occurred so generally after the carbonigenous era. This new era is distinguished by a paucity of organic remains, as might partly be expected from the appearances of disturbance, and the red tint of the rocks, the latter being communicated by a solution of oxide of iron, a substance unfavourable to animal life.

The second group is a limestone with an infusion of magnesia. It is developed less generally than some others, but occurs conspicuously in England and Germany. Its place, above the red sandstone, shews the recurrence of circumstances favourable to animal life, and we accordingly find in it not only zoophytes, conchifera, and a few tribes of fish, but some faint traces of land plants, and a new and startling appearance—a reptile of saurian (lizard) character, analogous to the now existing family called monitors. Remains of this creature are found in cupriferous (copper-bearing) slate connected with the mountain limestone, at Mansfield and Glucksbrunn, in Germany, which may be taken as evidence that dry land existed in that age near those places. The magnesia limestone is also remarkable as the last rock in which appears the leptæna, or producta, a conchifer of numerous species which makes a conspicuous appearance in all previous seas. It is likewise to be observed, that the fishes of this age, to the genera of which the names palæoniscus, catopterus, platysomus, &c., have been applied, vanish, and henceforth appear no more.

The third group, chiefly sandstones, variously coloured according to the amount and nature of the metallic oxide infused into them, shews a recurrence of agitation, and a consequent diminution of the amount of animal life. In the upper part, however, of this group, there are abundant symptoms of a revival of proper conditions for such life. There are marl beds, the origin of which substance in decomposed shells is obvious; and in Germany, though not in England, here occurs the muschelkalk, containing numerous organic remains, (generally different from those of the magnesian limestone,) and noted for the specimens of land animals, which it is the first to present in any considerable abundance to our notice.

These animals are of the vertebrate sub-kingdom, but of its lowest class next after fishes,—namely, reptiles,—a portion of the terrestrial tribes whose imperfect respiratory system perhaps fitted them for enduring an atmosphere not yet quite suitable for birds or mammifers. {97} The specimens found in the muschelkalk are allied to the crocodile and lizard tribes of the present day, but in the latter instance are upon a scale of magnitude as much superior to present forms as the lepidodendron of the coal era was superior to the dwarf club-mosses of our time. These saurians also combine some peculiarities of structure of a most extraordinary character.

The animal to which the name _ichthyosaurus_ has been given, was as long as a young whale, and it was fitted for living in the water, though breathing the atmosphere. It had the vertebral column and general bodily form of a fish, but to that were added the head and breast-bone of a lizard, and the paddles of the whale tribes. The beak, moreover, was that of a porpoise, and the teeth were those of a crocodile. It must have been a most destructive creature to the fish of those early seas.

The _plesiosaurus_ was of similar bulk, with a turtle-like body and paddles, shewing that the sea was its element, but with a long serpent-like neck, terminating in a saurian head, calculated to reach prey at a considerable distance. These two animals, of which many varieties have been discovered, constituting distinct species, are supposed to have lived in the shallow borders of the seas of this and subsequent formations, devouring immense quantities of the finny tribes. It was at first thought that no creatures approaching them in character now inhabit the earth; but latterly Mr. Darwin has discovered, in the reptile-peopled Galapagos Islands, in the South Sea, a marine saurian from three to four feet long.

The _megalosaurus_ was an enormous lizard—a land creature, also carnivorous. The _pterodactyle_ was another lizard, but furnished with wings to pursue its prey in the air, and varying in size between a cormorant and a snipe. Crocodiles abounded, and some of these were herbivorous. Such was the iguanodon, a creature of the character of the iguana of the Ganges, but reaching a hundred feet in length, or twenty times that of its modern representative.

There were also numerous _tortoises_, some of them reaching a great size; and Professor Owen has found in Warwickshire some remains of an animal of the batrachian order, {99} to which, from the peculiar form of the teeth, he has given the name of labyrinthidon. Thus, three of Cuvier’s four orders of reptilia (_sauria_, _chelonia_, and _batrachia_) are represented in this formation, the serpent order (_ophidia_) being alone wanting.

The variegated marl beds which constitute the uppermost group of the formation, present two additional genera of huge saurians,—the phytosaurus and mastodonsaurus.

It is in the upper beds of the red sandstone that beds of salt first occur. These are sometimes of such thickness, that the mine from which the material has been excavated looks like a lofty church. We see in the present world no circumstances calculated to produce the formation of a bed of rock salt; yet it is not difficult to understand how such strata were formed in an age marked by ultra-tropical heat and frequent volcanic disturbances. An estuary, cut off by an upthrow of trap, or a change of level, and left to dry up under the heat of the sun, would quickly become the bed of a dense layer of rock salt. A second shift of level, or some other volcanic disturbance, connecting it again with the sea, would expose this stratum to being covered over with a layer of sand or mud, destined in time to form the next stratum of rock above it.