In the old red sandstone, the marine plants, of which faint traces are observable in the Silurians, continue to appear. It would seem as if less change took place in the vegetation than in the animals of those early seas; and for this, as Mr. Miller has remarked, it is easy to imagine reasons. For example, an infusion of lime into the sea would destroy animal life, but be favourable to vegetation.
As yet there were no land animals or plants, and for this the presumable reason is, that no dry land as yet existed. We are not left to make this inference solely from the absence of land animals and plants; in the arrangement of the primary (stratified) rocks, we have further evidence of it. That these rocks were formed in a generally horizontal position, we are as well a.s.sured as that they were formed at the bottom of seas. But they are always found greatly inclined in position, tilted up against the slopes of the granitic ma.s.ses which are beneath them in geological order, though often shooting up to a higher point in the atmosphere. No doubt can be entertained that these granitic ma.s.ses, forming our princ.i.p.al mountain ranges, have been protruded from below, or, at least, thrust much further up, SINCE the deposition of the primary rocks. The protrusion was what tilted up the primary rocks; and the inference is, of course, unavoidable, that these mountains have risen chiefly, at least, since the primary rocks were laid down. It is remarkable that, while the primary rocks thus incline towards granitic nuclei or axes, the strata higher in the series rest against these again, generally at a less inclination, or none at all, shewing that these strata were laid down after the swelling mountain eminences had, by their protrusion, tilted up the primary strata. And thus it may be said an era of local upthrowing of the primitive and (perhaps) central matter of our planet, is established as happening about the close of the primary strata, and beginning of the next ensuing system. It may be called the Era of the Oldest Mountains, or, more boldly, of the formation of the detached portions of dry land over the hitherto watery surface of the globe--an important part of the designs of Providence, for which the time was now apparently come.
It may be remarked, that volcanic disturbances and protrusions of trap took place throughout the whole period of the deposition of the primary rocks; but they were upon a comparatively limited scale, and probably all took place under water. It was only now that the central granitic ma.s.ses of the great mountain ranges were thrown up, carrying up with them broken edges of the primary strata; a process which seems to have had this difference from the other, that it was the effect of a more tremendous force exerted at a lower depth in the earth, and generally acting in lines pervading a considerable portion of the earth"s surface. We shall by-and-by see that the protrusion of some of the mountain ranges was not completed, or did not stop, at that period. There is no part of geological science more clear than that which refers to the ages of mountains. It is as certain that the Grampian mountains of Scotland are older than the Alps and Apennines, as it is that civilization had visited Italy, and had enabled her to subdue the world, while Scotland was the residence of "roving barbarians." The Pyrenees, Carpathians, and other ranges of continental Europe, are all younger than the Grampians, or even the insignificant Mendip Hills of southern England. Stratification tells this tale as plainly as Livy tells the history of the Roman republic.
It tells us--to use the words of Professor Philips--that at the time when the Grampians sent streams and detritus to straits where now the valleys of the Forth and Clyde meet, the greater part of Europe was a wide ocean.
The last three systems--called, in England, the c.u.mbrian, Silurian, and Devonian, and collectively the palaeozoic rocks, from their containing the remains of the earliest inhabitants of the globe--are of vast thickness; in England, not much less than 30,000 feet, or nearly six miles. In other parts of the world, as we have seen, the earliest of these systems alone is of much greater depth--arguing an enormous profundity in the ocean in which they were formed.
SECONDARY ROCKS. ERA OF THE CARBONIFEROUS FORMATION. LAND FORMED.
COMMENCEMENT OF LAND PLANTS.
We now enter upon a new great epoch in the history of our globe.
There was now dry land. As a consequence of this fact, there was fresh water, for rain, instead of immediately returning to the sea, as formerly, was now gathered in channels of the earth, and became springs, rivers, and lakes. There was now a theatre for the existence of land plants and animals, and it remains to be inquired if these accordingly were produced.
The Secondary Rocks, in which our further researches are to be prosecuted, consist of a great and varied series, resting, generally unconformably, against flanks of the upturned primary rocks, sometimes themselves considerably inclined, at others, forming extensive basin-like beds, nearly horizontal; in many places, much broken up and shifted by disturbances from below. They have all been formed out of the materials of the older rocks, by virtue of the wearing power of air and water, which is still every day carrying down vast quant.i.ties of the elevated matter of the globe into the sea. But the separate strata are each much more distinct in the matter of its composition than might be expected. Some are siliceous or arenaceous (sandstones), composed mainly of fine grains from the quartz rocks--the most abundant of the primary strata. Others are argillaceous--clays, shales, &c., chiefly derived, probably, from the slate beds of the primary series. Others are calcareous, derived from the early limestone. As a general feature, they are softer and less crystalline than the primary rocks, as if they had endured less of both heat and pressure than the senior formation. There are beds (coal) formed solely of vegetable matter, and some others in which the main ingredient is particles of iron, (the iron black band.) The secondary rocks are quite as communicative with regard to their portion of the earth"s history as the primitive were.
The first, or lowest, group of the secondary rocks is called the Carboniferous Formation, from the remarkable feature of its numerous interspersed beds of coal. It commences with the beds of the MOUNTAIN LIMESTONE, which, in some situations, as in Derbyshire and Ireland, are of great thickness, being alternated with chert (a siliceous sandstone), sandstones, shales, and beds of coal, generally of the harder and less bituminous kind (anthracite), the whole being covered in some places by the millstone grit, a siliceous conglomerate composed of the detritus of the primary rocks. The mountain limestone, attaining in England to a depth of eight hundred yards, greatly exceeds in volume any of the primary limestone beds, and shews an enormous addition of power to the causes formerly suggested as having produced this substance. In fact, remains of corals, crinoidea, and sh.e.l.ls, are so abundant in it, as to compose three-fourths of the ma.s.s in some parts. Above the mountain limestone commence the more conspicuous COAL BEDS, alternating with sandstones, shales, beds of limestone, and ironstone. Coal is altogether composed of the matter of a terrestrial vegetation, trans.m.u.ted by pressure. Some fresh-water sh.e.l.ls have been found in it, but few of marine origin, and no remains of those zoophytes and crinoidea so abundant in the mountain limestone and other rocks.
Coal beds exist in Europe, Asia, and America, and have hitherto been esteemed as the most valuable of mineral productions, from the important services which the substance renders in manufactures and in domestic economy. It is to be remarked, that there are some local variations in the arrangement of coal beds. In France, they rest immediately on the granite and other primary rocks, the intermediate strata not having been found at those places. In America, the kind called anthracite occurs among the slate beds, and this species also abounds more in the mountain limestone than with us. These last circ.u.mstances only shew that different parts of the earth"s surface did not all witness the same events of a certain fixed series exactly at the same time. There had been an exhibition of dry land about the site of America, a little earlier than in Europe.
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 ma.s.ses 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 lat.i.tude 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 mult.i.tude of small islands, is considered probable, from similar vegetation being now found in such situations within the tropics. With regard to the circ.u.mstances under which the ma.s.ses 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 circ.u.mstances 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 acc.u.mulated 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 repet.i.tions of this process, the alternate layers of coal, sandstone, and shale, const.i.tuting 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 cla.s.ses of circ.u.mstances, 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 cla.s.ses of plants become decomposed in water in a much less s.p.a.ce of time than others, and it is remarkable that those which decompose soonest, are of the cla.s.ses found most rare, or not at all, in the coal strata. It is consequently to be inferred that there may have been gra.s.ses 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 ma.s.s 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 cla.s.ses 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-cla.s.s 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 lat.i.tudes.
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 (equisetaceae), 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 lat.i.tudes 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 (lycopodiaceae) are other plants of the present surface, usually seen in a lowly and creeping form in temperate lat.i.tudes, 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 equisetaceae, 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 naeggerathia,) besides a few not distinctly a.s.signable to any cla.s.s.
The dicotyledons of the coal are comparatively few, though on the present surface they are the most numerous sub-cla.s.s. Besides some of doubtful affinity, (annularia, asterophyllites, &c.,) there were a few of the pine family, which seem to have been the highest cla.s.s 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. .h.i.t 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 coniferae 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 coniferae 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 dest.i.tute 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 ma.s.ses 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 sh.e.l.l 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 cla.s.s 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 palaeothrissum, palaeoniscus, 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 circ.u.mstances 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 acc.u.mulations 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 cla.s.s 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 sh.e.l.ls.
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 circ.u.mstances 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, a.n.a.logous 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 leptaena, 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 palaeoniscus, 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 sh.e.l.ls 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 cla.s.s 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, const.i.tuting distinct species, are supposed to have lived in the shallow borders of the seas of this and subsequent formations, devouring immense quant.i.ties 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 const.i.tute 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 circ.u.mstances 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.
The plants of this era are few and un.o.btrusive. Equiseta, calamites, ferns, Voltzia, and a few of the other families found so abundantly in the preceding formation, here present themselves, but in diminished size and quant.i.ty.
This seems to be the proper place to advert to certain memorials of a peculiar and unexpected character respecting these early ages in the sandstones. So low as the bottom of the carboniferous system, slabs are found marked over a great extent of surface with that peculiar corrugation or wrinkling which the receding tide leaves upon a sandy beach when the sea is but slightly agitated; and not only are these ripple-marks, as they are called, found on the surfaces, but casts of them are found on the under sides of slabs lying above. The phenomena suggests the time when the sand ultimately formed into these stone slabs, was part of the beach of a sea of the carbonigenous era; when, left wavy by one tide, it was covered over with a thin layer of fresh sand by the next, and so on, precisely as such circ.u.mstances might be expected to take place at the present day. Sandstone surfaces, ripple-marked, are found throughout the subsequent formations: in those of the new red, at more than one place in England, they further bear impressions of rain-drops which have fallen upon them--the rain, of course, of the inconceivably remote age in which the sandstones were formed. In the Greensill sandstone, near Shrewsbury, it has even been possible to tell from what direction the shower came which impressed the sandy surface, the rims of the marks being somewhat raised on one side, exactly as might be expected from a slanting shower falling at this day upon one of our beaches. These facts have the same sort of interest as the season rings of the Craigleith conifers, as speaking of a parity between some of the familiar processes of nature in those early ages and our own.
In the new red sandstone, impressions still more important in the inferences to which they tend, have been observed,--namely, the footmarks of various animals. In a quarry of this formation, at Cornc.o.c.kle Muir, in Dumfriesshire, where the slabs incline at an angle of thirty-eight degrees, the vestiges of an animal supposed to have been a tortoise are distinctly traced up and down the slope, as if the creature had had occasion to pa.s.s backwards and forwards in that direction only, possibly in its daily visits to the sea. Some slabs similarly impressed, in the Stourton quarries in Cheshire, are further marked with a shower of rain which we know must have fallen AFTERWARDS, for its little hollows are impressed in the footmarks also, though more slightly than on the rest of the surface, the comparative hardness of a trodden place having apparently prevented so deep an impression being made. At Hessberg, in Saxony, the vestiges of four distinct animals have been traced, one of them a web-footed animal of small size, considered as a congener of the crocodile; another, whose footsteps having a resemblance to an impression of a swelled human hand, has caused it to be named the cheirotherium. The footsteps of the cheirotherium have been found also in the Stourton quarries above mentioned. Professor Owen, who stands at the head of comparative anatomy in the present day, has expressed his belief that this last animal was the same batrachian of which he has found fragments in the new red sandstone of Warwickshire. At Runcorn, near Manchester, and elsewhere, have been discovered the tracks of an animal which Mr. Owen calls the rynchosaurus, uniting with the body of a reptile the beak and feet of a bird, and which clearly had been a LINK between these two cla.s.ses.
If geologists shall ultimately give their approbation to the inferences made from a recent discovery in America, we shall have the addition of perfect birds, though probably of a low type, to the animal forms of this era. It is stated to be in quarries of this rock, in the valley of Connecticut, that footprints have been found, apparently produced by birds of the order grallae, or waders. "The footsteps appear in regular succession on the continuous track of an animal, in the act of walking or running, with the right and left foot always in their relative places. The distance of the intervals between each footstep on the same track is occasionally varied, but to no greater amount than may be explained by the bird having altered its pace. Many tracks of different individuals and different species are often found crossing each other, and crowded, like impressions of feet upon the sh.o.r.es of a muddy stream, where ducks and geese resort." {103} Some of these prints indicate small animals, but others denote birds of what would now be an unusually large size.
One animal, having a foot fifteen inches in length, (one-half more than that of the ostrich,) and a stride of from four to six feet, has been appropriately ent.i.tled, ornithichnites giganteus.
ERA OF THE OOLITE. COMMENCEMENT OF MAMMALIA.
The chronicles of this period consist of a series of beds, mostly calcareous, taking their general name (Oolite System) from a conspicuous member of them--the oolite--a limestone composed of an aggregation of small round grains or spherules, and so called from its fancied resemblance to a cl.u.s.ter of eggs, or the roe of a fish.