THE FERN FORESTS OF THE CARBONIFEROUS PERIOD.
Draw two lines on your map, the upper one running from the mouth of the St. Lawrence westward nearly to St. Paul on the Mississippi, and the lower one from the neighborhood of St. John's in Newfoundland running southwesterly about to the point where the Wisconsin joins the Mississippi, but jutting down to form an extensive peninsula comprising part of the States of Indiana and Illinois, and you include between them all of the United States which existed at the close of the Devonian period. The upper line rests against the granite hills dividing the Silurian and Devonian deposits of the British Possessions to the north from those of the United States to the south, Canada itself consisting, in great part, of the granite ridge.
How far the early deposits extended to the north of the Laurentian Hills, as well as the outline of that portion of the continent in those times, remains still very problematical; but the investigations thus far undertaken in those regions would lead to the supposition that the same granite upheaval which raised Canada stretched northward in a broad, low ridge of land, widening in its upper part and extending to the neighborhood of Bathurst Inlet and King William's Island, while on either side of it to the east and west the Silurian and Devonian deposits extended far toward the present outlines of the continent.
Indeed, our geological surveys, as well as the information otherwise obtained concerning the primitive condition of North America and the gradual accessions it has received in more recent periods, point to a very early circumscription of the area which, in the course of time, was to become the continent we now inhabit, with its modern features.[A]
[Footnote A: It would be impossible to encumber the pages of the Atlantic Monthly with references to all the authorities on which such geological results rest. They are drawn from the various State Surveys, including that of the mineral lands of Lake Superior, and other more general works on American geology.]
Not only from the geology of America, but from that of Europe also, it would seem that the position of the continents was sketched out very early in the progressive development of the physical constitution of our earth. It is true that in the present state of our knowledge such wide generalizations must be taken with caution, and held in abeyance to the additional facts which future investigations may develop. But thus far the results certainly do not sustain the theories which have lately found favor among geologists, of entire changes in the relative distribution of land and sea and in the connection of continents with one another; on the contrary, it would appear, that, in accordance with the laws of all organic progress, arising from a fixed starting-point and proceeding through regular changes toward a well-defined end, the continents have grown steadily and consistently from the beginning, through successive accessions in a definite direction, to their present form and Organic correlations. If, indeed, there is any meaning in the remarkably symmetrical combinations of the double twin continents in the Eastern Hemisphere, so closely soldered in their northern half, as contrasted with the single pair in the Western Hemisphere, isolated in their position, but so strikingly similar in their Outlines, they must be the result of a progressive and predetermined growth already hinted at in the relative position and gradual increase of the first lands raised above the level of the ocean.
However this may be, there can be no doubt that we now know with tolerable accuracy the limits of the land raised above the water at that period in the present United States. Let us see, then, what we inclose between oar two lines. We have Newfoundland and Nova Scotia, the greater part of New England, the whole of New York, a narrow strip along the north of Ohio, a great part of Indiana and Illinois, and nearly the whole of Michigan and Wisconsin.
Within this region lie all the Great Lakes. The origin of these large troughs, holding such immense sheets of fresh water, remains still the subject of discussion and investigation among geologists. It has been supposed that in the primitive configuration of the globe, when the formation of those depressions at the poles in which the Arctic seas are accumulated gave rise to a corresponding protrusion at the equator, the curve thus produced throughout the North Temperate Zone may have forced up the Canada granite, and have caused, at the same time, those rents in the earth's surface now filled by the Canada lakes; and this view is sustained by the fact that there is a belt of lakes, among which, however, the Canada lakes are far the largest, all around the world in that latitude. The geological phenomena connected with all these lakes have not, however, been investigated with sufficient accuracy and detail, nor has there been any comparison of them extensive and comprehensive enough to justify the adoption of any theory respecting their origin. In an excursion to Lake Superior, some years since, I satisfied myself that the position and outline of that particular lake had their immediate cause in several distinct systems of dikes which intersect its northern shore, and have probably cut up the whole tract of rock over the space now filled by that wonderful sheet of fresh water in such a way as to destroy its continuity, to produce depressions, and gradually create the excavation which now forms the basin of the lake. How far the same causes have been effectual in producing the other large lakes I am unable to say, never having had the opportunity of studying their formation with the same care.
The existence of the numerous smaller lakes running north and south in the State of New York, as the Canandaigua, Seneca, Cayuga, etc., is more easily accounted for. Slow and gradual as was the process by which all that region was lifted above the ocean, it was, nevertheless, accompanied by powerful dislocations of the stratified deposits, as we shall see when we examine them with reference to the local phenomena connected with them. To these dislocations of the strata we owe the transverse cracks across the central part of New York, which needed only the addition of the fresh water poured into them by the rains to transform them into lakes.
I shall not attempt any account of the differences between the animals of the Devonian period and those of the Silurian period, because they consist of structural details difficult to present in a popular form and uninteresting to all but the professional naturalist. Suffice it to say, that, though the organic world had the same general character in these two closely allied periods, yet its representatives in each were specifically distinct, and their differences, however slight, are as constant and as definitely marked as those between more widely separated creations.
At the close of the Devonian period, several upheavals occurred of great significance for the future history of America. One in Ohio raised the elevated ground on which Cincinnati now stands; another hill lifted its granite crest in Missouri, raising with it an extensive tract of Silurian and Devonian deposits; while a smaller one, which does not seem, however, to have disturbed the beds about it so powerfully, broke through in Arkansas. At the same time, elevations took place toward the East,—the first links, few and detached, in the great Alleghany chain which now raises its rocky wall from New England to Alabama.
In the Ohio hill, the granite did not break through, though the force of the upheaval was such as to rend asunder the Devonian deposits, for we find them lying torn and broken about the base of the hill; while the Silurian beds, which should underlie them in their natural position, form its centre and summit. This accounts for the great profusion of Silurian organic remains in that neighborhood. Indeed, there is no locality which forces upon the observer more strongly the conviction of the profusion and richness of the early creation; for one may actually collect the remains of Silurian Shells and Crustacea by cart-loads around the city of Cincinnati. A naturalist would find it difficult to gather along any modern sea-shore, even on tropical coasts, where marine life is more abundant than elsewhere, so rich a harvest, in the same time, as he will bring home from an hour's ramble in the environs of that city.
These elevations naturally gave rise to depressions between themselves and the land on either side of them, and caused also so many counter-slopes dipping toward the uniform southern slope already formed at the north. Thus between the several new upheavals, as well as between them all and the land to the north of them, wide basins or troughs were formed, inclosed on the south, west, and east by low hills, (for these more recent eruptions were, like all the early upheavals, insignificant in height,) and bounded on the north by the more ancient shores of the preceding ages.
These were the inland seas of the Carboniferous period. Here, again, we must infer the successive stages of a history which we can read only in its results. Shut out from the ocean, these shallow sea-basins were gradually changed by the rains to fresh-water lakes; the lakes, in their turn, underwent a transformation, becoming filled, in the course of centuries, with the materials worn away from their shores, with the débris of the animals which lived and died in their waters, as well as with the decaying matter from aquatic plants, till at last they were changed to spreading marshes, and on these marshes arose the gigantic fern-vegetation of which the first forests chiefly consisted. Such are the separate chapters in the history of the coal-basins of Illinois, Missouri, Pennsylvania, New England, and Nova Scotia. First inland seas, then fresh-water lakes, then spreading marshes, then gigantic forests, and lastly vast storehouses of coal for the human race.
Although coal-beds are by no means peculiar to the Carboniferous period, since such deposits must be formed wherever the decay of vegetation is going on extensively, yet it would seem that coal-making was the great work in that age of the world's physical history. The atmospheric conditions, so far as we can understand them, were then especially favorable to this result. Though the existence of such an extensive terrestrial vegetation shows conclusively that an atmosphere must have been already established, with all the attendant phenomena of light, heat, air, moisture, etc., yet it is probable that this atmosphere differed from ours in being very largely charged with carbonic acid.
We should infer this from the nature of the animals characteristic of the period; for, though land-animals were introduced, and the organic world was no longer exclusively marine, there were as yet none of the higher beings in whom respiration is an active process. In all warm-blooded animals the breathing is quick, requiring a large proportion of oxygen in the surrounding air, and indicating by its rapidity the animation of the whole system; while the slow-breathing, cold-blooded animals can live in an air that is heavily loaded with carbon. It is well known, however, that, though carbon is so deadly to higher animal life, plants require it in great quantities; and it would seem that one of the chief offices of the early forests was to purify the atmosphere of its undue proportion of carbonic acid, by absorbing the carbon into their own substance, and eventually depositing it as coal in the soil.
Another very important agent in the process of purifying the atmosphere, and adapting it to the maintenance of a higher organic life, is found in the deposits of lime. My readers will excuse me, if I introduce here a very elementary chemical fact to explain this statement. Limestone is carbonate of calcium. Calcium is a metal, fusible as such, and, forming a part of the melted masses within the earth, it was thrown out with the eruptions of Plutonic rocks. Brought to the air, it would appropriate a certain amount of oxygen, and by that process would become oxide of calcium, in which condition it combines very readily with carbonic acid. Thus it becomes carbonate of lime; and all lime deposits played an important part in establishing the atmospheric proportions essential to the existence of the warm-blooded animals.
Such facts remind us how far more comprehensive the results of science will become when the different branches of scientific investigation are pursued in connection with each other. When chemists have brought their knowledge out of their special laboratories into the laboratory of the world, where chemical combinations are and have been through all time going on in such vast proportions,—when physicists study the laws of moisture, of clouds and storms, in past periods as well as in the present,—when, in short, geologists and zoologists are chemists and physicists, and vice versa,—then we shall learn more of the changes the world has undergone than is possible now that they are separately studied.
It may be asked, how any clue can be found to phenomena so evanescent as those of clouds and moisture. But do we not trace in the old deposits the rainstorms of past times? The heavy drops of a passing shower, the thick, crowded tread of a splashing rain, or the small pinpricks of a close and fine one,—all the story, in short, of the rising vapors, the gathering clouds, the storms and showers of ancient days, we find recorded for us in the fossil rain-drops; and when we add to this the possibility of analyzing the chemical elements which have been absorbed into the soil, but which once made part of the atmosphere, it is not too much to hope that we shall learn something hereafter of the meteorology even of the earliest geological ages.
The peculiar character of the vegetable tissue in the trees of the Carboniferous period, containing, as it did, a large supply of resin drawn from the surrounding elements, confirms the view of the atmospheric conditions above stated; and this fact, as well as the damp, soggy soil in which the first forests must have grown, accounts for the formation of coal in greater quantity and more combustible in quality than is found in the more recent deposits. But stately as were those fern forests, where plants which creep low at our feet to-day, or are known to us chiefly as underbrush, or as rushes and grasses in swampy grounds, grew to the height of lofty trees, yet the vegetation was of an inferior kind.
There has been a gradation in time for the vegetable as well as the animal world. With the marine population of the more ancient geological ages we find nothing but sea-weeds,—of great variety, it is true, and, as it would seem, from some remains of the marine Cryptogams in early times, of immense size, as compared with modern sea-weeds. But in the Carboniferous period, the plants, though still requiring a soaked and marshy soil, were aërial or atmospheric plants: they were covered with leaves; they breathed; their fructification was like that which now characterizes the ferns, the club-mosses, and the so-called "horse-tail plants," (Equisetaceae,) those grasses of low, damp grounds remarkable for the strongly marked articulations of the stem.
These were the lords of the forests all over the world in the Carboniferous period. Wherever the Carboniferous deposits have been traced, in the United States, in Canada, in England, France, Belgium, Germany, in New Holland, at the Cape of Good Hope, and in South America, the general aspect of the vegetation has been found to be the same, though characterized in the different localities by specific differences of the same nature as those by which the various floras are distinguished now in different parts of the same zone. For instance, the Temperate Zone throughout the world is characterized by certain families of trees: by Oaks, Maples, Beeches, Birches, Pines, etc.; but the Oaks, Maples, Beeches, Birches, and the like, of the American flora in that latitude differ in species from the corresponding European flora. So in the Carboniferous period, when more uniform climatic conditions prevailed throughout the world, the character of the vegetation showed a general unity of structure everywhere; but it was nevertheless broken up into distinct botanical provinces by specific differences of the same kind as those which now give such diversity of appearance to the vegetation of the Temperate Zone in Europe as compared with that of America, or to the forests of South America as compared with those of Africa.
There can be no doubt as to the true nature of the Carboniferous forests; for the structural character of the trees is as strongly marked in their fossil remains as in any living plants of the same character. We distinguish the Ferns not only by the peculiar form of their leaves, often perfectly preserved, but also by the fructification on the lower surface of the leaves, and by the distinct marks made on the stem at their point of juncture with it. The leaf of the Fern, when falling, leaves a scar on the stem varying in shape and size according to the kind of Fern, so that the botanist readily distinguishes any particular species of Fern by this means,—a birth-mark, as it were, by which he detects the parentage of the individual. Another indication, equally significant, is found in the tubular structure of the wood in Ferns. On a vertical section of any well-preserved Fern-trunk from the old forests the little tubes may be seen very distinctly running up its length; or, if it be cut through transversely, they may be traced by the little pores like dots on the surface. Trees of this description are found in the Carboniferous marshes, standing erect and perfectly preserved, with trunks a foot and a half in diameter, rising to a height of many feet. Plants so strongly bituminous as the Ferns, when they equalled in size many of our present forest-trees, naturally made coal deposits of the most combustible quality. It is true that we find the anthracite coal of the same period with comparatively little bituminous matter; but this is where the bitumen has been destroyed by the action of the internal heat of the earth.
Next to the Ferns, the Club-Mosses (Lycopodiacae) seem to have contributed most largely to the marsh-forests. They were characterized, then, as now, by the small size of the leaves growing close against the stem, so that the stem itself, though covered with leaves, looks almost naked, like the stem of the Cactus. Beside these, there are the tree-like Equiseta, in which we find the articulations on the trunk corresponding exactly to those now so characteristic of those marsh-grasses which are the modern representatives of this family of plants, with cone-like fructifications on the summit of the stem.
I would merely touch here upon a subject which does not belong to my own branch of Natural History, but is of the greatest interest in botanical research, namely, the gradation of plants in the geological ages, and the combination of characters in some of the earlier vegetable forms, corresponding to that already noticed in the ancient animal types. For instance, in the Carboniferous period we have only Cryptogams, Ferns, Lycopodiacae, and Equisetaceae. In the middle geological ages, Conifers are introduced, the first flowering plant known on earth, but in which the flower is very imperfect as compared with those of the higher groups. The Coniferae were chiefly represented in the middle periods by the Cycadae, that peculiar group of Coniferae, resembling Pines in their structure, but recalling the Ferns by their external appearance. The stem is round and short, its surface being covered with scars similar to those of the Ferns; while on the summit are ten or more leaves, fan-like and spreading when their growth is complete, but rolled up at first, like Fern-leaves before they expand. Their fruit resembles somewhat the Pine-Apple.
The mode of growth of the Coniferae recalls a feature of the Equisetaceae also, in the tufts of little leaves which appear in whorls at regular intervals along the length of the stem in proportion as it elongates, reminding one of the articulations on the stem of the Equisetaceae. The first cone also appears on the summit of the stem, like the terminal cone in the Equisetaceae and the Club-Mosses. Thus in certain types of the vegetable, as well as the animal creation of earlier times, there was a continuation of features, afterwards divided and presented in separate groups. In the present times, no one of these families of plants overlaps the others, but each has a distinct individual character of its own.
At the close of the middle geological ages and the opening of the Tertiary periods, the Monocotyledons become abundant, the first plants with flower and inclosed seed, though with no true floral envelope: but not until the two last epochs of the Tertiary age do we find in any number the Dicotyledonous plants, in which flower and fruit rise to their highest perfection. Thus there has been a procession of plants from their earliest introduction to the present day, corresponding to their botanical rank as they now exist, so that the series of gradation in the Vegetable Kingdom, as well as the Animal Kingdom, is the same, whether founded upon succession in time or upon comparative structural rank.
Some attempt has been made to reproduce under an artistic form the aspect of the world in the different geological ages, and to present in single connected pictures the animal and vegetable world of each period. Professor F. Unger, of Vienna, has prepared a collection of fourteen such sketches, entitled, "Tableaux Physionomiques de la Végétation des Diverses Périodes du Monde Primitif."
First, we have the Devonian shores, with spreading fields of sea-weed and numbers of the club-shaped Algae of gigantic size. He has ventured, also, to represent a few trees, with scanty foliage; but I believe their existence at so early a period to be very problematical.
Next comes the Carboniferous forest, with still pools of water lying between the Fern-trees, which, much as they affect damp, swampy grounds, seem scarcely able to find foothold on the dripping earth. Their trunks, as well as those of the Club-Moss trees which make the foreground of the picture, stand up free from any branches for many feet above the ground, giving one a glimpse between them into the dim recesses of this quiet, watery wood, where the silence was unbroken by the song of birds or the hum of insects. We shall find, it is true, when we give a glance at the animals of this time, that certain insects made their appearance with the first terrestrial vegetation; but they were few in number and of a peculiar kind, such as thrive now in low, wet lands.
Upon this follow a number of sketches introducing us to the middle periods, where the land is higher and more extensive, covered chiefly with Pine forests, beneath which grows a thick carpet of underbrush, consisting mostly of Grasses, Rushes, and Ferns. Here and there one of the gigantic reptiles of the time may be seen sunning himself on the shore. One of these sketches shows us such a creature hungrily inspecting a pool where Crinoids, with their long stems, large, closely-coiled Chambered Shells, and Brachiopods, the Oysters and Clams of those days, offer him a tempting repast. Here and there a Pterodactyl, the curious winged reptile of the later middle periods, stretches its long neck from the water, and birds also begin to make their appearance.
After these come the Tertiary periods: the Eocene first, where the landscape is already broken up by hills and mountains, clothed with a varied vegetation of comparatively modern character. Lily-pads are floating on the stream which makes the central part of the picture; large herds of the Palaeotherium, the ancient Pachyderm, reconstructed with such accuracy by Cuvier, are feeding along its banks; and a tall bird of the Heron or Pelican kind stands watching by the water's edge. In the Miocene the vegetation looks still more familiar, though the Elephants roaming about in regions of the Temperate Zone, and the huge Salamanders crawling out of the water, remind us that we are still far removed from present times. Lastly, we have the ice period, with the glaciers coming down to the borders of a river where large troops of Buffalo are drinking, while on the shore some Bears are feasting on the remains of a huge carcass.
It is, however, with the Carboniferous age that we have to do at present, and I will not anticipate the coming chapters of my story by dwelling now on the aspect of the later periods. To return, then, to the period of the coal, it would seem that extensive freshets frequently overflowed the marshes, and that even after many successive forests had sprung up and decayed upon their soil, they were still subject to submergence by heavy floods. These freshets, at certain intervals, are not difficult to understand, when we remember, that, beside the occasional influx of violent rains, the earth was constantly undergoing changes of level, and that a subsidence or upheaval in the neighborhood would disturb the equilibrium of the waters, causing them to overflow and pour over the surface of the country, thus inundating the marshes anew.
That such was the case we can hardly doubt, after the facts revealed by recent investigations of the Carboniferous deposits. In some of the deeper coal-beds there is a regular alternation between layers of coal and layers of sand or clay; in certain localities, as many as ten, twelve, and even fifteen coal-beds have been found alternating with as many deposits of clay or mud or sand; and in some instances, where the trunks of the trees are hollow and have been left standing erect, they are filled to the brim, or to the height of the next layer of deposits, with the materials that have been swept over them. Upon this set of deposits comes a new bed of coal with the remains of a new forest, and. above this again a layer of materials left by a second freshet, and so on through a number of alternate strata. It is evident from these facts that there have been a succession of forests, one above another, but that in the intervals of their growth great floods have poured over the marshes, bringing with them all kinds of loose materials, such as sand, pebbles, clay, mud, lime, etc., which, as the freshets subsided, settled down over the coal, filling not only the spaces between such trees as remained standing, but even the hollow trunks of the trees themselves.
Let us give a glance now at the animals which inhabited the waters of this period. In the Radiates we shall not find great changes; the three classes are continued, though with new representatives, and the Polyp Corals are increasing, while the Acalephian Corals, the Kugosa and Tabulata, are diminishing. The Crinoids were still the most prominent representatives of the class of Echinoderms, though some resembling the Ophiurans and Echinoids (Sea-Urchins) began to make their appearance. The adjoining wood-cut represents a characteristic Crinoid of the Carboniferous age.
[Illustration]
Among the Mollusks, Brachiopods are still prominent, one new genus among them, the Productus, being very remarkable on account of the manner in which one valve rises above the other. The wood-cut below represents such a shell, looked at from the side of the flat valve, showing the straight cut of the line of juncture between the valves and the rising curve of the opposite one, which looks like a hooked beak when seen in profile.
[Illustration]
Other species of Bivalves were also introduced, approaching more nearly our Clams and Oysters, or, as they are called in scientific nomenclature, the Lamellibranchiates. They differ from the Brachiopods chiefly in the higher character of their breathing-apparatus; for they have free gills, instead of the net-work of vessels on the lining skin which serves as the organ of respiration in the Brachiopods. We shall always find, that, in proportion as the functions are distinct, and, as it were, individualized by having special organs appropriated to them, animals rise in the scale of structure. The next class of Mollusks, the Gasteropods, or Univalves, with spiral shells, were numerous, but, from their brittle character, are seldom found in a good state of preservation.
The Chambered Shells, or the Cephalopods, represented chiefly in the earlier periods by the straight Orthoceratites described in a previous article, are now curled in a close coil, and the internal structure of their chambers has become more complicated. The subjoined wood-cut represents a characteristic Chambered Shell of the Carboniferous age. Goniatites is the scientific name of these later forms. If we had looked for them in the Devonian period, we should have found many with looser coils than these, and some only slightly curved in the shape of a horn. These, as well as the perfectly straight forms, still exist in the coal period, but the Goniatites with close whorls are the more numerous and more characteristic.
[Illustration]
The Articulates have gained their missing class since the close of the Devonian period, for Insects have come in, and that division of the Animal Kingdom is therefore complete, and represented by three classes, as it is at present. Of the Worms little can be said; their traces are found as before, but they are very imperfectly preserved. There are still Trilobites, but they are very few in number, and other groups of Crustacea have been added.
One of the most prominent of these new types bears a striking resemblance to the Horse-Shoe Crab of present times.
[Illustration]
[Illustration]
I here present one of our common Horse-Shoe Crabs above one of these old-world Crustaceans, and it will be seen, that, while the latter preserves some of the Trilobitic characters, such as the marked articulations on the posterior part of the body and their division into three lobes, yet in the prominence of its anterior shield, its more elongated form, and tapering extremity, it resembles its modern representative. In some of them, however, there is no such sharp point as is here figured, and the body terminates bluntly. There were a large number of these Entomostraca in the Carboniferous period, a group which is chiefly represented among living Crustacea by an exceedingly minute kind of Shrimp; but in those days they were of the size of our Crabs and Lobsters, or even larger, and the Horse-Shoe Crab still maintains their claim to a place among the larger and more conspicuous members of the class.
The Insects were few, and, as I have said above, of a kind which seeks a moist atmosphere, or whose larvae live altogether in water. They are not usually well preserved, as will be seen from the broken character of the one here represented, although the wood-cut is made from a better specimen than is often found. We have, however, remains enough to establish unquestionably the fact of their existence in the Carboniferous period, and to show us that the type of Articulates was already represented by all its classes.
[Illustration]
Not so with the Vertebrates. Fishes abound, but their class still consists, as before, of the Ganoids, those fishes of the earlier periods built on the Gar-Pike and Sturgeon pattern, and the Selachians, represented now by Sharks and Skates. In the Carboniferous period we begin to find perfectly preserved specimens of the Ganoids, and the adjoining wood-cut represents such a one. Of the old type of Selachians we have again one lingering representative in our own times to give us the clue to its ancestors,—as the Gar-Pike explains the old Ganoids, and the Chambered Nautilus helps us to understand the Chambered Shells of past times. The so-called Port-Jackson Shark has features which were very characteristic of the Carboniferous Sharks and are lost in the modern ones, so that it affords us a sort of link, as it were, and a measure of comparison, between those now living and the more ancient forms. It is an interesting fact that this only living representative of the Carboniferous Shark should be found in New Holland, because it is there, in that isolated continent, left apart, as it would seem, for a special purpose, that we find reproduced for us most fully the character of the Animal Kingdom in earlier creations.
[Illustration]
The first Mammalia in the world were pouched animals, having that extraordinary attachment to the mother after birth which characterizes the Kangaroo. In New Holland almost all the Mammalia are pouched, and have also the imperfect organization of the brain, as compared with the other Mammalia, which accompanies that peculiar structural feature; and although the American Opossum makes an exception to the rule, it is nevertheless true that this type of the Animal Kingdom is now confined almost exclusively to New Holland. Whether this living picture of old creations in modern garb was meant to be educational for man or not, it is at least well that we should take advantage of it in learning all it has to teach us of the relations between the organic world of past and present times.
There were a great variety of the Selachians in the Carboniferous period. The wood-cuts below represent a tooth and a spine from one of the most characteristic groups, but I have not thought it worth while to enumerate or to figure others here, for there are no perfect specimens, and their structural differences consist chiefly in the various form and appearance of the teeth, scales, and spines, and would be uninteresting to most of my readers. I would refer the more scientific ones, who may care to know something of these details, to my investigations on Fossil Fishes, published many years since under the title of "Recherches sur les Poissons Fossiles."
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Although the Vertebrate division of the Animal Kingdom still waited for its higher classes, yet it had received one important addition since the Silurian and Devonian periods. The Carboniferous marshes were not without their reptilian inhabitants; but they were Reptiles of the lowest class, the so-called Amphibians, those which are hatched from the egg in an immature condition, undergoing metamorphosis after birth. They have no hard scales, and lay a large number of eggs. I am unable to present any figure of one of these ancient Reptiles, as they are found in so imperfect a state of preservation that no plates have been made from them. I would add in connection with this subject that I believe a large number of animals found in the Carboniferous deposits, and referred to the class of Reptiles, to be Fishes allied to Saurians.
Before leaving the Carboniferous period, let us see what territory the United States has conquered from the Ocean during that time. All its central portion, from Canada to Alabama, and from Western Iowa, Missouri, and Arkansas to Eastern Virginia, was raised above the water. But as yet the Alleghanies and the Rocky Mountains did not exist; a great gulf ran up to the mouth of the Ohio, for the Mississippi had not yet accumulated the soil for the fertile valley through which it was to take its southern course; the Coral-Builders had still their work to do in constructing the peninsula of Florida; and, indeed, all the borders of the continent of North America, as well as a large part of its Western territory, were still to be added. But although its central portion held its ground and was never submerged again, yet the continent was slowly subsiding during the middle geological periods, so that, instead of enlarging gradually by the increase of deposits, its limits remained much the same.
This accounts for the very scanty traces to be found in America of the secondary deposits; for the Permian, Triassic, and Jurassic beds, instead of being raised to form successive shores, along which their deposits could be accumulated in regular sequence, as had been the case with the Azoic, Silurian, and Devonian deposits in the northern part of the United States, were constantly sinking, so that the Triassic settled above the Permian, the Jurassic above the Triassic, and so on, each set of strata thus covering over and concealing the preceding one. Though we find the stratified rocks of these periods cropping out here and there, where some violent disturbance or the abrading action of water has torn asunder or worn away the overlying strata, yet we never find them consecutively over any extensive region; and it is not till the Cretaceous and earlier Tertiary periods that we find again a regular succession of deposits around the shores of the continent, marking its present outlines. It is, then, in Europe, where the sequence of their beds is most complete, that we must seek to decipher the history of the middle geological ages; and therefore, when I meet my readers again, it will be in the Old World of civilization, though more recent in its physical features than the one we leave.
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