Condensed Tabular View MesozoicClick on table to view larger version.
CHAPTER X.
THE NEOZOIC AGES.
Betweenthe Mesozoic and the next succeeding time which may be known as the Neozoic or Tertiary,[AG]there is in the arrangements of most geologists a great break in the succession of life; and undoubtedly the widespread and deep subsidence of the Cretaceous, followed by the elevation of land on a great scale at the beginning of the next period, is a physical cause sufficient to account for vast life changes. Yet we must not forget to consider that even in the Cretaceous itself there were new features beginning to appear. Let us note in this way, in the first place, the introduction of the familiar generic forms of exogenous trees. Next we may mention the decided prevalence of the modern types of coral animals and of a great number of modern generic forms of mollusks. Then we have the establishment of the modern tribes of lobsters and crabs, and the appearance of nearly all the orders of insects. Among vertebrates, the ordinary fishes are now introduced. Modern orders ofreptiles, as the crocodiles and chelonians, had already appeared, and the first mammals. Henceforth the progress of organic nature lies chiefly in the dropping of many Mesozoic forms and in the introduction of the higher tribes of mammals and of man.
[AG]The former name is related to Palæozoic and Mesozoic, the latter to the older terms Primary and Secondary. For the sake of euphony we shall use both. The term Neozoic was proposed by Edward Forbes for the Mesozoic and Cainozoic combined; but I use it here as a more euphonious and accurate term for the Cainozoic alone.
[AG]The former name is related to Palæozoic and Mesozoic, the latter to the older terms Primary and Secondary. For the sake of euphony we shall use both. The term Neozoic was proposed by Edward Forbes for the Mesozoic and Cainozoic combined; but I use it here as a more euphonious and accurate term for the Cainozoic alone.
It is further to be observed that the new things introduced in the later Mesozoic came in little by little in the progress of the period, and anticipated the great physical changes occurring at its close. On the other hand, while many family and even generic types pass over from the Mesozoic to the earlier Tertiary, very few species do so. It would seem, therefore, as if changes of species were more strictly subordinate to physical revolutions than were changes of genera and orders—these last overriding under different specific forms many minor vicissitudes, and only in part being overwhelmed in the grander revolutions of the earth.
Both in Europe and America there is evidence of great changes of level at the beginning of the Tertiary. In the west of Europe beds often of shallow-water or even fresh-water origin fill the hollows in the bent Cretaceous strata. This is manifestly the case with the formations of the London and Paris basins, contemporaneous but detached deposits of the Tertiary age, lying in depressions of the chalk. Still this does not imply much want of conformity, and according to the best explorers of those Alpine regions in which both the Mesozoic and Tertiary beds have been thrown up to great elevations, they are in the main conformable to one another. Something of thesame kind occurs in America. On the Atlantic coast the marine beds of the Older Tertiary cover the Cretaceous, and little elevation seems to have occurred Farther west the elevation increases, and in the upper part of the valley of the Mississippi it amounts to 1700 feet. Still farther west, in the region of the Rocky Mountains, there is evidence of elevation to the extent of as much as 7000 feet. Throughout all these regions scarcely any disturbance of the old Cretaceous sea-bottom seems to have occurred until after the deposition of the older Tertiary, so that there was first a slow and general elevation of the Cretaceous ocean bottom, succeeded by gigantic folds and fractures, and extensive extravasations of the bowels of the earth in molten rocks, in the course of the succeeding Tertiary age. These great physical changes inaugurated the new and higher life of the Tertiary, just as the similar changes in the Permian did that of the Mesozoic.
The beginning of these movements consisted of a great and gradual elevation of the northern parts of both the Old and New Continents out of the sea, whereby a much greater land surface was produced, and such changes of depth and direction of currents in the ocean as must have very much modified the conditions of marine life. The effect of all these changes in the aggregate was to cause a more varied and variable climate, and to convert vast areas previously tenanted by marine animals into the abodes of animals and plants of the land, and of estuaries, lakes, and shallow waters. Still, however, very large areasnow continental were under the sea. As the Tertiary period advanced, these latter areas were elevated, and in many cases were folded up into high mountains. This produced further changes of climate and habitat of animals, and finally brought our continents into all the variety of surface which they now present, and which fits them so well for the habitation of the higher animals and of man.
The thoughtful reader will observe that it follows from the above statements that the partial distribution and diversity in different localities which apply to the deposits of such ages as the Permian and the Trias apply also to the earlier Tertiary; and as the continents, notwithstanding some dips under water, have retained their present forms since the beginning of the Tertiary, it follows that these beds are more definitely related to existing geographical conditions than are those of the older periods, and that the more extensive marine deposits of the Tertiary are, to a great extent, unknown to us. This has naturally led to some difficulty in the classification of Neozoic deposits—those of some of the Tertiary ages being very patchy and irregular, while others spread very widely. In consequence of this, Sir Charles Lyell, to whom we owe very much of our definite knowledge of this period, has proposed a subdivision based on the percentage of recent and fossil animals. In other words, he takes it for granted that a deposit which contains more numerous species of animals still living than another, may be judged on that account to bemore recent. Such a mode of estimation is, no doubt, to some extent arbitrary; but in the main, when it can be tested by the superposition of deposits, it has proved itself reliable. Further, it brings before us this remarkable fact, that while in the older periods all the animals whose remains we find are extinct as species, so soon as we enter on the Neozoic we find some which still continue to our time—at first only a very few, but in later and later beds in gradually increasing percentage, till the fossil and extinct wholly disappear in the recent and living.
The Lyellian classification of the Tertiary will therefore stand as in the following table, bearing in mind that the percentage of fossils is taken from marine forms, and mainly from mollusks, and that the system has in some cases been modified by stratigraphical evidence:—
Post-pliocene, including that which immediately precedes the Modern. In this the shells, etc., are recent, the Mammalia in part extinct.
Pliocene, or more recent age. In this the majority of shells found are recent in the upper beds. In the lower beds the extinct become predominant.
Miocene, or less recent. In this the large majority of shells found are extinct.
Eocene, the dawn of the recent. In this only a few recent shells occur.
If we attempt to divide the Tertiary time into ages corresponding to those of the older times, we are metby the difficulty that as the continents have retained their present forms and characters to a great extent throughout this time, we fail to find those evidences of long-continued submergences of the whole continental plateaus, or very large portions of them, which we have found so very valuable in the Palæozoic and Mesozoic. In the Eocene, however, we shall discover one very instructive case in the great Nummulitic Limestone. In the Miocene and Pliocene the oscillations seem to have been slight and partial. In the Post-pliocene we have the great subsidence of the glacial drift; but that seems to have been a comparatively rapid dip, though of long duration when measured by human history; not allowing time for the formation of great limestones, but only of fossiliferous sands and clays, which require comparatively short time for their deposition If then we ask as to the duration of the Neozoic, I answer that we have not a definite measure of its ages, if it had any; and that it is possible that the Neozoic may have as yet had but one age, which closed with the great drift period, and that we are now only in the beginning of its second age. Some geologists, impressed with this comparative shortness of the Tertiary, connect it with Mesozoic, grouping both together. This, however, is obviously unnatural. The Mesozoic time certainly terminated with the Cretaceous, and what follows belongs to a distinct aeon.
But we must now try to paint the character of this new and peculiar time; and this may perhaps be bestdone in the following sketches: 1. The seas of the Eocene. 2. Mammals from the Eocene to the Modern. 3. Tertiary floras. 4. The Glacial period. 5. The Advent of Man.
The great elevation of the continents which closed the Cretaceous was followed by a partial and unequal subsidence, affecting principally the more southern parts of the land of the northern hemisphere. Thus, a wide sea area stretched across all the south of Europe and Asia, and separated the northern part of North America from what of land existed in the southern hemisphere. This is the age of the great Nummulitic Limestones of Europe, Africa, and Asia, and the Orbitoidal Limestones of North America. The names are derived from the prevalence of certain forms of those humble shell-bearing protozoa which we first met with in the Laurentian, and which we have found to be instrumental in building up the chalk, theForaminiferaof zoologists. (Fig. p. 243.) But in the Eocene the species of the chalk were replaced by certain broad flat forms, the appearance of which is expressed by the term nummulite, or money-stone; the rock appearing to be made up of fossils, somewhat resembling shillings, sixpences, or three-penny pieces, according to the size of the shells, each of which includes a vast number of small concentric chambers, which during life were filled with the soft jelly of the animal. The nummulite limestone was undoubtedly oceanic, and the other shells contained in it are marine species. After what we have alreadyseen we do not need this limestone to convince us of the continent-building powers of the oceanic protozoa; but the distribution of these limestones, and the elevation which they attain, furnish the most striking proofs that we can imagine of the changes which the earth’s crust has undergone in times geologically modern, and also of the extreme newness of man and his works. Large portions of those countries which constitute the earliest seats of man in Southern Europe, Northern Africa, and Western and Southern Asia, are built upon the old nummulitic sea-bottom. The Egyptians and many other ancient nations quarried it for their oldest buildings. In some of these regions it attains a thickness of several thousand feet, evidencing a lapse of time in its accumulation equal to that implied in the chalk itself. In the Swiss Alps it reaches a height above the sea of 10,000 feet, and it enters largely into the structure of the Carpathians and Pyrenees. In Thibet it has been observed at an elevation of 16,500 feet above the sea. Thus we learn that at a time no more geologically remote than the Eocene Tertiary, lands now of this great elevation were in the bottom of the deep sea; and this not merely for a little time, but during a time sufficient for the slow accumulation of hundreds of feet of rock, made up of the shells of successive generations of animals. If geology presented to us no other revelation than this one fact, it would alone constitute one of the most stupendous pictures in physical geography which could be presented to the imagination. I begleave here to present to the reader a little illustration of the limestone-making Foraminifera of the Cretaceous and Eocene seas. In the middle above is a nummulite of the natural size. Below is another, sliced to show its internal chambers. At one side is a magnified section of the common building stone of Paris, the milioline limestone of the Eocene, so called from its immense abundance of microscopic shells of the genus Miliolina. At the other side is a magnified section of one of the harder varieties of chalk, ground so thin as to become transparent,[AH]and mounted in Canada balsam. It shows many microscopic chamberedshells of Foraminifera. These may serve as illustrations of the functions of these humble inhabitants of the sea as accumulators of calcareous matter. It is further interesting to remark that some of the beds of nummulitic limestone are so completely filled with these shells, that we might from detached specimens suppose that they belonged to sea-bottoms whereon no other form of life was present. Yet some beds of this age are remarkably rich in other fossils. Lyell states that as many as six hundred species of shells have been found in the principal limestone of the Paris basin alone; and the lower Eocene beds afford remains of fishes, of reptiles, of birds, and of mammals. Among the latter are the bones of gigantic whales, of which one of the most remarkable is the Zeuglodon of Alabama, a creature sometimes seventy feet in length, and which replaces in the Tertiary the great Elasmosaurs and Ichthyosaurs of the Mesozoic, marking the advent, even in the sea, of the age of Mammals as distinguished from the age of Reptiles.
[AH]As for instance that of the Giant’s Causeway, Antrim.
[AH]As for instance that of the Giant’s Causeway, Antrim.
FORAMINIFERAL ROCK-BUILDERS.A. Nummulites lævigata—Eocene.B. The same, showing chambered interior.C. Milioline limestone, magnified—Eocene, Paris.D. Hard Chalk, section magnified—Cretaceous.
FORAMINIFERAL ROCK-BUILDERS.
A. Nummulites lævigata—Eocene.B. The same, showing chambered interior.C. Milioline limestone, magnified—Eocene, Paris.D. Hard Chalk, section magnified—Cretaceous.
This fact leads us naturally to consider in the second place the mammalia, and other land animals of the Tertiary. At the beginning of the period we meet with that higher group of mammals, not pouched, which now prevails. Among the oldest of these Tertiary beasts areCoryphodon, an animal related to the Modern Tapirs, andArctocyon, a creature related to the bears and racoons. These animals represent respectively the Pachyderms, or thick-skinned mammals, and the ordinary Carnivora. Contemporary withor shortly succeeding these, were species representing the Rodents, or gnawing animals, and many other creatures of the group Pachydermata, allied to the Modern Tapirs and Hogs, as well as several additional carnivorous quadrupeds. Thus at the very beginning of the Tertiary period we enter on the age of mammals, It may be well, however, to take these animals somewhat in chronological order.
If the old Egyptian, by quarrying the nummulite limestone, bore unconscious testimony to the recent origin of man (whose remains are wholly absent from the Tertiary deposits), so did the ancient Britons and Gauls, when they laid the first rude foundations of future capitals on the banks of the Thames and of the Seine. Both cities lie in basins of Eocene Tertiary, occupying hollows in the chalk. Under London there is principally a thick bed of clay, the “London clay” attaining a thickness of five hundred feet. This bed is obviously marine, containing numerous species of sea shells; but it must have been deposited near land, as it also holds many fossil fruits and other remains of plants to which we shall refer in the sequel, and the bones of several species of large animals. Among these the old reptiles of the Mesozoic are represented by the vertebrae of a supposed “sea snake” (Palæophis) thirteen feet long, and species of crocodile allied both to the alligators and the gavials. But besides these there are bones of several animals allied to the hog and tapir, and also a species of opossum, These remains must be drift carcases from neighbouringshores, and they show first the elevation of the old deep-sea bottom represented by the chalk, so that part of it became dry land; next, the peopling of that land by tribes of animals and plants unknown to the Mesozoic; and lastly, that a warm climate must have existed, enabling England at this time to support many types of animals and plants now proper to intertropical regions. As Lyell well remarks, it is most interesting to observe that these beds belong to the beginning of the Tertiary, that they are older than those great nummulite limestones to which we have referred, and that they are older than the principal mountain chains of Europe and Asia. They show that no sooner was the Cretaceous sea dried from off the new land, than there were abundance of animals and plants ready to occupy it, and these not the survivors of the flora and fauna of the Wealden, but a new creation. The mention of the deposit last named places this in a striking light. We have seen that the Wealden beds, under the chalk, represent a Mesozoic estuary, and in it we have the remains of the animals and plants of the land that then was. The great Cretaceous subsidence intervened, and in the London clay we have an estuary of the Eocene. But if we pass through the galleries of a museum where these formations are represented, though we know that both existed in the same locality under a warm climate, we see that they belong to two different worlds, the one to that of the Dinosaurs, the Ammonites, the Cycads, and the minute Marsupials of the Mesozoic, the other to that of the Pachyderms, the Palms, and the Nautili of the Tertiary.
The London clay is lower Eocene; but in the beds of the Isle of Wight and neighbouring parts of the South of England, we have the middle and upper members of the series. They are not, however, so largely developed as in the Paris basin, where, resting on the equivalent of the London clay, we have a thick marine limestone, the Calcaire Grossier, abounding in marine remains, and in some beds composed of shells of foraminifera. The sea in which this limestone was deposited, a portion no doubt of the great Atlantic area of the period, became shallow, so that beds of sand succeeded those of limestone, and finally it was dried up into lake basins, in which gypsum, magnesian sediments, and siliceous limestone were deposited. These lakes or ponds must at some period have resembled the American “salt-licks,” and were no doubt resorted to by animals from all the surrounding country in search of the saline mud and water which they afforded. Hence in some marly beds intervening between the layers of gypsum, numerous footprints occur, exactly like those already noticed in the Trias. Had there been a Nimrod in those days to watch with bow or boomerang by the muddy shore, he would have seen herds of heavy short-legged and three-hoofed monsters (Palæotherium), with large heads and long snouts, probably scantily covered with sleek hair, and closely resembling the Modern Tapirs of South America and India, laboriously wading through the mud, and grunting with indolent delight as they rolled themselves in the cool saline slime. Others more light andgraceful, combining some features of the antelope with those of the Tapir (Anoplotherium) ran in herds over the drier ridges, or sometimes timidly approached the treacherous clay, tempted by the saline waters. Other creatures representing the Modern Damans or Conies—“feeble folk” which, with the aspect of hares, have the structure of Pachyderms—were also present. Creatures of these types constituted the great majority of the animals of the Parisian Eocene lakes; but there were also Carnivorous animals allied to the hyæna, the wolf, and the opossum, which prowled along the shores by night to seize unwary wanderers, or to prey on the carcases of animals mired in the sloughs. Wading birds equal in size to the ostrich also stalked through the shallows, and tortoises crawled over the mud.
Lyell mentions the discovery of some bones of one of these gigantic birds (Gastornis) in a bed of the rolled chalk flints which form the base of the Paris series, resting immediately on the chalk; one of the first inhabitants perhaps to people some island of chalk just emerged from the waters, and under which lay the bones of the mighty Dinosaurs, and in which were embedded those of sea birds that had ranged, like the albatross and petrel, over the wide expanse of the Cretaceous ocean. These waders, however, like the tortoises and crocodiles and small marsupial mammals, form a link of connection in type at least between the Eocene and the Cretaceous, for bones of wading birds have been found in the Greensandsindicating their existence before the close of the Mesozoic.
The researches of Baron Cuvier in the bones collected in the quarries of Montmartre were regarded as an astonishing triumph of comparative anatomy; and familiar as we now are with similar and yet more difficult achievements, we can yet afford to regard with admiration the work of the great French naturalist as it is recorded in its collected form in his “Recherches sur les Ossemens Fossiles,” published in 1812. His clear and philosophical views as to the plan perceptible in nature, his admirable powers of classification, his acute perception of the correlation of parts in animals, his nice discrimination of the resemblances and differences of fossil and recent structures, and of the uses of these,—all mark him as one of the greatest minds ever devoted to the study of natural science. It is obvious, that had his intellect been occupied by the evolutionist metaphysics which pass for natural science with too many in our day, he would have effected comparatively little; and instead of the magnificent museum in the “Règne Animal” and the “Ossemens Fossiles,” we might have had wearisome speculations on the derivation of species. It is reason for profound thankfulness that it was not so; and also that so many great observers and thinkers of our day, like Sedgwick, Murchison, Lyell, Owen, Dana, and Agassiz, have been allowed to work out their researches almost to completion before the advent of those poisonedstreams and mephitic vapours which threaten the intellectual obscuration of those who should be their successors.
If we pass from the Eocene to the Miocene, still confining ourselves mainly to mammalian life, we find three remarkable points of difference—(1) Whereas the Eocene mammals are remarkable for adherence to one general type, viz., that group of pachyderms most regular and complete in its dentition, we now find a great number of more specialised and peculiar forms; (2) We find in the latter period a far greater proportion of large carnivorous animals; (3) We find much greater variety of mammals than either in the Eocene or the Modern, and a remarkable abundance of species of gigantic size. The Miocene is thus apparently the culminating age of the mammalia, in so far as physical development is concerned; and this, as we shall find, accords with its remarkably genial climate and exuberant vegetation.
In Europe, the beds of this age present, for the first time, examples of the monkeys, represented by two generic types, both of them apparently related to the modern long-armed species, or Gibbons. Among carnivorous animals we have cat-like creatures, one of which is the terribleMachairodus, distinguished from all modern animals of its group by the long sabre-shaped canines of its upper jaw, fitting it to pull down and destroy those large pachyderms which could have easily shaken off a lion or a tiger. Here also we have the elephants, represented byseveral species now extinct; the mastodon, a great, coarsely-built, hog-like elephant, some species of which had tusks both in the upper and lower jaw; the rhinoceros, the hippopotamus, and the horse, all of extinct species. We have also giraffes, stags, and antelopes, the first ruminants known to us, and a great variety of smaller and less noteworthy creatures. Here also, for the first time, we find the curious and exceptional group of Edentates, represented by a large ant-eater. Of all the animals of the European Miocene, the most wonderful and unlike any modern beast, is the Dinotherium, found in the Miocene of Epplesheim in Germany; and described by Kaup. Some doubt rests on the form and affinities of the animal; but we may reasonably take it, as restored by its describer, and currently reproduced in popular books, to have been a quadruped of somewhat elephantine form. Some years ago, however, a huge haunch bone, supposed to belong to this creature, was discovered in the South of France; and from this it was inferred that the Dinothere may have been a marsupial or pouched animal, perhaps allied in form and habits to the kangaroos. The skull is three feet four inches in length; and when provided with its soft parts, including a snout or trunk in front, it must have been at least five or six feet long. Such a head, if it belonged to a quadruped of ordinary proportions, must represent an animal as large in proportion to our elephant as an elephant to an ox. But its sizeis not its most remarkable feature. It has two large tusks firmly implanted in strong bony sockets; but they are attached to the end of the lower jaw and point downward at right angles to it, so that the lower jaw forms a sort of double-pointed pickaxe of great size and strength. This might have been used as a weapon; or, if the creature was aquatic, as a grappling iron to hold by the bank, or by floating timber; but more probably it was a grubbing-hoe for digging up roots or loosening the bases of trees which the animal might afterward pull down to devour them. However this may be, the creature laboured under the mechanical disadvantage of having to lift an immense weight in the process of mastication, and of being unable to bring its mouth to the ground, or to bite or grasp anything with the front of its jaws. To make up for this, it had muscles of enormous power on the sides of the head attached to great projecting processes; and it had a thick but flexible proboscis, to place in its mouth the food grubbed up by its tusks. Taken altogether, the Dinothere is perhaps the most remarkable of mammals, fossil or recent; and if the rest of its frame were as extraordinary as its skull, we have probably as yet but a faint conception of its peculiarities. We may apply to it, with added force, the admiring ejaculation of Job, when he describes the strength of the hippopotamus, “He is the chief of the ways of God. He who made him, gave him his sword.”
MIOCENE MAMMALS OF THE EASTERN CONTINENT.In the foregroundElephas,Ganesa,Hydracotherium,Dinotherium,Machairodus,Mastodon longirostris. In the middle distance,Apes, twoAnoplotheres,Palæotherium,Xiphodon, andSivatherium. Sequoias and Fan Palm in the background.
MIOCENE MAMMALS OF THE EASTERN CONTINENT.
In the foregroundElephas,Ganesa,Hydracotherium,Dinotherium,Machairodus,Mastodon longirostris. In the middle distance,Apes, twoAnoplotheres,Palæotherium,Xiphodon, andSivatherium. Sequoias and Fan Palm in the background.
In Asia, the Siwalik hills afforded to Falconer and Cautley one of the most remarkable exhibitions of Miocene animals in the world. These hills form a ridge subordinate to the Himalayan chain; and rise to a height of 2,000 to 3,000 feet. In the Miocene period, they were sandy and pebbly shores and banks lying at the foot of the then infant Himalayas, which, with the table-lands to the north, probably formed a somewhat narrow east and west continental mass or large island. As a mere example of the marvellous fauna which inhabited this Miocene land, it has afforded remains of seven species of elephants, mastodons, and allied animals; one of them, theE. Ganesa, with tusks ten feet and a half long, and twenty-six inches in circumference at the base. Besides these there are five species of rhinoceros, three of horse and allied animals, four or more of hippopotamus, and species of camel, giraffe, antelope, sheep, ox, and many other genera, as well as numerous large and formidable beasts of prey. There is also an ostrich; and, among other reptiles, a tortoise having a shell twelve feet in length, and this huge roof must have covered an animal eighteen feet long and seven feet high. Among the more remarkable of the Siwalik animals is theSivatherium, a gigantic four-horned antelope or deer, supposed to have been of elephantine size, and of great power and swiftness; and to have presented features connecting the ruminants and pachyderms. Our restoration of this creature is to some extent conjectural; and a remarkably artistic, and probably more accurate, restoration of theanimal has recently been published by Dr. Murie, in the Geological Magazine. We justly regard the Mammalian fauna of modern India as one of the noblest in the world; but it is paltry in comparison with that of the much more limited Miocene India; even if we suppose, contrary to all probability, that we know most of the animals of the latter. But if we consider the likelihood that we do not yet know a tenth of the Miocene animals, the contrast becomes vastly greater.
Miocene America is scarcely behind the Old World in the development of its land animals. From one locality in Nebraska, Leidy described in 1852 fifteen species of large quadrupeds; and the number has since been considerably increased. Among these are species of Rhinoceros, Palæotherium, and Machairodus; and one animal, the Titanotherium, allied to the European Anoplothere, is said to have attained a length of eighteen feet and a height of nine, its jaws alone being five feet long.
In the illustration, I have grouped some of the characteristic Mammalian forms of the Miocene, as we can restore them from their scattered bones, more or less conjecturally; but could we have seen them march before us in all their majesty, like the Edenic animals before Adam, I feel persuaded that our impressions of this wonderful age would have far exceeded anything that we can derive either from words or illustrations. I insist on this the more that the Miocene happens to be very slenderlyrepresented in Britain; and scarcely at all in north-eastern America; and hence has not impressed the imagination of the English race so strongly as its importance justifies.
The next succeeding period, that of the Pliocene, continues the conditions of the last, but with signs of decadence. Many of the old gigantic pachyderms have disappeared; and in their stead some familiar modern genera were introduced. The Pliocene was terminated by the cold or glacial period, in which a remarkable lowering of temperature occurred over all the northern hemisphere, accompanied, at least in a portion of the time, by a very general and great subsidence, which laid all the lower parts of our continents under water. This terminated much of the life of the Pliocene, and replaced it with boreal and Arctic forms, some of them, like the great hairy Siberian mammoth and the woolly rhinoceros, fit successors of the gigantic Miocene fauna. How it happened that such creatures were continued during the Post-pliocene cold, we cannot understand till we have the Tertiary vegetation before us. It must suffice now to say, that as the temperature was modified, and the land rose, and the Modern period was inaugurated, these animals passed away, and those of the present time remained.
Perhaps the most remarkable fact connected with this change, is that stated by Pictet, that all the modern European mammals are direct descendants of Post-pliocene species; but that in the Post-pliocenethey were associated with many other species; and these, often of great dimensions, now extinct. In other words, the time from the Pliocene to the Modern, has been a time of diminution of species, while that from the Eocene to the Miocene was a time of rapid introduction of new species. Thus the Tertiary fauna culminated in the Miocene. Yet, strange though this may appear, Man himself, the latest and noblest of all, would seem to have been a product of the later stages of the time of decadence. I propose, however, to return to the animals immediately preceding man and his contemporaries, after we have noticed the Tertiary flora and the Glacial period.
CHAPTER XI.
THE NEOZOIC AGES (continued).
Plant-lifein the Tertiary approaches very nearly to that of the Modern World, in so far as its leading types are concerned; but in its distribution geographically it was wonderfully different from that with which we are at present familiar. For example, in the Isle of Sheppey, at the mouth of the Thames, are beds of “London clay,” fall of fossil nuts; and these, instead of being hazel nuts and acorns, belong to palms allied to species now found in the Philippine Islands and Bengal, while with them are numerous cone-like fruits belonging to the Proteaceæ (banksias, silver-trees, wagenbooms, etc.), a group of trees now confined to Australia and South Africa, but which in the Northern Hemisphere had already, as stated in a previous paper, made their appearance in the Cretaceous, and were abundant in the Eocene. The state of preservation of these fruits shows that they were not drifted far; and in some beds in Hampshire, also of Eocene age, the leaves of similar plants occur along with species of fig, cinnamon, and other forms equally Australian or Indian. In America, especially in the west, there are thick and widely-distributed beds of lignite or imperfect coal of the Eocene period; but the plants foundin the American Eocene are more like those of the European Miocene or the Modern American flora, a fact to which we must revert immediately.
In Europe, while the Eocene plants resemble those of Australia, when we ascend into the Miocene they resemble those of America, though still retaining some of the Australian forms. In the leaf-beds of the Isle of Mull,—where beds of vegetable mould and leaves were covered up with the erupted matter of a volcano belonging to a great series of such eruptions which produced the basaltic cliffs of Antrim and of Staffa,—and at Bovey, in Devonshire, where Miocene plants have accumulated in many thick beds of lignite, the prevailing plants are sequoias or red-woods, vines, figs, cinnamons, etc. In the sandstones at the base of the Alps similar plants and also palms of American types occur. In the Upper Miocene beds of [OE]ningen in the Rhine valley, nearly five hundred species of plants have been found, and include such familiar forms as the maples, plane-trees, cypress, elm, and sweet-gum, more American, however, than European in their aspect. It thus appears that the Miocene flora of Europe resembles that of America at pre sent, while the Eocene flora of Europe resembles that of Australia, and the Eocene flora of America, as well as the modern, resembles the Miocene of Europe. In other words, the changes of the flora have been more rapid in Europe than in America and probably slowest of all in Australia. The Eastern Continent has thus taken the lead in rapidity ofchange in the Tertiary period, and it has done so in animals as well as in plants.
The following description of the flora of Bovey is given, with slight alteration, in the words of Dr. Heer, in his memoir on that district. The woods that covered the slopes consisted mainly of a huge pine-tree (sequoia), whose figure resembled in all probability its highly-admired cousin, the giant Wellingtonia of California. The leafy trees of most frequent occurrence were the cinnamon and an evergreen oak like those now seen in Mexico. The evergreen figs, the custard apples, and allies of the Cape jasmine, were rarer. The trees were festooned with vines, beside which the prickly rotang palm twined its snake-like form. In the shade of the forest throve numerous ferns, one species of which formed trees of imposing grandeur, and there were masses of under-wood belonging to various species of Nyssa, like the tupelos and sour-gums of North America. This is a true picture, based on actual facts, of the vegetation of England in the Miocene age.
But all the other wonders of the Miocene flora are thrown into the shade by the discoveries of plants of this age which have recently been made in Greenland, a region now bound up in what we poetically call eternal ice, but which in the Miocene was a fair and verdant land, rejoicing in a mild climate and rich vegetation. The beds containing these specimens occur in various places in North Greenland; and the principal locality, Atane-Kerdluk, is in lat. 70 N.and at an elevation of more than a thousand feet above the sea. The plants occur abundantly in sandstone and clay beds, and the manner in which delicate leaves and fruits are preserved shows that they have not been far water-borne, a conclusion which is confirmed by the occurrence of beds of lignite of considerable thickness, and which are evidently peaty accumulations containing trunks of trees. The collections made have enabled Heer to catalogue 137 species, all of them of forms proper to temperate, or even warm regions, and mostly American in character. As many as forty-six of the species already referred to as occurring at Bovey Tracey and [OE]ningen occur also in the Greenland beds. Among the plants are many species of pines, some of them of large size; and the beeches, oaks, planes, poplars, maples, walnuts, limes, magnolias, and vines are apparently as well represented as in the warm temperate zone of America at the present day. This wonderful flora was not a merely local phenomenon, for similar plants are found in Spitzbergen in lat. 78° 56'. It is to be further observed, that while the general characters of these ancient Arctic plants imply a large amount of summer heat and light, the evergreens equally imply a mild winter. Further, though animal remains are not found with these plants, it is probable that so rich a supply of vegetable food was not unutilised, and that we shall some time find that there was an Arctic fauna corresponding to the Arctic flora. How such a climate could exist in Greenland and Spitzbergen isstill a mystery. It has, however, been suggested that this effect might result from the concurrence of such astronomical conditions in connection with the eccentricity of the earth’s orbit as would give the greatest amount of warmth in the Northern Hemisphere with such distribution of land and water as would give the least amount of cold northern land and the most favourable arrangement of the warm surface currents of the ocean.[AI]
[AI]Croll and Lyell.
[AI]Croll and Lyell.
Before leaving these Miocene plants, I must refer to a paragraph which Dr. Heer has thought it necessary to insert in his memoir on the Greenland flora, and which curiously illustrates the feebleness of what with some men passes for science. He says: “In conclusion, I beg to offer a few remarks on the amount of certainty in identification which the determination of fossil plants is able to afford us. We know that the flowers, fruits, and seeds are more important as characteristics than the leaves. There are many genera of which the leaves are variable, and consequently would be likely to lead us astray if we trusted in them alone. However, many characters of the form and venation of leaves are well-known to be characteristic of certain genera, and can therefore afford us characters of great value for their recognition.” In a similar apologetic style he proceeds through several sentences to plead the cause of his Greenland leaves. That he should have to do so is strange, unless indeed the botany known to those forwhom he writes is no more than that which a school-girl learns in her few lessons in dissecting a buttercup or daisy. It is easy for scientific triflers to exhibit collections of plants in which species of different genera and families are so similar in their leaves that a careless observer would mistake one for the other, or to get up composite leaves in part of one species and in part of another, and yet seeming the same, and in this way to underrate the labours of painstaking observers like Heer. But it is nevertheless true that in any of these leaves, not only are there good characters by which they can be recognised, but that a single breathing pore, or a single hair, or a few cells, or a bit of epidermis not larger than a pin’s head, should enable any one who understands his business to see as great differences as a merely superficial botanist would see between the flower of a ranunculus and that of a strawberry. Heer himself, and the same applies to all other competent students of fossil plants, has almost invariably found his determinations from mere fragments of leaves confirmed when more characteristic parts were afterwards discovered. It is high time, in the interests of geology, that botanists should learn that constancy and correlation of parts are laws in the plant as well as in the animal; and this they can learn only by working more diligently with the microscope. I would, however, go further than this, and maintain that, in regard to some of the most important geological conclusions to be derived from fossils, even the leaves of plants are vastly moree valuable than the hard parts of animals. For instance, the bones of elephants and rhinoceroses found in Greenland would not prove a warm climate; because the creatures might have been protected from cold with hair like that of the musk-sheep, and they might have had facilities for annual migrations like the bisons. The occurrence of bones of reindeer in France does not prove that its climate was like that of Lapland; but only that it was wooded, and that the animals could rove at will to the hills and to the coast. But, on the other hand, the remains of an evergreen oak in Greenland constitute absolute proof of a warm and equable climate; and the occurrence of leaves of the dwarf birch in France constitutes a proof of a cool climate, worth more than that which can be derived from the bones of millions of reindeer and musk-sheep. Still further, in all those greater and more difficult questions of geology which relate to the emergence and submergence of land areas, and to the geographical conditions of past geological periods, the evidence of plants, especially when rooted in place, is of far more value than that of animals, though it has yet been very little used.
This digression prepares the way for the question: Was the Miocene period on the whole a better age of the world than that in which we live? In some respects it was. Obviously there was in the Northern Hemisphere a vast surface of land under a mild and equable climate, and clothed with a rich and varied vegetation. Had we lived in the Miocene, we mighthave sat under our vine and fig-tree equally in Greenland and Spitzbergen and in those more southern climes to which this privilege is now restricted. We might have enjoyed a great variety of rich and nutritive fruits, and, if sufficiently muscular, and able to cope with the gigantic mammals of the period, we might have engaged in either the life of the hunter or that of the agriculturist under advantages which we do not now possess. On the whole, the Miocene presents to us in these respects the perfection of the Neozoic time, and its culmination in so far as the nobler forms of brute animals and of plants are concerned. Had men existed in those days, however, they should have been, in order to suit the conditions surrounding them, a race of giants; and they would probably have felt the want of many of those more modern species belonging to the flora and fauna of Europe and Western Asia on which man has so much depended for his civilization. Some reasons have been adduced for the belief that in the Miocene and Eocene there were intervals of cold climate; but the evidence of this may be merely local and exceptional, and does not interfere with the broad characteristics of the age as sketched above.
The warm climate and rich vegetation of the Miocene extended far into the Pliocene, with characters very similar to those already stated; but as the Pliocene age went on, cold and frost settled down upon the Northern Hemisphere, and a remarkable change took place in its vegetable productions. Forexample, in the somewhat celebrated “forest bed” of Cromer, in Norfolk, which is regarded as Newer Pliocene, we have lost all the foreign and warm-climate plants of the Miocene, and find the familiar Scotch firs and other plants of the Modern British flora. The animals, however, retain their former types; for two species of elephant, a hippopotamus, and a rhinoceros are found in connection with these plants. This is another evidence, in addition to those above referred to, that plants are better thermometers to indicate geological and climatal change than animals. This Pliocene refrigeration appears to have gone on increasing into the next or Post-pliocene age, and attained its maximum in the Glacial period, when, as many geologists think, our continents were, even in the temperate latitudes, covered with a sheet of ice like that which now clothes Greenland. Then occurred a very general subsidence, in which they were submerged under the waters of a cold icy sea, tenanted by marine animals now belonging to boreal and arctic regions. After this last great plunge-bath they rose to constitute the dry land of man and his contemporaries. Let us close this part of the subject with one striking illustration from Heer’s memoir on Bovey Tracey. At this place, above the great series of clays and lignites containing the Miocene plants already described, is a thick covering of clay, gravel, and stones, evidently of much later date. This also contains some plants; but instead of the figs, and cinnamons, and evergreen oaks, they are the pettydwarf birch of Scandinavia and the Highland hills, and three willows, one of them the little Arctic and Alpine creeping willow. Thus we have in the south of England a transition in the course of the Pliocene period, from a climate much milder than that of Modern England to one almost Arctic in its character.
Our next topic for consideration is one of the most vexed questions among geologists, the Glacial period which immediately preceded the Advent of Man. In treating of this it will be safest first to sketch the actual appearances which present themselves, and then to draw such pictures as we can of the conditions which they represent. The most recent and superficial covering of the earth’s crust is usually composed of rock material more or less ground up and weathered. This may, with reference to its geological character and origin, be considered as of three kinds. It may be merely the rock weathered and decomposed to a certain extentin situ; or it may be alluvial matter carried or deposited by existing streams or tides, or by the rains; or, lastly, it may be material evidencing the operation of causes not now in action. This last constitutes what has been called drift or diluvial detritus, and is that with which we have now to do. Such drift, then, is very widely distributed on our continents in the higher latitudes. In the Northern Hemisphere it extends from the Arctic regions to about 50° of north latitude in Europe, and as low as 40° in North America; and it occurs south of similar parallels in the Southern Hemisphere. Farthertowards the equator than the latitudes indicated, we do not find the proper drift deposits, but merely weathered rocks or alluvia, or old sea bottoms raised up. This limitation of the drift, at the very outset gives it the character of a deposit in some way connected with the Polar cold. Besides this, the general transport of stones and other material in the northern regions has been to the south; hence in the Northern Hemisphere this deposit may be called theNorthernDrift.
If now we take a typical locality of this formation, such, for instance, as we may find in Scotland, or Scandinavia, or Canada, we shall find it to consist of three members, as follows:—
This arrangement may locally be more complicated, or it may be deficient in one of its members. The boulder clay may, for example, be underlaid by stratified sand or gravel, or even by peaty deposits; it may be intermixed with layers of clay or sand; the stratified clay or the boulder clay may be absent, or may be uncovered by any upper member. Still we may take the typical series as above stated, and inquire as to its characters and teaching.
The lower member, or boulder clay, is a very remarkable kind of deposit, consisting of a paste which may graduate from tough clay to loose sand,and which holds large angular and rounded stones or boulders confusedly intermixed; these stones may be either from the rocks found in the immediate vicinity of their present position, or at great distances. This mass is usually destitute of any lamination or subordinate stratification, whence it is often calledUnstratifiedDrift, and is of very variable thickness, often occurring in very thick beds in valleys, and being comparatively thin or absent on intervening hills. Further, if we examine the stones contained in the boulder clay, we shall find that they are often scratched or striated and grooved; and when we remove the clay from the rock surfaces on which it rests, we find these in like manner striated, grooved and polished. These phenomena, viz., of polished and striated rocks and stones, are similar to those produced by those great sliding masses of ice, the glaciers of Alpine regions, which in a small way and in narrow and elevated valleys, act on the rocks and stones in this manner, though they cannot form deposits precisely analogous to the boulder clay, owing to the wasting away of much of the finer material by the torrents, and the heaping of the coarser detritus in ridges and piles. Further, we have in Greenland a continental mass, with all its valleys thus filled with slowly-moving ice, and from this there drift off immense ice-islands, which continue at least the mud-and-stone-depositing process, and possibly also the grinding process, over the sea bottom. So far all geologists are agreed; but herethey diverge into two schools. One of these, than of the Glacier theorists, holds that the boulder clay is the product of land-ice; and this requires the supposition that at the time when it was deposited the whole of our continents north of 40° or 50° was in the condition of Greenland at present. This is, however, a hypothesis so inconvenient, not to say improbable, that many hesitate to accept it, and prefer to believe that in the so-called Glacial period the land was submerged, and that icebergs then as now drifted from the north in obedience to the Arctic currents, and produced the effects observed. It would be tedious to go into all the arguments of the advocates of glaciers and icebergs, and I shall not attempt this, more especially as the only way to decide the question is to observe carefully the facts in every particular locality, and inquire as to the conclusions fairly deducible. With the view of aiding such a solution, however, I may state a few general principles applicable to the appearances observed. We may then suppose that boulder clay may be formed in three ways. (1) It may be deposited on land, as what is called the bottom moraine of a land glacier. (2) It may be deposited in the sea when such a glacier ends on the coast. (3) It may be deposited by the melting or grounding on muddy bottoms of the iceberg masses floated off from the end of such a glacier. It is altogether likely, from the observations recently made in Greenland, that in that country such a deposit is being formed in allthese ways. In like manner, the ancient boulder clay may have been formed in one or more of these ways in any given locality where it occurs, though it may be difficult in many instances to indicate the precise mode. There are, however, certain criteria which may be applied to the determination of its origin, and I may state a few of these, which are the results of my own experience. (1) Where the boulder clay contains marine shells, or rounded stones which if exposed to the air would have been cracked to pieces, decomposed, or oxidized, it must have been formed under water. Where the conditions are the reverse of these, it may have been formed on land. (2) When the striations and transport of materials do not conform to the levels of the country, and take that direction, usuallyN.E.andS.W., which the Arctic current would take if the country were submerged, the probability is that it was deposited in the sea. Where, however, the striation and transport take the course of existing valleys, more especially in hilly regions, the contrary may be inferred. (3) Where most of the material, more especially the large stones, has been carried to great distances from its original site, especially over plains or up slopes, it has probably been sea-borne. Where it is mostly local, local ice-action may be inferred. Other criteria may be stated, but these are sufficient for our present purpose. Their application in every special case I do not presume to make; but I am convinced that when applied to those regions in Eastern America with which I amfamiliar, they necessitate the conclusion that in the period of extreme refrigeration, the greater part of the land was under water, and such hills and mountains as remained were little Greenlands, covered with ice and sending down glaciers to the sea. In hilly and broken regions, therefore, and especially at considerable elevations, we find indications ofglacieraction; on the great plains, on the contrary, the indications are those ofmarineglaciation and transport. This last statement, I believe, applies to the mountains and plains of Europe and Asia as well as of America.
This view requires not only the supposition of great refrigeration, but of a great subsidence of the land in the temperate latitudes, with large residual islands and hills in the Arctic regions. That such subsidence actually took place is proved, not only by the frequent occurrence of marine shells in the boulder clay itself, but also by the occurrence of stratified marine clays filled with shells, often of deep-water species, immediately over that deposit. Further, the shells, and also occasional land plants found in these beds, indicate a cold climate and much cold fresh water pouring into the sea from melting ice and snow. In Canada these marine clays have been traced up to elevations of 600 feet, and in Great Britain deposits of this kind occur on one of the mountains of Wales at the height of 1300 feet above the level of the sea. Nor is it to be supposed that this level marks the extreme height of the Post-pliocene waters, for driftmaterial not explicable by glaciers, and evidences of marine erosion, occur at still higher levels, and it is natural that on high and exposed points fewer remains of fossiliferous beds should be left than in plains and valleys.
At the present day the coasts of Britain and other parts of Western Europe enjoy an exceptionally warm temperature, owing to the warm currents of the Atlantic being thrown on them, and the warm and moist Atlantic air flowing over them, under the influence of the prevailing westerly winds. These advantages are not possessed by the eastern coast of North America, nor by some deep channels in the sea, along which the cold northern currents flow under the warmer water. Hence these last-mentioned localities are inhabited by boreal shells much farther south than such species extend on the coasts and banks of Great Britain. In the Glacial period this exceptional advantage was lost, and while the American seas, as judged by their marine animals, were somewhat colder than at present, the British seas were proportionally much more cooled down. No doubt, however, there were warmer and colder areas, determined by depth and prevailing currents, and as these changed their position in elevation and subsidence of the land, alternations and even mixtures of the inhabitants of cold and warm water resulted, which have often been very puzzling to geologists.
I have taken the series of drift deposits seen in Britain and in Canada as typical, and the previousdiscussion has had reference to them. But it would be unfair not to inform the reader that this succession of deposits after all belongs to the margins of our continents rather than to their great central areas. This is the case at least in North America, where in the region of the great lakes the oldest glaciated surfaces are overlaid by thick beds of stratified clay, without marine fossils, and often without either stones or boulders, though these sometimes occur, especially toward the north. The clay, however, contains drifted fragments of coniferous trees. Above this clay are sand and gravel, and the principal deposit of travelled stones and boulders rests on these. I cannot affirm that a similar succession occurs on the great inland plains of Europe and Asia: but I think it probable that to some extent it does. The explanation of this inland drift by the advocates of a great continental glacier is as follows: (1) In the Pliocene period the continents were higher than at present, and many deep valleys, since filled up, were cut in them. (2) In the Post-pliocene these elevated continents became covered with ice, by the movement of which the valleys were deepened and the surfaces striated. (3) This ice-period was followed by a depression and submergence, in which the clays were deposited, filling up old channels, and much changing the levels of the land. Lastly, as the land rose again from this submergence, sand and gravel were deposited, and boulders scattered over the surface by floating ice.
The advocates of floating ice as distinguished from a continental glacier, merely dispense with the latter, and affirm that the striation under the clay, as well as that connected with the later boulders, is the effect of floating bergs. The occurrence of so much drift wood in the clay favours their view, as it is more likely that there would be islands clothed with trees in the sea, than that these should exist immediately after the country had been mantled in ice. The want of marine shells is a difficulty in either view, but may be accounted for by the rapid deposition of the clay and the slow spreading of marine animals over a submerged continent under unfavourable conditions of climate.
In any case the reader will please observe that theorists must account for both the interior and marginal forms of these deposits. Let us tabulate the facts and the modes of accounting for them.