[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 for whom 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 more e 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 might have 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. For example, 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 petty dwarf 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 extent in 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. Farther towards 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 the Northern Drift.

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:—

  1. 3. Superficial Sands or Gravels.
  2. 2. Stratified Clays.
  3. 1. Till or Boulder Clay.

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 called Unstratified Drift, 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 here they 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 all these 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, usually N.E. and S.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 am familiar, 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 of glacier action; on the great plains, on the contrary, the indications are those of marine glaciation 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 drift material 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.