« AnteriorContinua »
At the beginning of the third Mediterranean stage the sea once more broadened out, covering even these • Pontic beds' with truly marine strata, the fauna of which approaches more closely than hitherto to that of the existing Mediterranean. Since this time, however, many changes, climatal and geographical, have taken place in the European area, but we can only mention one or two of the most remarkable. The deposits of the third Mediterranean stage, which corresponds generally with the Lower Pliocene in other regions, show by their position that the vertical movements of the sea-margin have amounted in some places to little less than 4000 feet, and by their fauna that only a limited number of the southern forms which had been driven away by the changes leading to the Pontic' condition were able to regain their footing in the Mediterranean area. In fact, the approach of conditions unfavourable to their existence, and probably a gradual refrigeration of climate, are indicated by the general aspect of the fauna of the third stage. This becomes more conspicuous in the passage to the fourth (or Upper Pliocene) stage, in which they are still further reduced in number; and 'we meet with a completely new element which has hitherto been foreign to the Mediterranean, namely, a series of northern immigrants. These, no doubt, signify the gradual approach of that age of severe and widespread cold known as the Glacial epoch.
All through the first and second Mediterranean stages the Ægean region stood above sea-level. During the third it was the site of a deep fresh-water lake and formed part of a large land area, linking the existing continent of Europe with Asia Minor, to the south of which lay the Mediterranean of that age. Then, during the fourth stage, the southern part of this continental area subsided-the volcanic line of the Cyclades probably indicating the position of the fracture, along which eruptions and earthquakes continue to the present day; and the sea penetrated as far northwards as Melos, Rhodes and the eastern part of Cos. Finally, in very recent times, the remainder of the Ægean area subsided, and its waters spread .far and wide over the Pontic basin and even overflowed the otherwise regular shores of this sea into the Sea of Azov. Nor is the Ægean
the only region of comparatively recent subsidence. The island of Malta, the Adriatic and the Tyrrhenian seas, all, in one way or another, bear witness to fractures and downward movements in those parts of the crust. In addition to this, there has been a sinking, not always uniform, of the sea-line; and, though the Professor evidently mislikes the idea of upheavals of the land, we must not forget that the latter is also possible.
The same principles of interpretation are extended to the larger ocean basins. They also are great sunken areas, the continents assuming the character of horsts,' in each case, no doubt, complicated, and not the result of a single movement or necessarily simultaneous. A short inspection, indeed, suffices to show that great differences exist between the Atlantic and Indian Oceans on the one hand and the Pacific Ocean on the other. The Pacific is bordered by long mountain chains and surrounded by volcanoes; there is no such boundary to either of the other seas. The mountains limiting the Pacific are found on examination to be folded ranges, which, though presenting some differences on the east and on the west, agree in turning their folds towards the depths of the sea.
But away from the eastern coastline the structure of the Eurasian continent is extremely complex; and the trends of its folds are more or less transverse to the ocean borders. These do much to determine the boundary of the Atlantic. The eastern margin, instead of being limited by a mountain chain, cuts transversely across the above-named folds and receives from them its well-known irregular outlines. This, no doubt, is true; but we must not forget that deep indentations, such as occur on the coasts of western Scandinavia and western Patagonia, are often due to a change in the sea-level, which many geologists would attribute to a subsidence of the land. The western coast of the Atlantic, though the Alleghany ranges run parallel with it, shows, when it is traced along the two continents, a similar diversity of trend.
The associations of movements on the land and in the ocean are less clear, and apparently belong to more remote dates in the Atlantic and Indian Oceans than they do in the Pacific. In the vast expanse of this sea, islands, either volcanic or coral reefs (the latter probably often founded on extinct cones) are numerous; and some
; of the most profound abysses are parallel with and not far away from greatly folded areas. Here there is a region of collapses on a grander scale and more recent than in the Atlantic. Movements have also occurred, as we have seen, on the continents. It is indeed true that 'where the long street roars hath been The stillness of the central sea'; but its depths have nowhere equalled the abysses of the great oceans. Even the sea in which the white chalk of northern France and parts of Britain was formed, though unusually free from sediment, was probably nowhere so much as 2000 feet in depth, which is not one-sixth of that attained by the greater part of the bed in the Atlantic and the Pacific. In one sense these oceans are very old, because they include basins which may go back to a remote antiquity, and may even be indicative, as some eminent astronomers suppose, of masses flung off into space when the earth's crust was still weak and the rotation about its axis was much more rapid than at the present day. In outline, however, they are more recent. The Indian Ocean, according to Suess, is perhaps the oldest, and had been formed by the subsidence of the greater part of a continent, named Gondwanaland by geologists, which united India and Africa, and of which the Archæan rocks of the Seychelles and Mauritius may be remnants. The Atlantic results from the gradual enlargement of two gulfs, which projected north and south from that old Mediterranean of which we have already spoken, and may date from the middle of the Tertiary era, while the Pacific may have undergone great changes during still later times. In regard to these matters evidence is difficult to obtain. Something may be inferred, as mentioned above, from islands which consist of very ancient rocks and from the presence, in unaltered sedimentary strata, now widely separated, of closely allied plants or animals which are incapable of crossing a broad expanse of sea; but, as a rule, the ocean keeps its secrets.
Prof. Suess returns to this subject, or rather sums up a long series of descriptions, in the opening chapter of his third volume. In language unusually terse and lucid he states his ideas of how he conceives the earth's features to have been formed. Could we see it in the same way
Vol. 216.-No. 431.
as the moon, we should perceive a great difference between the two. The moon is without those long continuous systems of folds which form the mountain chains of the earth. It is without clouds or seas, and gives no sign of stratified deposits. Its surface is studded with the craters of extinct volcanoes, broad rather than high; some many leagues in diameter, others more nearly comparable with those of Hawaii, besides smaller parasitic craters, evidently of later origin. Its crust also, apparently scoriaceous, shows, in its fissures of contraction and crater rills, signs of progressive solidification. On the earth's crust the volcanoes are mostly grouped along circular areas, some of which are those of folded ranges, while others, such as the volcanic lines of Mexico and South America, together with the Ethiopian faulttroughs, cut them at all angles and maintain an independent course. This, at first sight, seems an anomaly, but it is capable of explanation, because the trend-lines of mountain systems are the results, as we have already intimated, of something more than a simple lateral pressure. The geologist who would interpret the meaning of the wrinkles and scars on the earth's time-worn face must ascertain the plan of these trend-lines. Lack of information about some important regions made this almost impossible when Prof. Suess began his task; but so much knowledge has been acquired during the interval between the second and third volumes, that it has been done in the latter, though, as he frankly states, some of his conclusions can be only provisional.
Turning, then, to the trend-lines of Europe, where we are on surer ground, we find as a result of our study that, as a general truth, all the Archæan rocks have suffered folding or an equivalent compression. Later rocks also may be greatly plicated, but this is more local; while their strata, when comparatively undisturbed, are almost always found to rest on a denuded surface of folded ancient rocks. In our own country *the gneisses of the Hebrides were folded and levelled before the Torridon sandstone was deposited upon their remains. Over this sandstone the Caledonian mountain-flakes were thrust from the south-east in pre-Devonian times. Towards the close of the Carboniferous epoch the folding of the Armorican and Variscan arcs took place. When these arcs
had been broken up into horsts there occurred still further to the south, and hemmed in by these horsts, the formation of the Alps' (Suess, iii, 5). They also indicate a triple movement. Combined with the Jura, they form parts of the southernmost, innermost and most recent of those crescentic systems of folds which have arisen, one after the other, across Central Europe.' That alone indicates the necessity for a second factor in mountain-making-something to resist as well as something to push; and this is found, as we have already stated, in the great and sometimes half-buried land-masses on the other side of the yielding area. Augmented in Palæozoic ages by the results of the Armorican and Caledonian foldings, this great mass of crust was too solid to yield to the pressures from the Mediterranean region, to the border zone of which the puckering was restricted. The trend-lines also, as Prof. Suess points out, are affected by the basin of the Atlantic. The arc of Gibraltar and that of the Lesser Antilles, lying almost symmetrically, are bent completely round as they approach the Atlantic region, as though some unknown and mysterious force prevented their entry into this domain. Not only so, but the basin of Asturias shows that, prior to the former arc, a similar kind of flexure existed eight degrees further north. How such curves could be successively formed, unless we assume the existence of some kind of wave propagating itself freely through the crust of the earth, seems to be inexplicable.
These considerations justify the inference that there are tracts of the earth's surface which have been protected from extensions of the sea and from mountainbuilding for a very long period. Here, then, terrestrial faunas would find places of refuge; here they would increase and multiply in more favourable circumstances; here also, in consequence of modification of their environment such as general climatal change, processes of hybridisation, and other disturbances of a uniformity of descent, they would develope new species and genera. Four such asylums may be roughly outlined: (1) Laurentia, including a large portion of North America, extending up to the Arctic Ocean; (2) Angaraland, or the tableland of East Siberia, with, possibly, parts of China ; (3) Gondwana