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a supplementary volume containing the index and some plates is still in the press. That will make the book much more useful, for at present the student has no other guide through its rather tortuous course than a too brief summary of its contents, which is in this case the less helpful because, in a work which has been nearly thirty years in publication, recurrences, alterations and additions to earlier material have become inevitable.
No armchair geologist could have written such a work as Das Antlitz'; and Prof. Suess, before undertaking it, worked as a field geologist and widened his experiences by travel. Beginning, in 1849, by investigating the early Palæozoic strata and their foundation of Archæan crystalline rocks in Bohemia, he turned next year to the Alps, where he was introduced to some of their problems by the distinguished Swiss geologists, Bernhard Studer and Escher von der Linth. He subsequently travelled in other parts of Europe. Meanwhile, in more than one quarter, efforts were being made to systematise the facts already ascertained and to interpret their significance. One of our greatest English geologists, Sir Henry de la Beche, had already, as Prof. Suess remarks, struck the right note when, in speaking of South Wales, he declared that the foldings of its mountains imply adaptation to a complicated lateral pressure. That is now generally agreed. When a fairly thick crust had formed on the surface of a globe, once molten, this, as cooling progressed, was bound to pucker like the skin of an apple which has been kept through the winter. But though the wrinkles on the earth's face are the signs of its age, they are less easily explained than in the case of the fruit; for a mountain system is often the result of more than a single disturbance and of movements affecting a far wider area.
Their nature Prof. Suess expounds in the earlier part of his great work. After a discussion, perhaps needlessly prolonged, of the effects of floods, he passes on to earthquakes, which indicate disturbances in the outer crust. These are often quite independent of volcanic explosions, but they are frequent in mountain districts, where the folded rocks are almost certain to be in a state of strain; and, even when occurring in a lowland, can often be reason
ably attributed to movements in ancient rocks concealed beneath a superficial covering of more modern deposits. The earth's crust also is not so uniform as the skin of an apple. It is far from certain that, at the beginning, it would be homogeneous or would solidify simultaneously; for the molten surface would more probably begin to freeze at the poles. Thus the crust would be unequal in hardness and in power of resistance to the strain of contraction. Besides this, we cannot even be sure that the globe originally took the form of a spheroid of rotation. If it did not, and if it began to cool 'in an asymmetrical condition,' the stresses set up would soon become very great.
Prof. Sollas, in his essay on 'The Figure of the Earth,' has lucidly stated the results which might be expected to follow both in a pear-shaped figure, suggested (in 1903) by Mr J. H. Jeans, and in a tetrahedral one, as proposed by Mr Lowthian Green so long ago as 1873. The subject is too intricate for discussion here, but it is remarkable that the position of the mainland masses and ocean basins, as Prof. Suess has pointed out, is explicable by the contraction of a mass which was from the first rather asymmetrical in form; and of these basins the Atlantic and Indian oceans seem to indicate areas of depression much more ancient than that of the Pacific. In any case, when once a tolerably thick crust has formed, further cooling of the interior, with a consequent decrease in its volume, must produce strains in that crust, which must either accommodate itself to the inner mass, and thus become puckered, or be detached from it and overarch an internal cavity. In the latter case gravitation will sooner or later cause a collapse of the crust; and the broken portions will sink downwards until they are again supported by the interior. In other words, as Prof. Suess expresses it, the tensions resulting from the loss of volume in our planet are resolved into tangential and radial components; the one producing more or less horizontal, the other more or less vertical, displacements of its crust.
Since the close packing of material, brought about by the thrusting, adds to the solidity of the portion of crust thus affected, we might expect to find the collapses not infrequently the later in time. That, according to
Mr Clarence King, is the case in the geological province of the Great Basin of North America; the one movement, producing packing and plication, presumably occurred in post-Jurassic times; the other, with displacements strictly vertical, presumably within the Tertiary. By the second movement a large area was broken up into great crustal blocks, some of which slipped down in a vertical direction more than others, till they were again brought to rest—a result which may have been accelerated by the former squeezing up the more plastic inner material into the cavities beneath the latter. In such case the blocks which have been left behind in the downward movement will appear to have risen; the sea may overflow the others, and the ultimate result be a number of masses of older rocks rising like islands above newer ones. Such insulated masses, the 'horsts' of Prof. Suess, play, as we shall see, a very important part as factors in the crust of the earth, for the most striking results of tangential action occur in areas which, at an earlier date, have been profoundly modified by downward movements.
It is, however, possible that in some cases, as we shall see, the two movements may alternate, and an actual collapse of the crust not be the only cause of the one which produces ordinary faults. If a strip of crust be broken into blocks and these wedged together, they will occupy a longer space than they originally did-as we can see by imagining the alternate voussoirs of an arch to slip inwards while the others retain their position. So long ago as 1868, Canon J. M. Wilson, writing in the 'Geological Magazine,' showed that contortion was due to the subsidence of a curved surface, and faulting to its elevation. Hence, since we interpret folding to mean compression, it seems natural to infer from faulting an actual, not a relative elevation; that is, to regard folding as the result of stress, and faulting as the result of strain. If so, Prof. Suess' idea of the unequal dropping down of great crustal blocks, though generally true, may not be sufficiently comprehensive; and it would be more correct to say that, while contraction, when localised, causes the intense plication of mountain chains, it sometimes produces very low arches in a much wider area. It is therefore possible that, as Prof. Lapworth has suggested, broad zones of elevation and depression may
alternate (making allowance for local irregularities) in the crust of the earth as a whole.
Here we may refer to some experiments described in 1879 by the late Prof. Daubrée of Paris, which, perhaps, have hardly received the attention they deserve. Taking one of the large inflated balls of india-rubber, then common playthings with children, he applied a stiffening varnish to alternate longitudinal zones; after which he allowed a little of the gas to escape. The surface was no longer a sphere, the stiffened zones rising above the others in low, arched areas, bounded by circles of longitude and puckered repeatedly parallel with those of latitude. On these strain must have acted in one direction and stress in another at right angles to it. When a broad equatorial zone was similarly treated, arching and puckering occurred at right angles to their former directions. To the objections arising from these experiments we must add one of a more general character. We cannot but doubt whether a lens-shaped cavity would be so readily formed by the still plastic nucleus of the earth tearing itself away, in contracting, from the already solidified crust, as by a strain set up by the application of a gradually increasing lateral pressure to a similar section of that crust.
Thus Prof. Suess, as we think, unduly discredits movements indicative of an actual rise of the land. Dealing with this point, he selects for special examination two instances, one the Pacific coast of Chili, the other the Atlantic coast of Scandinavia with the British Isles. As regards the former, the account given by Charles Darwin in his Geological Observations' is so precise, and shows him to have been so fully on his guard against possible sources of error, that we may fairly demand more convincing evidence than is cited by Prof. Suess; and the other example, especially in regard to our own islands, was so fully discussed in 1904 by Sir A. Geikie in a presidential address to the Geological Society, that it may be enough to say that the old beaches and other seamarks in Scandinavia can hardly be explained by a uniform subsidence of the sea-level. In the southern part they are generally absent in the immediate neighbourhood of the sea, though making their appearance up the fjords, thus indicating that the land has risen more
rapidly in an eastern than in a northern direction. But north of Trondhjem they become more and more conspicuous-raised beaches, sometimes carved by the waves into two or three distinct terraces, or ice-worn rocks similarly grooved, being often visible; while the well-marked beaches within the limits of the Alten Fjord, as has been repeatedly shown during the last sixty years, are distinctly more elevated in one part than in the other.
Thus, while admitting that Prof. Suess succeeds in showing that a rise and fall of the land has sometimes been asserted on insufficient evidence, we cannot but hold this to have been a more important factor in altering the face of the earth than he is willing to concede. The dropping down' of comparatively large tracts of the earth's surface, by which countries have been separated and a hilly lowland converted into an island-studded sea, would be more easily brought about, for the reason already indicated, by the preliminary formation of a low arch in the earth's crust. But at any rate we may safely conclude that mountain-making is a consequence of localised folding, and that volcanic activity is generally associated with areas of vertical displacement or, in other words, with ordinary faulting.
To work out these leading ideas is the task undertaken in 'The Face of the Earth'; and it demands a careful study of the geological history of every portion of that face. The Professor illustrates it by the physical history of the Alps, which sweep in a great curve round the lowland of northern Italy, beginning to the west of Vienna and terminating abruptly against the sea in the neighbourhood of Genoa. But further study shows that this chain cannot be separated, on the one hand, from the Dalmatian Alps east of the Adriatic and, on the other, from the Apennines; and a wider survey indicates that other chains, such as the Carpathians, the Crimean hills and the Caucasus, towards the east, the Pyrenees on the west, and the Atlas on the south or south-west, appear to be related to the deep basin or basins of the Mediterranean. For such intense folding as the Alps display-where a strip of lowland not less than 200 miles broad is supposed to have been reduced by packing to about 130 miles-something more is necessary than a lateral thrust, due in some way or other to contraction of the globe. Such a thrust