“The internal tangential stress in the matter within the lithosphere may increase so much that it can no longer be supported. If this happens a series of local fractures will take place, continuing until the lithosphere is again adjusted much more nearly to a figure of equilibrium, which will be less oblate than the original figure. The effect on the distribution of land and water will be that the depth of the ocean will increase rather rapidly and spasmodically in lower latitudes and diminish in higher latitudes.
“Accordingly, the kind of geological change which the theory of tidal friction would lead us to expect is a sort of rhythmic sequence, involving long periods of comparative quiescence, marked by what Suess calls ‘positive movements of the strand,’ in the higher latitudes, and ‘negative movements’ in the lower, alternating with comparatively short periods of greater activity, marked by rise of the land around the poles and subsidences in the equatorial regions.”
The main periods of adjustment under this scheme fall at the beginning and end of the Proterozoic, in the Permo-Carboniferous and in the Quaternary. The two latter at least were periods of great earth-movement, while the two former were also continental periods, since the land-masses were large and high enough to develop ice-sheets.
The difficult question raised by the low latitudes in which the Pre-Cambrian and Permo-Carboniferous glaciations were chiefly developed cannot yet be regarded as solved, but the geological facts speak strongly in favour of ice-sheets rather than mountain glaciers, and practically speaking it is meteorologically impossible for large ice-sheets to extend to sea-level in the Tropics while the rest of the world enjoys a temperate climate. The only escape seems to be to assume a position of the South Pole somewhere between Africa, India and Australia throughout the whole of the Proterozoic and Palæozoic periods. On the other hand, from the Jurassic onwards, there is no real support to the hypothesis that the positions of the poles were other than they are now. Wegener’s explanation of the Quaternary Ice Age we have seen to be untenable. The period of transition appears to lie in the later Permian and Triassic. The Proterozoic and Permo-Carboniferous glacial periods were much less definite in the north than in the south-east; but such as they were they appear to have been most severe in the east of North America, where the ice was coming from the north; there are also some glacial traces in Europe. This indicates that the position of the North Pole cannot have been in the North Pacific Ocean, which is antipodal to the South Indian Ocean. Hence it seems that what we have to consider is not so much the wanderings of the poles at large among the continents as the break-off at the close of the Palæozoic period of portions of the Antarctic continent and their drift northwards towards the equator. Without going into the mathematics of the question, it seems just possible that the periodic overloading of circumpolar continents by large ice-masses could have this effect in the course of time,[2] but the suggestion is put forward tentatively for consideration rather than as a definite hypothesis. We must be thankful that in the next chapter we are on safer ground.
BIBLIOGRAPHY
Coleman, A. P. “Climates and physical conditions of the early Pre-Cambrian.” Geol. Mag. (6), Vol. 1, 1914, p. 466.
Eckardt, W. R. “Paläoklimatologie.” Sammlung Goschen, Leipzig, 1910.
Geikie, J. “The Evolution of Climate.” Edinburgh, Scot. Geogr. Mag. (6), 1890, p. 57.
Grabau, A. W. “Principles of Stratigraphy.” New York, 1913, pp. 74, et seq.