Page:The origin of continents and oceans - Wegener, tr. Skerl - 1924.djvu/148

The third phenomenon belonging here is the displacement of the poles of rotation in the course of the earth’s history. In Chapter VI we have deduced the position of the poles in the Carboniferous period; it was essentially different from that of to-day. We certainly do not know whether the position is also altered in relation to the interior of the earth, or if, as many authors contend, the crust only has been displaced. Presumably both occur. But, whichever be true, we need in each case the assumption that the globe, or a part of it, is viscous. This is quite obvious in a shifting of the crust; but in the matter of the displacement of the pole relative to the interior of the earth, we also need a viscous earth. Laplace has shown that the axes in a rigid earth cannot be displaced. His idea indicates that in this case the axis of maximum inertia is firmly fixed through the protuberance at the equator, so that even the greatest displacement of continents and other geological phenomena could not move it to any marked extent. At the same time the axis of rotation also must be confined to this position, allowing for the small Euler perturbation. It is otherwise, when the earth is a viscous body. Lord Kelvin says about this assumption^[1]: “We may not merely admit, but assert as highly probable, that the axis of maximum inertia and axis of rotation, always very near one another, may have been in ancient times very far from their present geographical position, and may have gradually shifted through 10, 20, 30, 40, or more degrees, without there being at any time any perceptible sudden disturbance of either land or water.” Rudzki^[2] also says: “In case the palæontologists ever come to the conclusion that the

1. Sir William Thompson, Report of Section of Mathematics and Physics, p. 11. Report of British Association, 1876.
2. Rudzki, loc. cit., p. 209.