Page:The origin of continents and oceans - Wegener, tr. Skerl - 1924.djvu/184

158 for it only means a small step forward if the mountain chains of the Himalayas are explained by a gigantic thrusting-forward of a long portion of the crust, the southern end of which, India of the present day, once lay near to Madagascar.

This development, step by step, of the theory of horizontal thrusting, has been accompanied by a series of other attempts to explain mountain-building, according to which the heaving-up of the mountains takes place through internal forces, such as volcanic agencies, the pressure of growing crystals, chemical transformations, or the space requirements of volcanic intrusions. Although the occasional co-operation of such causes should not be denied, it appears to me hopeless to seek herein the main causes of mountain-building. Hence we cannot go any further into these ideas.

Gravity measurements are important for the understanding of folding processes. Koszmat has investigated them for Central Europe in an interesting work, from which the map of Fig. 31 is taken. The gravity values actually observed are, as usual, reduced in such a way that we may suppose that the whole relief of the earth was planed down to sea-level and the measurement was carried out at this zero altitude; that is, besides the reduction to sea-level, the influence of the mass above it is also taken away from the result. The observed value so reduced is then compared