Page:The origin of continents and oceans - Wegener, tr. Skerl - 1924.djvu/171

Rh that all layers beneath the level of the deep-sea floor are pressed down by the elevation of this mountain mass (compare the section on mountain-building which follows), then the neighbouring ocean-floor must be also dragged down with it. There is a still further cause for the sinking of the continental margin, namely, the melting of the downward directed mountain folds and the carrying off, by the westerly drift of the block, of the molten masses eastwards, where they are partly elevated, according to our assumptions, in the Abrolhos Bank. The continental margin must thereby sink and the adjacent sima be dragged down with it.

However, all these ideas on the nature of the oceanic deeps are still in need of thorough confirmation by further more accurate investigation, especially by gravity measurements. On this point there are to be found up to the present, to my knowledge, only Hecker’s observations on the Tonga deep, which gave a strong disturbance of gravity (∆ g = −0.25, in comparison with +0.13 to 0.22 on the Tonga plateau).

This would agree with our idea that here isostatic compensation has not yet been effected by the inflow of the sima. But it would be of great importance if, as a result of still further observations on other deeps, we could understand more accurately the nature of these interesting disturbances of gravity.