Page:The origin of continents and oceans - Wegener, tr. Skerl - 1924.djvu/199



the margin of a continental block there lies below the floor of the ocean an approximately vertical parting plane between the sial and sima which does not correspond to the natural layer-like arrangement of light and heavy materials, but which only exists because of the solidity of the block of sial. On this account special forces are exerted here which strive to bring about the natural arrangement of the masses, and are therefore in opposition to the molecular forces of the block. In this connection there are a whole series of phenomena, which will be dealt with in the following pages.

As Schiötz first recognized from the gravity measurements taken on the Fram which was drifting over the margin of the continental shelf of the Polar seas, and Helmert deduced in detail later, the pendulum observations on the margins of the continental blocks show a characteristic disturbance of gravity, which is reproduced from Helmert in Fig. 37. As the coast is approached from the land the gravity increases to a maximum on the shore-line itself, then rapidly falls and reaches a minimum at the place below which