Page:The origin of continents and oceans - Wegener, tr. Skerl - 1924.djvu/48

24 had succeeded in determining the gravity on board ship by means of simultaneous readings on a mercury barometer and on a boiling-point thermometer, in a manner suggested by Mohn, in place of the pendulum, which is not applicable in this case, he was able to carry out the measurements during several journeys on the Atlantic, Indian and Pacific Oceans, and so obtained definite results. The evident mass-defects of the ocean basins must thus—conversely to the case of the mountain chains—be compensated by a subterranean excess of mass. The manner in which these subterranean excesses and defects can be explained has given rise to many different conjectures in the course of time. Pratt thought of the earth’s crust as a dough-like mass, originally of the same thickness everywhere, expanded upwards in continents and compressed in the oceanic areas. This idea was further developed by Hayford and Helmert and applied generally to the interpretation of the gravity observations.

Recently, however, another view, which had already been expressed by Airy in 1859, has become prominent mainly through Schweydar’s work. According to this view, the continents swim as lighter blocks on the heavier deeper material. Heim was certainly the first to assume that this less dense crust is thickened under mountains, and that the heavy magma was forced to greater depths there (compare Fig. 3). Conversely, this crust must be especially thin under the oceans (according to the displacement theory it is quite absent). The more recent evolution of this doctrine of isostasy has concerned itself mainly with the question of the extent of its validity. For the