Oceanic Deeps and the Thickness of the Continents

Abstract

IN a recent paper dealing inpart with the structure of the lithosphere (Geological Magazine, June 1927) I assumed the substratum of the floor of the Pacific to be mainly eclogite, and I hinted at the possibility that the great bordering deeps of that ocean might mark the sites of great intrusions of peridotite, This speculation, however, fails to accord with the requirements of isostasy and is therefore unsatisfactory. An analysis made by Hiller (Genlands Beiträge, 1927, p. 279) of the velocities of tong waves which had followed widely different paths around the upper layers of the lithosphere iseems, moreover, to rule out the identification of the Pacific sima with eclogite. For waves having a period of 18 to 20 seconds, Hiller finds velocities of 3.7 km. per sec. in the material of the Pacific floor, and 2.9 in that of Europe, Asia, and America. He adds that these are the respective velocities to be expected jn sirna and sial. Cormparison of the ratio Of these velocities (1.27) with that of the corresponding P- or S-wave velocities in gabbro and granite (1.23), indicates that the floor of the Pacific down to a depth of the same order as that of the sial of the continents behaves as gabbro would do.