Abnormal Densities in the Earth's Crust Disclosed by Analysis of Geodetic Data

Abstract

THE first comprehensive test of the Theory of Isostasy was rhade by Hayford of the United States Coast and Geodetic Survey,* when he used the latitude and longitude determined in the United States by astronomical observations and by direct measurements in the form of triangulation. Hayford assumed that, under each topographic feature, there is a deficiency of mass exactly equal to the mass of the feature, that the deficiency is confined to a layer terminating at a uniform depth below sea-level, and that the deficiency is uniformly distributed vertically. It was necessary to assume some such ideal conditions in order to make feasible the vast amount of computations involved. Besides, it was as reasonable a set of conditions as any other which might have been adopted. Hayford's researches indicated that the theory of isostasy is substantially true. This is shown by the fact that he obtained deflections of the vertical only ten per cent. as large on an average as computed deflections on the rigid Earth hypothesis. The work done in India by Burrard and his assistants, and in the United States by the author, has so extended the knowledge, gained from Hayford's work, of the isostatic conditions of the Earth's crust that we may now draw some quite definite conclusions.