Constitution of the Earth

Abstract

COMPARING the position of geophysics now with what existed in 1910, while we are struck by the great development that has taken place, we are equally struck, on looking more closely, by the fact that most of the theoretical advances are due, not to specifically new methods, but to the fuller application of methods that were already known. The work of Kelvin and Sir George Darwin on the rigidity of the earth, and on the evolution of the earth-moon system under the action of tidal friction, was already classical; Darwin's theory of the stresses needed to support continents and mountains was thirty years old; the existence of isostatic compensation, and the two alternative explanations of Pratt and Airy, had been known for fifty years, Stokes's theory of the determination of the figure of the earth from observations of gravity for sixty, and Poisson's theory of the longitudinal and transverse waves in an elastic solid for eighty. Dr. C. Davison, still with us, had put the thermal contraction theory of mountain formation on a quantitative basis in 1887, and Wiechert had shown how to reconcile the earth's ellipticity and precessional constant on the assumption of a thick rocky shell surrounding a dense metallic core. The existence of a change in properties in the crust in the continents at some small depth had already been inferred from geological considerations by Suess.