Surface Structure beneath the Pacific

Abstract

IN a letter in NATURE of November 2, p.723, it is stated by Mr. H. F. Baird, acting director of Apia Observatory, that I have found from the velocities of seismic surface waves evidence of the existence of a layer of granite some 10 km. thick below the Pacific Ocean. This statement, based on an abstract only of my paper, is liable to be quite misleading. One extract from the actual paper will make the position clear: ” Our ignorance of the structure of the Pacific floor gives no ground for applying the theory of a double surface-layer; it might be worth applying a single layer theory if we had any idea of the properties of that layer. The order of magnitude of the thickness of an equivalent single layer of granite can be found.” If that layer happens to be granite, then the theory gives 10 km. with fair accuracy; if diorite, the thickness is about 16 km.