The Compressibilities of Dunite and of Basalt Glass and Their Bearing on the Composition of the Earth.

In a former publication from this Laboratory, results on the compressibility of various minerals and rocks were presented.' At the time when these measurements were made the importance of the physical properties of the ultrabasic rocks was not fully appreciated. Realizing that too little emphasis had been placed on these materials we have now made a number of measurements on a rock approaching olivine in composition. For reasons which will be given later, we have also made some measurements on a sample of basaltic glass. Method.-The method used in measuring the compressibility of solids has already been described in detail.2 The material to be investigated is subjected to pressure in a steel cylinder or bomb. The specimen, usually cylindrical in form, is entirely surrounded by a liquid, and pressure is produced by forcing a piston into the bomb. The compressibility is determined by a series of measurements of (1) the pressure, which is measured by means of a resistance gauge, and (2) the piston-displacement as read on a dial micrometer. A preliminary experiment using a solid of known compressibility, namely soft steel, serves to compensate for the distortion of the bomb and for the compressibility of the liquid used to transmit pressure. From these readings the change in volume of the specimen at various pressures is calculated from the formula: