Advanced Summary The following discussion seeks to determine inductively the structural character of the lithosphere at depths beyond the reach of observation. The mechanical state of the outer shell of the lithosphere is discussed on the basis of Adams’ experiments with rocks under confining pressures, which demonstrate that pressure increases the internal friction and consequently the absolute strength of solid rocks. The conclusion reached is that the rate of increase of absolute strength is very great in the first few miles below the surface; that below 10 miles the gain decreases rapidly, but there is, nevertheless, a continued increase in strength with increasing depth under normal conditions of pressure’ and temperature in the lithosphere. Pressure alone can not promote mobility. High temperature in excess of the normal is essential to mobility or fusion. The absolute strength of rock does not, however, increase as rapidly as the load with increasing depth. The relative . . .