248 B. WILLIS ^DISCOIDAL STRUCTURE OF THE LITHOSPHERE 



which demonstrate that pressure increases the internal friction and con- 

 sequently 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 strength of rock — that 

 is, the ratio of the absolute strength to the superincumbent load — there- 

 fore decreases with increasing depth. The relative strength thus becomes 

 equal to 1 at 40 miles or less below the surface, and below that depth 

 is a fraction; that is to say, the rock is potentially crushed. 



According to this analysis, the "zone of flowage" lies much deeper 

 than was estimated by Van Hise. Furthermore, emphasis is laid upon 

 the fact that the flow of a solid takes place either by recrystallization or 

 by shearing, and thus differs markedly from liquid flow. No matter how 

 slight the relative strength of the absolutely strong rock may be, it can 

 not transmit pressures hydrostatically, and the assumption of hydrostatic 

 pressure, which underlies many discussions of the mechanical state of the 

 lithosphere, including isostasy, is an assumption of conditions which can 

 exist only if the rock be molten. 



In order to analyze the relative effects of temperature and pressure, a 

 comparison is made between the moduli of thermal expansion and the 

 moduli of compression of rocks, plate-glass being taken as a representative 

 substance. Quantitative values are not determinable, since the moduli 

 at high temperatures and pressures are unknown. It is shown, however, 

 that heat and pressure, in constant and unrelaxing opposition, maintain 

 the rock in a sensitive elastic state, such that it responds instantly by 

 change of volume to any variation of pressure or temperature. 



Isostasy is discussed as an essential working hypothesis. Among the 

 several forms which the hypothesis has assumed during the past thirty 

 years, that one is preferred which was formulated by Gilbert and which 

 postulates isostatic equilibrium among large masses, but recognizes effect- 

 ive rigidity of the crust as the condition of support of smaller irregulari- 

 ties of the earth^s surface. 



The demonstrated strength of the crust puts out of consideration the 

 effects of erosion and deposition as a cause of deformation. 



Hayford's mathematical deductions are believed to be inconsistent with 

 the facts of geologic history. Exception is taken to the assumption of uni- 



