THE STRENGTH OF THE EARTH'S CRUST 503 



should be distinguished from rigidity. If a body can resist even 

 small shearing stresses for an indefinite period, it has the essential 

 properties of a solid and not a gas. If it possesses real rigidity, 

 even if it should be true that under relief from pressure the sub- 

 stance would turn into a gas, yet such relief cannot take place and 

 it is a confusion of terms to speak of the substance as a gas when 

 exhibiting to a striking degree the essential qualities of a solid. 

 This distinction between viscosity and rigidity is of first importance, 

 yet is not mentioned by Arrhenius. Although undercooling of 

 a fluid into a glass gives rise to the elastic properties of a solid, 

 it has not been shown that increase of pressure, however great, 

 upon a gas above the critical temperature would transform in- 

 creasing fluid viscosity into soHd rigidity and plasticity such as is 

 exhibited by the earth. 



As to the hypothesis that the crust is a semipermeable mem- 

 brane, permitting a free downward seepage of ocean water, but 

 little need be said, since this is a subject which has been much 

 discussed in recent years and is now largely discarded by geologists. 

 The evidence against it is varied. Petro logic study shows the 

 deep rocks to be impermeable and unaltered; beyond a shallow 

 depth they are dry, and their gaseous and liquid occlusions are 

 held unchanged for geologic ages. Unsound conclusions have been 

 built upon the behavior of steam within porous sandstones, com- 

 bined with confusion of the rate of diffusion under enormous 

 pressure-gradients in the laboratory with enormous pressures, but 

 low pressure-gradients within the crust. Furthermore volcanoes 

 are not restricted to the vicinity of the sea and their emanations 

 are not of the proper composition to have been derived from ocean 

 waters. As Suess has said, volcanoes are not nourished by the 

 sea, but every volcanic eruption adds to the waters of the ocean. 



The paper under discussion was written by a scientist who has 

 done much exact work in physical chemistry, but who in passing 

 to geologic thinking has adopted the habit of an earlier generation 

 — a habit of speculative thought, suggested by chemical and physi- 

 cal concepts and not verified by a study of the earth. The form 

 of present geologic investigations has, however, advanced to the 

 quantitative stage, although the data are often so inexact that the 



