AND ITS INHABITANTS 41 



state, shift into positions which ease the strain, and again enter 

 into a crystalline solid condition. The proof that such a pro- 

 cess exists in the earth is based on several lines of evidence. 



First, evidence of a broad isostatic equilibrium notwith- 

 standing the agencies of mountain folding, of erosion, and of 

 sedimentation, all of which work through geologic time tend- 

 ing to destroy those relations of elevation which are needed 

 to maintain isostasy, giving equal pressures by broad crustal 

 areas of unlike density upon the yielding zone below. 



Second, the evidence of increasing temperature with depth, 

 giving temperatures close to those of fusion at depths below 

 40 to 50 miles. 



Third, the evidence from tides and earthquakes that the 

 earth as a whole is more rigid than steel and cannot possess a 

 fluid shell beneath the crust. 



Fourth, the physical principle that at temperatures close 

 to fusion a crystalline substance is incapable of supporting 

 permanent shearing stresses, but yields slowly by recrystalliza- 

 tion, notwithstanding the fact that under short stresses the 

 same substance may be as rigid as steel. 



The conclusion to which this argument leads is that an outer 

 crust or litho sphere, the rock sphere, 50 to 75 miles thick and 

 very strong, is marked by broad variations in density amount- 

 ing to as much as 5 per cent, and more local variations up to 

 10 per cent, which correspond to the broader relief of the 

 earth's surface. Below this lies a thick, hot, basic, rigid yet 

 weak shell which the writer has named the asthenosphere, 1 

 the sphere of weakness. The problem of the origin of the 

 ocean basins and of continental platforms resolves itself con- 

 sequently into the origin of the density differences in the 

 lithosphere and the maintenance of the heated and weak con- 

 dition in the asthenosphere. 



7 Barrell, Joseph, "The Strength of the Earth's Crust." Jour. Geology, vols. 

 22, 23, 1914, 1915. 



