THE STRENGTH OF THE EARTH'S CRUST 441 



torn is excessive and the motion requires more internal work. Much 

 of its mass is permanently well below the freezing temperature. 

 These are the factors which determine the steepness of the surface 

 gradient. The asthenosphere by contrast should be deep and the 

 differential motions within it necessary to satisfy isostasy would be 

 correspondingly small. The temperature through a wide zone 

 should be that of fusion under the hydrostatic pressures prevailing. 

 The solid rock should be sodden with occluded gases, giving mobility 

 to the growing fluid and ready to play their part in assisting recrys- 

 tallization. Such a physical condition, as long as there is a continu- 

 ous solid, would exhibit perfect elasticity and high rigidity during 

 the passage of transverse vibrations, yet would slowly yield to pro- 

 longed shearing stresses, even though these were very small in 

 amount. 



RELATIONS OF IGNEOUS ACTIVITY TO ASTHENOSPHERE AND 

 LITHOSPHERE 



The argument has led to the view that the asthenosphere is a 

 region where the temperature curve becomes tangent to the fusion 

 curve, but that a condition of solidity is maintained by the recurrent 

 elimination of that material which becomes molten. The impor- 

 tance of such a process, maintaining the solidity of the earth, has 

 been dwelt upon by Chamberlin, especially as accounting for the 

 overwhelming igneous activity of Archeozoic time. In lessened 

 measure it applies to all later times as well. 



Becker has held that the bottom of the zone of isostatic com- 

 pensation is the depth at which the temperature curve approached 

 nearest to the fusion curve, and he was the first to connect in this 

 way the geodetic evidence with a temperature relation.^ But 

 Becker does not conceive of actual, permanent contact of the two 

 curves as occurring, and took the depth of nearest approach to 

 fusion as 122 km. This follows from Hayford's hypothesis of a 

 uniform distribution for isostatic compensation, but in the present 

 work there has been found reason for believing that compensation 

 fades out through a greater depth; the strength, as measured by 



^ "Age of a Cooling Globe in Which the Initial Temperature Increases Directly 

 as the Distance from the Surface," Science, XXVII (1908), 227-33, 392; "The Age 

 of the Earth," Smithsonian Miscellaneous Collections, LVI, No. 6 (1910), 1-28. 



