THE STRENGTH OF THE EARTH'S CRUST 443 



The temperature gradient under this view should naturally vary 

 widely from place to place and from time to time. Igneous activity 

 is the effective means by which heat is brought up from depths 

 which on account of the slowness of conduction would be otherwise 

 thermally isolated from the outer crust. Offset against this, cool- 

 ing by conduction advances downward from the surface, dissipating 

 not only the heat of local radio-active origin but that excess rising 

 from the depths. The heat of the crust is not then a continually 

 ebbing residuum from a primal molten state, but represents rather 

 an oscillating ebb and flow, one of the balances of nature main- 

 tained through geologic time. 



If this view be true — -that the invasive igneous rocks have been 

 an important factor in determining the amount and distribution of 

 heat in the crust — it is doubtful if any sound arguments can be 

 derived from the study of the present gradients as to the initial 

 temperature. This conclusion is similar to the change of viewpoint 

 in other lines of geology. It was once thought that the composition 

 of the present atmosphere and the character of present climates were 

 steps in a simple and continuous series of changes passing from 

 primal conditions to a future in which the water would be absorbed 

 into the earth and its surface transformed into a frozen desert. 

 Now, however, it is generally recognized that since the earliest 

 known times the surface conditions have been in a state of oscillat- 

 ing equilibrium. The argument of this section leads toward the 

 view that this is true for the physical conditions within lithosphere 

 and asthenosphere also. 



[To be concluded] 



