THE GEOSPHERES 243 



been detected in all igneous rocks, especially 

 in pegmatites, and the quantity in the crust 

 as a whole is believed by R. J. Strutt to be 

 sufficient to account for the temperature 

 gradient, and to indicate that the acid crust 

 cannot be more than forty miles in thickness, 

 otherwise the outflow of heat would be greater 

 than the amount actually observed. It has 

 already been pointed out that there is more 

 radio-active matter in the red clays of the 

 ocean deeps than in any continental strata. 

 We cannot tell why this should be so, but 

 there are indications that it is very closely 

 connected with the rate of deposition of the 

 marine deposits. 



