THE STRENGTH OF THE EARTH'S CRUST 47 



theless to imply a strength of the crust about twice that assumed 

 as a maximum by Hoskins as a basis for his calculation. There 

 are several contributing factors which may explain the disagree- 

 ment between the figures obtained by observation of the deltas 

 and the calculation given by Hoskins and others: First, part of the 

 stress is transmitted laterally to some ex;tent into the deeper layers, 

 but as the diameter of the loaded area is four times its depth this 

 can be a partial explanation only and has, furthermore, been allowed 

 for. Second, part of the stress may be transmitted into the deeper 

 earth below the 76-mile zone of isostatic compensation. This is 

 about equivalent to third, that the zone of isostatic compensation 

 may extend deeper, at least locally, and fade out more after the 

 suggestion made by Chamberlin.^ Fourth, a consideration which 

 the writer regards as most important is that the crust may in reality 

 possess greater crushing strength than the 25,000 pounds per inch 

 postulated by Hoskins. At the time that Hoskins made this cal- 

 culation it seemed that this figure was the highest which could be 

 chosen, since it is higher in fact than the crushing strength of the 

 average surface rock when subjected for even a short time to com- 

 pression in a testing machine, and in the earth the stresses must be 

 carried for indefinite periods. The experiments by Adams^ have 

 shown, however, that under the conditions of cubic compression 

 which exist in the earth the rocks are capable of sustaining for 

 indefinite times far higher stress differences than they could bear 

 even for a short time when subjected to stress in one direction only, 

 as at the surface of the earth. These experiments showed that: 



At ordinary temperatures but under the conditions of hydrostatic pressure 

 or cubic compression which exist within the earth's crust, granite will sustain 

 a load of nearly 100 tons to the square inch, that is to say, a load rather more 

 than seven times as great as that which will crush it at the surface of the earth 

 under the conditions of the usual laboratory test. 



Under the conditions of pressure and temperature which are believed to 

 obtain within the earth's crust empty cavities may exist in granite to a depth 

 of at least 11 miles. ^ 



^ Jour. Geol., XV (1907), 76. 



^ "An Experimental Contribution to the Question of the Depth of the Zone of 

 Flow in the Earth's Crust," Jour. Geol., XX (1902), 97-118. 

 ■5 Op. cit., p. 117. 



