THE STRENGTH OF THE EARTH'S CRUST 43 



sustain a stress-difference of 1,750 kg. per sq. cm. (25,000 pounds 

 per square inch), and selected specimens show ultimate breaking 

 strength approaching 2,800 kg. per sq. cm. For stresses of geologic 

 endurance and in the heterogeneous outer crust it is probable, how- 

 ever, that stress limits should be chosen below 1,750 kg. per sq. cm. 



The work of Adams and King has shown that small cavities 

 in granite are not closed when the rock is subjected to the pressure 

 and temperature normally existing in the earth at a depth of 11 

 miles.^ The presence of occluded gases acting through great 

 lengths of time, by facilitating recrystallization, might affect this 

 result of laboratory experiments, but the capacity of dry rock to 

 sustain even greater cubic pressures without yielding seems to make 

 safe the conclusion that except in the presence of magmatic emana- 

 tions the crust at a depth of 11 miles (17.7 km.) is able to bear 

 a stress difference of 100,000 pounds per square inch and is at 

 least four times as strong as rock close to the surface. 



At twice this depth, however, the temperatures become such 

 that if it were not for the great pressures even dry rocks would 

 approach a molten condition. The presence of high temperatures 

 and of gases which may act as crystallizers presumably becomes 

 dominant at such depths over the effects of the increasing pressures. 

 We many conclude, therefore, that the maximum strength of the 

 crust in regions free from igneous activity is found at levels above 

 rather than below 40 km. and may lie between 20 and 30 km. deep. 



To bring to a focus this discussion a tabulation of ratios of 

 strengths for increasing depths may be given, as derived from 

 the strength curves F', F" of Fig. 18, the standard being taken as 

 the strength of surface rocks. By giving them merely as ratios 

 and stating that the average strength of the solid rocks at the 

 surface is itself an uncertain quantity owing to complications of 

 structure and composition, the appearance of an undue certainty is 

 avoided. 



The general conclusion which stands out from this tabulation is 

 that the weakest part of the asthenosphere is of the order of one 

 one-hundredth of the maximum strength of the lithosphere and is 

 perhaps only a twenty-fifth of that of massive surface rocks. Its 



^ Jour. GeoL, XX (1912), 97-138. 



