THE STRENGTH OF THE EARTH'S CRUST 509 



of a thousandth part of that which granite at the surface of the 

 earth can sustain. Furthermore, even if the stresses were greater 

 and could be used as a measure of strength, this would apply to 

 sudden stresses only and the results obtained from elastic vibra- 

 tions could not be used safely as a means of determining the 

 strength under long-enduring stresses. Thus the evidence from 

 both tides and earthquakes is negative in regard to the existence 

 of an asthenosphere. They show only that it is not fluid and that 

 it is not markedly unlike the rest of the earth in its elastic properties. 



HIGH, BUT VARIABLE, ELASTIC LIMIT WITHIN THE UPPER LITHOSPHERE 



The experiments by F. D. Adams showed that under conditions 

 of cubic compression rocks became far stronger than when sub- 

 jected to compression, as at the surface of the earth, in one direction 

 only. When a cylinder of Westerly granite was incased in a steel 

 jacket and then subjected to heavy pressure upon its ends, a small 

 cavity within the specimen just began to break down under a stress- 

 difference of between 160,000 and 200,000 pounds per square inch, 

 about six to eight times the strength possessed by this rock under 

 surface conditions. At a temperature of 550° C, a temperature 

 calculated to exist at a depth of 11 miles below the earth's surface, 

 small cavities remained open when submitted to considerably 

 greater pressures than occur from the overlying load at this 

 depth.^ 



Adams' experiments and King's calculations are most important 

 and show without doubt that the more superficial parts of the 

 earth, to a depth of ten to fifteen miles at least, are far stronger 

 than had been supposed; but they apply to the temperature and 

 pressure gradients in places of geologic quiet, not to regions under- 

 going igneous intrusion and crustal deformation. Then the tem- 

 peratures may become far higher and the crust surcharged with 

 magmatic gases. Yet it is under these conditions especially, of 

 geologic activity as contrasted to geologic quiet, that regional meta- 

 morphism and rock flowage proceeds. Still less does this experi- 

 mental work prove a great strength of the crust at depths of more 



'Louis Vessot King, "On the Limiting Strength of Rocks under Conditions of 

 Stress Existing in the Earth's Interior," Jour. GeoL, XX (191 2), 136, 137. 



