302 HAYFORD— THE EARTH FROM [April 24, 



an earlier part of this address that information as to the distribu- 

 tion of density in the earth is necessary in order to make safe 

 progress in learning other things about the earth. 



Is the earth competent to withstand without slow yielding the 

 stress-differences due to the weight of continents and mountains, 

 the isostatic compensations being considered? From the computa- 

 tions by Darwin and Love, considered in the light of the assump- 

 tions made by them to simplify the computations, I estimate that 

 it is probable that the actual mountains and continents with all 

 their irregularities of shape and elevation possibly produce stress- 

 differences in some few places as great as four tons per square inch, 

 and certainly produce stress-differences at many places as great as 

 two tenths of a ton per square inch. The material would certainly 

 yield slowly under such stress-differences especially when they per- 

 sist continuously over long periods of time and throughout large 

 regions. Four tons per inch is the breaking or rupture load for 

 good granite, one of the strongest materials existing in the earth in 

 large quantities. Two tenths of a ton per square inch is the safe 

 working load used by engineers for good granite. There is abun- 

 dant evidence from laboratory tests that the so-called yield point on 

 which the engineer bases his estimate of safe working load for a 

 given material is a function of the length of time the load is applied 

 and the delicacy of the test. The longer the time of application and 

 the more refined the test to determine the permanent yield the lower 

 the observed yield point. In the case of the test in progress in the 

 earth the time of application is indefinitely long and the test is ex- 

 tremely refined inasmuch as the minimum rate of yielding which 

 may be detected is exceedingly small. 



If an engineer wishes to know whether a bridge, or foundation, 

 or building, or railroad rail is yielding under stress-differences 

 which have been brought to bear upon it he looks for evidence of 

 distress, for rivet heads popped off, scaling from the surface, 

 settling, cracks, or even changes in microscopic structure. The 

 geologists have made very extensive corresponding examinations of 

 the earth. Everywhere they find evidence that the earth has yielded. 

 On the one fourth of the earth's surface exposed to examination, 

 the land, there is no part for which the evidence does not indicate 



