THE STRENGTH OF THE EARTH'S CRUST 731 



follow Darwin's law except in so far as the great rigidity of the 

 outer crust will permit it to sustain loads after the manner of a 

 continuous beam. It is shown, however, that the outer crust is in- 

 efficient as a beam, so that the results of applying Darwin's analysis 

 will probably not be greatly modified. 



It is found that the departures from isostasy are such as throw 

 great stress-differences upon the zone of compensation, here called 

 the lithosphere. The maximum stresses imposed by the loads 

 found to exist within the United States lie furthermore within 

 the outer two-thirds of that zone. The stress-differences due to 

 this cause reach maxima probably between 3,000 and 5,000 pounds 

 per square inch. 



Within the asthenosphere, on the contrary, the stresses caused 

 by the departures from isostasy are very small, under the United 

 States the stress-differences at depths of from 400 to 600 km. 

 reaching maxima probably between 500 and 600 pounds per square 

 inch, between a sixth and tenth of those existing at higher levels. 



Now the nature of those geologic actions which oppose isostasy, 

 both the great compressive movements and the great cycles of 

 erosion and sedimentation, are such that they tend to destroy the 

 isostatic adjustments of whole continents and large parts of conti- 

 nents. By these broad actions they tend to bring larger and larger 

 stress-differences upon the zone lying more than 200 km. deep. 

 The limitation of their action as shown by the dominance of regional 

 isostasy is therefore to be regarded as an effect of weakness in that 

 zone. This then is another proof of the reality of an astheno- 

 sphere. The proof in Part VI depended upon the dynamics 

 necessary for isostatic undertow; the proof in this part depends 

 upon the limitations of stress with depth as measured by the 

 existing departures from isostasy. 



This is as far as the present fragmentary data and imperfect 

 theory can safely go, but in order to visualize the arguments a 

 curve of strength is given which shows how great a falling-off of 

 strength there is from the upper part of the lithosphere to the middle 

 of the asthenosphere. Below, the strength undoubtedly again in- 

 creases, but the evidence for that is supplied by other Hnes of 

 research than that opened by the geodetic data. 



