286 Nature and Bearings of Isostasy. 



of the variations of density between the simple ideal of 

 Harford and the facts of nature, it would appear that 

 compensation is not uniform with depth, but occurs 

 mostly within the outer hundredth of the earth's radius, 

 forty miles, tapering out to a depth several times as 

 great and reaching in some regions perhaps to 200 miles. 



Lines of evidence other than those derived from geod- 

 esy have a bearing on the degree of isostatic warping in 

 response to load. The warpings of the crust beneath 

 the burden of the Pleistocene ice sheets offer interesting 

 studies in this connection. The warpings showed a lag 

 and followed broader lines than the contours of the 

 individual lobes of ice. The burdens of ice sustained 

 by the crust and the regional yielding under greater 

 loads correspond in order of magnitude with the capacity 

 to support the loads of sediment deposited by the Nile 

 and the Niger, as shown by the constructional form of 

 these typical deltas, but the yielding to larger loads im- 

 posed by mountain systems. The capacity of a land sur- 

 face to remain stationary during a cycle of erosion is 

 also to be noted. These various geologic tests of the 

 incompleteness and regional nature of isostasy are in 

 accord with the geodetic evidence as here interpreted. 



Another type of problem consists in the determination, 

 by means of the departures from isostasy, of the loca- 

 tion and magnitude of great bodies of rock of abnormally 

 high or low density. Beneath the plains in the center 

 of the continent these internal loads, both positive and 

 negative, are found to impose burdens on the crust as 

 great as those given by uncompensated mountain masses. 

 For illustration, a great body of heavy rock, presumably 

 gabbro, equivalent to a disc approximating 120 miles in 

 diameter and a thickness of perhaps three miles, is thus 

 found to underlie an area in central Texas. In the 

 neighborhood of Seattle, on the other hand, a remarkable 

 deficiency of density suggests the possibility of the exist- 

 ence there of a massive abyssal body of magma, a batho- 

 lith still unsolidified. 



IVe come next to some of the larger bearings of isos- 

 tasy. Its existence and degree of perfection having been 

 determined, it may be used in turn to throw light on 

 many problems of the earth's interior. Chief among 

 these perhaps is the evidence of isostasy on the distri- 

 bution of strength. 



