Status of the Theory of Isostasy. 331 



effective compensation is above a depth of 100 miles. 

 There are here an infinite number of gradations, but 

 they are restricted within definite limits, and the several 

 types shown in curves B, C, and the upper part of 13 

 may be taken as representative. 



It is seen that the hypothesis of uniform distribution, 

 curve B, is the simplest mathematically and readily con- 

 verted into distributions C and D. It is therefore a con- 

 venient form for using with the geodetic data. It is 

 not, however, for that reason more probable. In fact, 

 the indications are that C is more probable than B, and 

 D minus the tail may be better than C. 



The preceding discussion has been based on the solu- 

 tion which gave 113-7 kilometers, 70-7 miles, as the most 

 probable depth for compensation if uniformly distributed. 

 The later results have decreased this to 100 kilometers, 

 60 miles, and the compensation is brought that much 

 nearer to the surface. There appear to be good reasons, 

 however, partly geodetic, partly geologic, for regarding 

 the depth as notably different in different regions. For 

 example, in Hayford's publication of 1909 on deflections 

 of the vertical, group 8, including parts of Utah, Nevada 

 and California, indicated, on the basis of 42 residuals, 

 a depth of compensation much less than that of the 

 central United States. The same appears as the result 

 of Bowie's latest discussion of the anomalies of gravity. 

 On the geologic side it is seen that the great Cordilleran 

 plateaus have in recent geologic time been the seat of 

 regional igneous activity and block faulting on a vast 

 scale. Intrusions of magma and the heat which must 

 accompany them have set apart the Cordilleran province 

 from the regions of geologic quiet, and may have de- 

 creased the densities in the outer crust sufficiently to 

 account for the recent regional elevation above its own 

 former level and the present level of other portions of 

 the continent. The intra-continental compensation may 

 therefore be of a different nature and developed at a 

 higher level than the compensation which separates con- 

 tinents from oceans. 



In conclusion, the density of the crust is presumably 

 irregular in depth as well as in distribution, but it is 

 seen to be essentially a phenomenon of the outer fiftieth 

 of the earth's radius. The density variations which lead 

 to the major reliefs of the surface are merely dermal 

 features disconnected with the great body of the earth. 



Am. Jour. Sci.— Fourth Series, Vol. XLVIIT, No. 2*6. October, 191'.). 

 23 



