Ransomed.] 



The Great Valley. 



401 



servatism and by the tendency to limit such isostatic adjustments 

 to the larger inequalities of the earth's surface; yet it nevertheless 

 reveals a considerable change of view from that held in 1 886. 



Geikie,* in the last edition of his Text-book (1893). dismisses 

 the theory of isostasy in a rather summary manner. He says: " To 

 suppose . . . that the removal and deposit of a few thousand 

 feet of rock should so seriously affect the equilibrium of the crust as 

 to cause it to sink and rise in proportion, would evince such a 

 mobility in the earth as could not fail to manifest itself in a far more 

 powerful way under the influence of lunar and solar attraction. 

 That there has always been the closest relation between upheaval 

 and denudation on the one hand, and subsidence and deposition on 

 the other, is undoubtedly true. But denudation has been one of 

 the consequences of upheaval, and deposition has been kept up 

 only by continual subsidence." 



An exceedingly important paper dealing with the problem from 

 the physical side is the one on a series of transcontinental gravity 

 determinations by Putnam f with the appended notes by Gilbert. 

 According to Mr. Putnam, "the result of this series would 

 seem to lead to the conclusion that general continental elevations 

 are compensated by a deficiency of density in the matter below sea- 

 level, but that local topographical irregularities, whether elevations 

 or depressions, are not compensated for, but are maintained by the 

 partial rigidity of the earth's crust." % Mr. Gilbert sums up his dis- 

 cussion of these observations and their bearing on the question of 

 isostasy as follows: "The measurements of gravity appear far more 

 harmonious when the method of reduction postulates isostasy than 

 whep it postulates high rigidity. Nearly all the local peculiarities 

 of gravity admit of simple and rational explanation on the theory 

 that the continent as a whole is approximately isostatic, and that 

 the interior plain is almost perfectly isostatic. Most of the devia- 



* Text-book of Geology, 3d ed., 1893, p. 296. 



t Results of a Transcontinental Series of Gravity Measurements, by G. R. 

 Putnam. 



Notes on the Gravity Determinations reported by Mr. G. R. Putnam, by 

 G. K. Gilbert, Rull. Phil. Soc, Wash., Vol. XIII, pp. 31-76. 

 %Loc cit., p. 51. 



