Ransome. I 



The Great Valley. 



399 



This is the first recorded attempt to secure a definite quantita- 

 tive measure of the crust's rigidity from geological observations, 

 and its amount is in striking contrast with many of the views pre- 

 viously noted, and would seem to limit rather narrowly the applica- 

 tion of the hypothesis to ordinary geological problems. Gilbert 

 does not say how the 600 cubic miles of load should be distributed. 



The second edition of Osmond Fisher's well-known work* 

 appeared in this same year, marshaling a numerous array of facts 

 in support of isostasy, but not differing very essentially as regards 

 this theory from the first edition. 



Reyer, in 1892,! brings forward several objections against the 

 theory of isostasy (Onerar-hypothese), while at the same time admit- 

 ting that the earth's crust may be depressed if the sediments can 

 first accumulate to a sufficient depth. Delta deposits may subside 

 through the compression and squeezing seaward of their unconsoli- 

 dated lower beds, without any sinking of the underlying solid crust. 

 Subsidence is often more rapid than sedimentation, while enormous 

 loads, especially volcanic accumulations, frequently fail to depress 

 the crust beneath them. Eroded regions often sink in spite of 

 denudation. 



In the same year appeared McGee'sJ paper on the Gulf of Mex- 

 ico, in which he regards the problem from the purely geologic 

 standpoint, and presents that aspect of it with greater detail than 

 any previous writer. This paper is the best presentation of the 

 theory of isostasy as limited to the geological field, albeit carrying 

 the idea of a sensitive crust to a somewhat extreme degree. Owing 

 to its ready accessibility, only the writer's conclusions need be given 

 here. They are as follows: "(1) The direct data of modern times 

 indicate that deposition and isostatic subsidence are not only related 

 sequentially, but that under favorable conditions they are quantita- 

 tively equal or sub-equal. (2) This measure of isostasy [the Gulf 

 of Mexico] is consistent with the direct data, both quantitative and 

 qualitative, yielded by other noteworthy deposition tracts of the 



* Physics of the Earth's Crust, 2d ed., 1SS9. 



tUrsachen der Deformationen und der Gebirgsbildung. Leipzig, 1892. 

 :fThe Gulf of Mexico as a Measure of Isostasy, Am. lour. Sec, 3 ser, Vol. 

 XLIV, pp. 177-192. 



