W .1 MCGEE — A MEASURE OP ISOSTASY. 503 



deposition of the Columbia and Lafayette formations and during 1 1 it* high-level 

 periods represented by the degradation of both these format ions, the continent was 

 warped in curiously consistent fashion ; during both low-level periods there was 

 an axis of maximum subsidence approximately marked by the cities of Charleston 

 and Memphis and an axis of minimum subsidence approximately marked by cape 

 I latteras ; and during both high-level periods the ( !harleston-Memphis axis was one 

 of maximum uplift, and the Hatteras axis one of minimum uplift — i. e., the former 

 axis was one of maximum and the latter of minimum movement throughout the 

 oscillations. Now, this warping is so related to the varying configuration and un- 

 equal density of the southeastern portion of the continent as to suggest that it was 

 produced by changes in stresses growing out of the varying degrees of submergence, 

 [f the hypothesis be established, the efficiency of isostatic action will become so 

 extended as to demand recognition among the more important, though always 

 secondary (or consequent), agencies of mountain-building and continent-lifting.* 



A spirited discussion followed the reading of the paper, participated 

 in by G. K. Gilbert, E. W. Claypole, I. C. White and the author. Pro- 

 lessor White remarked : 



My studies of the valleys of certain rivers in the Appalachian region have led to 

 similar conclusions concerning the susceptibility of the terrestrial crust to changing 

 loads. Several instances of warping apparently caused by subsidence due to load- 

 ing have come to my knowledge, the North Susquehanna valley between Pittston 

 ami Bloomsburg being a conspicuous example. 



Mr. ( rilbert said : 



The communication is an important contribution to the subject, in that it recog- 

 nizes the limitations of isostatic action. The phenomena of orogeny and epei- 

 rogeny are too complex for complete explanation by the single cause of loading and 

 unloading, which are really conservative processes; and research concerning the 

 primary causes is facilitated by definition of those of secondary character. 



Professor Claypole remarked: 



While forced to express admiration for the exhaustive, able and eloquent state- 

 ment just presented, 1 am impelled also to point out certain objections to the theory 

 that given areas sink because of loading, while contiguous areas rise because of un- 

 loading. If the theory were true, the coordinated process would tend to keep rivers 

 and other geographic features indefinitely in their places, while in reality they are 

 constantly shifting. It seems to me that under this theory the true order of the 

 processes is reversed ; that in point of fact areas of deposition become such by 

 reason of subsidence, and that contiguous areas are degraded because of elevation. 

 Again, it seems to me that the argument proves too much — that the subsidence of 

 the Netherland and New Jersey coasts is too great to he produced by the relatively 

 slight deposition now taking place on the sea bottoms. On the other hand, the 

 theory fail- to account for the origin of such great features of the earth's surface as 

 the < lull' of Mexico and the Rocky mountains ; so that some more general forces 

 would sei-in to he re 1 1 1 li re 1 1 to e\ plain the 1 1 io\eii lent s of continents and sea bottoms. 



A theory thai needs to he eked out with another seems si I peril llou s. Moreover. 



I lie greater pari of the paper in printed in full in the Am. Journ. Sei., vol xliv, pp. it: 192, 1892, 



