STRATTGRAPHTC classification DIASTROPITIC CRITERIA 441 



palachian troughs by overthrusting of deposits of originally more eastern 

 basins. Such complicated and excessive overthrusting seems to have 

 occurred in southeastern Tennessee and northwestern Georgia, where the 

 deposits of the Athens trough are in contact with those of the Clinton 

 trough. As here outlined the structural history of the Appalachian 

 region appears to be essentially similar to that ascribed to the Alps by 

 the most advanced European geologists. 



The principles and processes involved in the above-suggested hypothesis 

 nave been discussed by many writers, notably Hayford, Hecker, and 

 Chamberlin and Salisbury. They are essentially the same as those at the 

 basis of Willis's "theory of continental structure."^^ Following Hayford. 

 he regards the lithosphere as heterogeneous in constitution — that is, as 

 "composed of lighter and heavier bodies," the former (positive areas) 

 tending to rise, the latter (negative areas) to sink. These differences 

 in density are postulated as original differences. The oceanic basins 

 being regarded as permanently sunken areas, they are naturally assumed 

 to comprise bodies of maximum density, while the continental areas 

 which have been oftenest or longest land are those of minimum density. 

 The important feature in this connection is the probable fact that the 

 areas of relative density exert a lateral pressure on the lighter areas, and 

 consequently that the continents have been narrowed by compression 

 originating beneath the surrounding oceans. "The pressures are at- 

 tributed to deep-seated sub oceanic spread'' and their mechanical effects 

 "are seen in numerous shearing planes on which, under appropriate 

 stress, the masses rose as on an inclined plane." 



The modified hypothesis as above outlined accepts the essential prin- 

 ciple of Willis's theory, namely, "suboceanic spread" due to deficiency 

 of density in the positive continental masses, and differs chiefly in that 

 it adds, in order to explain conditions not contemplated by Willis, the 

 idea of faulting at the margin of the continental shelf. The displace- 

 ment of this fault is supposed to be increased periodically, and as deeper 

 and deeper zones are affected in the progress of the suboceanic spread 

 new tangential planes of movement are formed in the adjacent positive 

 areas; and the successively lower planes emerge farther and farther 

 inland. 



Whatever the relative altitudes of the eastern and western parts of 

 Appalachia may have been in Paleozoic ages, two facts give decisive 

 evidence in favor of the greater elevation being in the west during the 

 Newark and Cretaceous periods. The first of these is the development of 



<5 Bull. Geological Society of America, vol. 18, 1007, pp. 389-412. 

 XXX— Bull. Geol. Soc. Am., Vol. 22, 1910 



