DEVELOPMENT OF THE DISCOIDAL HYPOTHESIS 277 



masses be of subcontinental size, their individual movements may have 

 been obscured, to some extent, by the movements of the continent as a 

 whole ; but the individuality of the larger subcontinental elements should 

 appear in the record as a more or less pronounced tendency on the part 

 of the heavier individuals to lie relatively lower, and on the part of the 

 lighter individuals to stand relatively higher, each with reference to the 

 other. The evident effect would be that the former would become more 

 deeply buried beneath continental and marine sediments, and that the 

 latter would be more deeply eroded. 



Let us call the heavier masses negative elements of the lithosphere, the 

 lighter masses positive elements; then we may say: A negative element 

 is indicated as underlying an area which has been the scene of long- 

 continued or repeated deposition, taking the geologic record as a whole, 

 whereas the positive element is presumably the underbody of an area 

 which has more commonly been subject to erosion than to deposition. 



The preceding conclusion was stated in 1907 in the reverse order of 

 argument. Isostasy was not so firmly established then on geodetic and 

 geologic opinion as it now is, and the geologic evidence of sediments 

 versus unconformities was appealed to as demonstrating the existence of 

 negative and positive continental elements, whose diverse tendencies were 

 interpreted in terms of isostatic balance.^^ Observational evidence of 

 the existence of isostatic balance to the degree determined by Gilbert and 

 Barrell is now practically conclusive and requires, in the opinion of the 

 writer, the recognition of continental elements which diifer as to density. 



In plate 11 is given an analysis of North America into its negative and 

 positive elements on the basis of sediments and unconformities. It thus 

 rests primarily on the facts of geology and presents the geologic argument 

 independently of the isostatic. It might be modified in outline by com- 

 parison with the distribution of gravity anomalies, but were there ma- 

 terial contradiction the geologic evidence, where definite, should, in my 

 opinion, be given greater weight, because the evidences of deposition and 

 unconformity are unequivocal and represent an original, persistent con- 

 dition, whereas the evidences of gravity anomalies are capable of several 

 interpretations and may correspond to recent changes only. 



A study of Asia, according to the view of continental structure here 

 presented, yielded as a result the distinction of positive and negative 

 elements.^* South America appears to comprise at least four positive 



33 Bailey Willis : A theory of continental structure. Bull. Geol, Soc. Am., vol. 18, 

 1907, pp. 389-412. 



3* Bailey Willis : Research in China. Carnegie Institution of Washington, Publication 

 No. 54. vol. ii, 1907, pp. 115-123 and plate 8. 



