CHAP. XV.] PERMANENCE OP CONTINENTS. 325 



originally solidified ; and this inference must of course be 

 accepted or rejected, according as physicists may decide 

 upon the probability or improbability of the hypothesis 

 upon which it depends. Now this hypothesis is itself 

 based on the assumption that the earth was originally, and 

 is now, of unlike composition along different radii, or on 

 different sides ; the continental portions of the crust, 

 according to Professor Dana, being composed of denser 

 materials than the oceanic. 



Mr. W. O. Crosby l has ably discussed this question ; he 

 points out that this initial assumption is entirely unsup- 

 ported by evidence, and that even if it were to be admitted, 

 Professor Dana's hypothesis involves so many other uncer- 

 tainties and improbabilities that it would not be worthy of 

 acceptance unless it was strongly confirmed by geological 

 evidence. 



Dr. Wallace takes very different ground, and boldly 

 endeavours to support the theory of permanence by dis- 

 cussing the geological evidence, but he too has been 

 answered by Mr. Crosby (loc. cit.), by Mr. G-. S. Gardner, 2 

 and by Professor Hull. 3 



Dr. Wallace's chief arguments, as stated in his " Island 

 Life," may be summarized as follows : 



1. That the modern oceanic deposits are different from 

 any that occur among the rocks composing continental 

 land. 



2. That the formations which compose our continents 

 are chiefly shore-deposits, and that the constant neigh- 

 bourhood of land is proved by the frequency of estuarine 

 and lacustrine deposits. 



3. That formations of similar constitution and contem- 



1 " Geol. Mag.," Dec. 2, vol. x. (1883), p. 241. 



2 Ibid., Dec. 2, vol. viii. (1881), p. 241, and Dec. 2, vol. ix. 

 p. 546. 



3 " Contributions to the Phys. Hist, of Brit. Isles," chap. iv. 



