1883.] 455 [Crosby. 



areas form the continents ; but Professor Le Conte says they 

 form the ocean-bottoms. Professors Dana and Le Conte agree, 

 however, upon the main point against which the principal argu- 

 ments of this paper are advanced ; namely, that the continents 

 •and ocean-basins are permanent, their present positions being 

 those which they occupied at the beginning of geological time. 



We pass now to a more detailed examination of the two main 

 theories, with the view to determining which one, in the light of 

 our present knowledge has the best claim to the title of the true 

 theory of continents and ocean-basins. 



1. Continents and ocean-basins are upward and down- 

 ward bendings of the earth's crust. 



This theory harmonizes perfectly with the known facts of ele- 

 vation and subsidence ; and it is the only theory that does offer 

 an adequate explanation of this important class of phenomena. 

 It permits the interchange of continents and seas ; a relation 

 which is supported by a vast mass of geological data, and against 

 which no sound argument has yet been urged. The principal, 

 and, so far as I am aware, the only serious objection to this the- 

 ory is that brought forward or at least specially insisted upon by 

 Le Conte. 1 He says that the great crust arches could not sus- 

 tain themselves for a moment, even if the crust were several hun- 

 dred miles thick ; but the continental arches would break down, 

 and the oceanic arches would break up and restore a level surface. 

 But he goes still further and remarks : — "So great is this force 

 tending to the general form of equilibrium that, even if the earth 

 as a whole were rigid as a solid globe of glass (that is, had no 

 plastic stratum), it could not resist it. Hence he is driven to the 

 conclusion, already expressed in his theory of continents, that 

 there is the same amount of matter along each of the terrestrial 

 radii ; since, if there were not, the continents, although they are 

 parts of a globe continuously solid and as rigid as glass, would, 

 according to his view, settle down after the manner of the ine- 

 qualities of a mass of pitch. If, then, we can show that the masses 

 of the terrestrial raclii are not equal Professor Le Conte's conti- 

 nents are left absolutely without support. Now nothing could be 

 easier than to demonstrate this. For, although we may readily 

 grant that, leaving out of view the equatorial protuberance due 



1 Amer. Jour. Sci. (3), vol. IV, p. 346. 



