82 ERNEST H. L. SCHWARZ 



jected to deformation, the underlying idea being that earth-move- 

 ments, such as those which produce mountain-folds, are confined 

 to the regions of the globe that have been, or are, above the level 

 of the ocean. The average specific gravity of continental types 

 of rock is taken to be 2 . 68. We owe this term to the theory of 

 Dana, Wallace, and others, that the oceans and continents have 

 remained pretty much in the same places during geological time. 

 The opposite of continental rock is, obviously, an oceanic rock, 

 though this term is never actually used, the meaning being con- 

 veyed by various phrases, such as "the rocks forming the ocean 

 floor," or "the sub-oceanic crust." Now, since the continents 

 as a whole are less elevated above mean sea-level than the ocean 

 floors are depressed below it, and since the density of water is less 

 than half that of rock, it is obvious that if the densities of the con- 

 tinents and ocean floors were the same, the water would be attracted 

 by the land-masses and heaped up along the shores. Such is actu- 

 ally assumed to be the case by Suess in the introduction to his Antlitz 

 der Erde, but Fisher adduces very strong arguments in favor of 

 the view that, setting aside rotational effects, the sea-level is sensibly 

 spherical. In the latter case, the rocks of the sub-oceanic crust 

 must be denser than the continental types to keep the water in 

 place, and it is assumed that they are composed of basic original 

 magma, and of basaltic flows from submarine volcanoes, which were 

 more dense than the material of the continental crust. ^ The spe- 

 cific gravity of the rocks composing the ocean floor is taken to 

 be 2.96. Thus it appears that in respect to the density of the sub- 

 oceanic floor and the consequent form of its hydrosphere we have 

 two rival schools, the one maintaining that the ocean surface forms 

 a series of troughs between the continents, the other that the hydro- 

 sphere is sensibly spherical. This point must be settled before 

 we can arrive at any comprehensive and satisfactory explanation 

 of the geology of the earth as a whole; and, as far as I know, 

 the materials at present available are not sufficient to enable 

 one to give an irrefutable answer one way or the other. Even if 

 Stokes, Pratt, and Faye, quoted by Fisher, go too far in one direc- 

 tion, and Fischer, Hann, and Listing, quoted by Suess, go too far 



I Fisher, Physics of the Earth's Crust, chap. 25, and Appendix, p. 16. 



