Crosby.] 462 [November 7, 



ture, usually remains fluent long enough to permit more or less 

 crystallization and rarely retains the vitreous texture. Hence 

 vitreous basalt or tachylite is a rare rock, although vitreous rhy- 

 olite or obsidian is abundant. 



In short, this theory requires an unproved and improbable con- 

 stitution of the earth's crust ; but, if the existence of this indis- 

 pensable condition were demonstrated, the known facts with re- 

 gard to the solidification of basic and siliceous rocks would 

 apparently lead us to a conclusion diametrically opposed to that 

 reached by Professor Dana ; namely, that the lighter and more 

 slowly conducting areas would become solid first, forming the 

 continents, while the denser and more rapidly conducting areas 

 would be the last to solidify and would form the ocean-floor. 

 In connection with his theory, Professor Dana offers the follow- 

 ing explanation of the fact that the land is mainly in the north- 

 ern hemisphere ; the southern hemisphere is composed of heavier 

 material than the northern, and consequently the ocean is at- 

 tracted in the former direction. But it will not escape observa- 

 tion that this admission that the densest matter is under the sea 

 sustains the point made in the last paragraph and is a direct con- 

 tradiction of the most essential part of Professor Dana's theory 

 of the origin of continents. 



Professor Dana says that his theory accounts for the abrupt 

 slopes of the continental borders, the ocean deepening rapidly 

 and not gradually, after we cross the true edge of the continent. 

 But it seems to the present writer that this is just what Profes- 

 sor Dana's theory does not account for. For, if we were to admit 

 that the earth is of unlike composition on different sides, it would 

 certainly be contrary to all analogy to suppose that the areas of 

 unlike composition are sharply marked off from each other ; and 

 yet the steep slopes of the oceanic depressions, according to this 

 theory, require an abrupt change in radial contraction and conse- 

 quently in conductivity and composition. 



The existence of a plastic zone beneath the crust is inconsist- 

 ent not only with the supposition that there is a marked absence 

 of uniformity in the composition of the crust, as has been pointed 

 out ; but it is also believed to be inconsistent with another essen- 

 tial feature of Professor Dana's theory ; namely, that the existing 

 relief of the crust has originated in the unequal radial contraction 



