Crosby.] 454 [November 7, 



caused its surface to sink below that of the continents ; and the 

 depressions thus developed became the future ocean-basins, 

 which, like the continents, are, according to this theory necessa- 

 rily of a permanent character. Indeed, it is a plain deduction 

 from Professor Dana's hypothesis that the existing continents 

 and oceans are as old as the earth's crust. Although subject to 

 extreme changes of level and frequently submerged, yet no por- 

 tion of the continent has ever become the site of the deep central 

 ocean ; nor has any portion of the floor of the abyssal sea ever 

 been elevated to form continental land. In the beginning, the 

 continents were narrow and the oceans shallow ; and during the 

 course of geologic time the continents have become constantly 

 $ wider and the oceans deeper. 



Closely related to Professor Dana's theory is that held by Arch- 

 deacon Pratt and Professor Le Conte. The following statement 

 of this theory is given in Le Conte's own words : 1 — " Continen- 

 tal surfaces and ocean-bottoms are due to unequal radial contrac- 

 tion of the earth in its secular cooling. It is evident that in such 

 secular cooling and contraction, unless the earth were perfectly 

 homogeneous, some parts being more conductive would cool and 

 contract more rapidly in a radial direction than others. Thus 

 some radii would become shorter than others. The more con- 

 ductive, rapidly contracting portions, with the shorter radii 

 would become sea-bottoms ; and the less conductive, less rapidly 

 contracting portions, with the longer radii, land-surfaces. In 

 other words, the solid eaith becomes slightly deformed and the 

 water collects in the depressions." Le Conte and Pratt further 

 hold that the quantity of matter along each of the terrestrial 

 radii was not only originally, but is yet, essentially equal ; the 

 matter being denser along the shorter oceanic than along the 

 longer continental radii." 



Certain passages in Le Conte's writings lead one to infer that 

 he regards his theory as essentially similar to Professor Dana's. 

 Nevertheless, the language quoted above shows that the differ- 

 ence is fundemental. They agree in requiring a heterogeneous 

 earth as a basis for unequal radial contraction and fixed conti- 

 nents ' but beyond that they are diametrically opposed. For 

 Professor Dana says that the more rapidly conducting and cooling 

 i Amer. Jour. Sci. (3), vol. iv, p 352. 



