1883.] 463 [Crosby. 



of the areas of dissimilar composition. To appreciate this point 

 we have only to conceive separate blocks of the crust floating on 

 the mobile layer, and to reflect that they must be of nearly the 

 same density as the layer in which they are immersed. If we 

 suppose the liquid to be one-tenth heavier than the blocks, it is 

 clear that blocks forty miles thick would rise four miles, or one- 

 tenth their thickness, out of the liquid ; while blocks ten miles 

 thick would project only one mile ; a difference of thirty miles in 

 the thickness of the blocks corresponding to a difference of three 

 miles in their altitudes above the surface of the liquid. But, if 

 the blocks are connected to form a continuous crust, the principle 

 of flotation will hold just as truly, the upper surfaces of the thick 

 and thin portions being much more nearly in one plane than the 

 under surfaces. Therefore the development of the existing relief 

 of the earth's surface in accordance with Professor Dana's theory 

 would require an excess of contraction in the oceanic as comp- 

 pared with the continental areas of more nearly thirty miles than 

 three miles as Professor Dana has estimated, and this enormous 

 contraction would correspond to a continental crust nearly four 

 hundred miles thick. In fact, this theory is entirely inconsistent 

 with a floating crust ; but it demands, on the contrary, that the 

 earth should be solid from centre to circumference, at least under 

 the continents ; and yet Professor Dana admits that the Rocky 

 Mountains have been elevated 8,000 to 10,000 feet since Creta- 

 ceous time, and that the eastern part of the continent subsided 

 40,000 feet during the Paleozoic era, and so on. 



But, ignoring for the present the principle of flotation, let us 

 assume that the theory in question is sound up to this point, and 

 inquire whether unequal contraction of the continental and oce- 

 anic areas could produce the depressions of the earth's surface. 

 We will suppose, with Professor Dana, that average rocks con- 

 tract eight per cent between the liquid and solid states — much 

 less than Bischof 's experiments show, but more in harmony with 

 the most recent determinations ; and make the extremely favor- 

 able though improbable supposition that the oceanic areas re- 

 mained liquid until the continents became entirely solid. Now 

 Professor Dana says the average depth of the depressions is 

 three miles, equal to the contraction resulting from the cooling 

 and solidification of about thirty-eight miles of rock. There is 



