Crosby, j 464 [November 7, 



one point of vital importance, however, which has been entirely- 

 overlooked ; namely, the transference of material, as the result 

 of denudation, from the continents to the sea-floors. If, as this 

 theory requires, the continents and oceans are fixed, this action 

 must have been always in the same direction. Now few geolo- 

 gists estimate the thickness of the stratified rocks at less than ten 

 miles; but it will be claimed by those believing in the theory that 

 the sediments must be much thinner over the floor of the central 

 ocean ; therefore we will assume five miles as the average thick- 

 ness for the globe, and allow that they are three-fourths on the 

 continents now. But to restore to the continents what they have 

 lost according to this view would increase the height of the land 

 and the general inequality of the surface at least five miles, which, 

 added to the existing relief of three miles, gives eight miles as 

 the excess of contraction of oceanic over continental areas, cor- 

 responding to a crust 100 miles thick. Remembering, however, 

 that according to Professor Dana, the most of this detritus was 

 derived from much smaller continents, say one-half as large, and 

 it is seen that the excess of contraction of the oceanic areas cor- 

 responds to a crust over 200 miles thick. It seems no exagger- 

 ation, therefore, to say that a clear statement of this part of the 

 theory is sufficient to refute it. 



As already pointed out, Professor Dana's theory affords no ad- 

 equate explanation of the known facts of elevation and subsi- 

 dence. His writings often appear ambiguous and contradictory 

 upon this point since, though generally admitting the existence 

 of a plastic zone, he often argues as if the continents at least 

 were solid to the earth's centre. 



No proposition in geology is more firmly established than this : 

 during the whole of geological time the earth's crust has been 

 subject to extensive and wide-spread oscillations; and, as already 

 pointed out, we know beyond a doubt that these movements are 

 still in progress. Geologists do not now generally believe that 

 the profound subsidences permitting the deposition of thick sed- 

 imentary formations are produced by these same sediments; but 

 they rather agree with Professor Dana that the oscillations are 

 due to lateral pressure and go on independently of the sedimen- 

 tary process. But this view certainly does not harmonize well 

 with the notion that these great vertical movements of the crust 



