244 W. O. Crosby—Origin of Continents. 
southern hemisphere is composed of heavier material than the 
northern, and consequently the ocean is attracted in the former 
direction. But it will not escape observation that this admission 
that the densest matter is under the sea sustains the point made in 
the last paragraph, and is a direct contradiction of the most essential 
part of Prof. Dana’s theory of the origin of continents. 
Prof. Dana says that his theory accounts for the abrupt slopes of 
the continental borders, the oceans deepening rapidly and not 
gradually after we cross the true edge of the continents. But it 
seems to the present writer that this is just what Prof. Dana’s 
theory does not account for. For, if we were to admit that the 
earth is of different composition on different sides, it would certainly 
be contrary to all analogy to suppose that the areas of different 
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 consequently in 
conductivity and composition. 
Let us next inquire whether unequal contraction of the continental 
and oceanic areas could produce the depressions of the earth’s surface. 
We will suppose, with Professor Dana, that average rocks contract 
eight per cent. between the liquid and solid states, and make the 
extremely favourable supposition that the oceanic areas remained 
liquid until the continents became entirely solid. Now Prof. Dana 
says the average depth of the depressions is three miles, equal to 
the contraction resulting from the solidification of about thirty- 
eight miles of rock. There is one point of vital importance, how- 
ever, which has been entirely neglected, viz. the transference of 
material, as the result of denudation, from the continents to the sea- 
floors. If the continents and oceans are fixed, this action must 
have been always in the same direction. Few geologists estimate 
the average 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 thickness 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 supposition 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, corresponding to a 
crust 100 miles thick. Remembering, however, that, according to 
Prof. Dana, 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 corresponds to a crust over 200 miles 
thick. It seems no exaggeration, therefore, to say that a clear 
statement of this part of the theory is sufficient to refute it. 
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 widespread oscillations, and we know beyond a 
doubt that these movements are still in progress. Geologists do not 
