DEVELOPMENT OF OCEAN BASINS. 117 



the depressions were at first so shallow that the primeval ocean may 

 have been universal, but the process of greater downward contraction 

 continuing, the ocean basins would become deeper and the less con- 

 tracted portions of the lithosphere would appear as land. The process 

 still continuing, the land would grow higher and more extensive and 

 the ocean basins deeper and less extended throughout all geological 

 time. On the whole, in spite of many oscillations, with increase and 

 decrease of land, to be spoken of later, and in spite, too, of exterior agen- 

 cies by erosion and sedimentation tending constantly to counteract these 

 effects, such has been, I believe, the fact throughout all geological history. 



It is evident, also, that on this view, since the same causes which orig- 

 inally formed the ocean basins have continued to operate in the same 

 places, the positions of these greatest inequalities of the lithosphere have 

 not substantially changed. This is the doctrine of the permanency of 

 oceanic basins and continental masses, first announced by Dana. Some 

 modification of this idea will come up under another head. 



The objection which may be — which has been — raised against this 

 view is that such heterogeneity as is here supposed, in a fused mass and 

 therefore in a mass solidified from a state of fusion, is highly improbable, 

 not to say impossible. This objection, I believe, will disappear when 

 we remember the very small differences in conductivity, and therefore 

 in contraction, that we are here dealing with ; small, I mean, in compari- 

 son with the size of the earth. This is evident when we consider the 

 inequalities of the earth's surface. The mean depth of the ocean is 

 about two and one-half miles ; the mean height of the land, about one- 

 third of a mile. The mean inequality of the lithosphere, therefore, is 

 less than three miles. This is 13 1 00 of the radius of the earth — less than 

 y^q of an inch (an almost imperceptible quantity) in a globe two feet in 

 diameter. I believe that a perfectly spheroidal ball of plastic clay 

 allowed to dry, or even a spheroidal ball of red-hot copper allowed to 

 cool, would show more deformation by contraction than the lithosphere 

 of the earth in its present condition. It is true the inequalities are more 

 accentuated in some places, especially on the margins of the continental 

 areas ; but this is due to another cause, mountain-making, to be taken 

 up later. 



Another objection will doubtless occur to the thoughtful geologist. It 

 would seem at first sight on this view that ocean areas cooling most rap- 

 idly ought to be the first to form a solid crust, and the crust (if there be 

 any interior liquid still remaining) ought to be thickest, and therefore 

 least subject to volcanic activity, there; but, on the contrary, we find 

 that it is just in these areas that volcanoes are most abundant and active. 

 It is for this reason that Dana believed that land areas were the first and 



