23G EARTH-CRUST MOVEMENTS AND THEIR CAUSES. 



alone would probably be very small but for the concurrence of another 

 and much greater cause, viz, the greater conductivity of the same areas. 

 Conductivity is not, indeed, strictly proportional to destiny; but in a 

 general way it is so. It is certain, therefore, that the denser areas 

 would be also the more conductive, and therefore the more rapidly 

 cooling and contracting areas. This would again increase, and in this 

 case progressively increase, the depression of the e areas. The two 

 causes — destiny aud conductivity, isostasy and contraction — would con- 

 cur, but the latter would be far the greater, because indefinitely cumu- 

 lative. The originally evenly spheroidal lithosphere would thus be 

 deformed or distorted, aud the distortion, fixed by solidification, would 

 be continually increased until now. When the earth cooled sufficiently 

 to precipitate atmospheric vapor the watery envelope thus formed 

 would accumulate in the basins of the lithosphere and form the oceans. 

 It is possible, and even probable, that the depressions were at first so 

 shallow that the primeval ocean may have been universal, but the proc- 

 ess of greater downward contraction continuing, the ocean basins 

 would become deeper and the less contracted 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 laud, to be spoken of 

 later, and in spite, too, of exterior agencies by erosion and sedimenta- 

 tion 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 

 originally formed the ocean basins have continued to operate in the 

 same places, the positions of these greatest inequalities of the litho- 

 sphere 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 dis- 

 appear when we remember the very small differences in conductivity, 

 and therefore in contraction, that we are here dealing with; small, I 

 mean, in comparison 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 2£ miles; the mean height of the land about £ of 

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

 3 miles. This is y^o of the radius of the earth — less than T ^„ of an inch 

 (an almost imperceptible quantity) in a globe 2 feet in diameter. I 

 believe that a perfect 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 lithospere of the earth in its 



