168 



GENERAL FORM AND STRUCTURE OF THE EARTH. 



level, could sustain itself for a moment. The only condition under 

 which such inequalities could sustain themselves on a supporting liquid 



is the existence 

 of inequalities on 

 '§ the. under surface 

 of the crust next 



Fig. 133. 



-Diagram illustrating 

 Crust on 



the Conditions of Equilibrium of a Solid +>,,-. li^m'-q oii-m' 

 a Liquid Interior. me squill, Simi- 



lar to those on 



the upper surface, but in reverse, as shown in Fig. 133. And these 

 lower or under-surface inequalities would have to be repeated not only 

 for the largest inequalities, viz., continental surfaces and ocean-bottoms, 

 but also for great mountain plateaus. And thus the hypothesis seems 

 to break down with the weight of its own assumption.* 



Besides, we have already given good reasons (pages 85-87) for be- 

 lieving that the earth is substantially solid. Upon the hypothesis of a 

 substantially solid earth, we explain the great inequalities constituting 

 continental surfaces and ocean-bottoms by unequal radial contraction 

 of the earth in its secular cooling. 



The earth was undoubtedly at one time an incandescent liquid 

 globe. It then, as we believe, cooled to a substantial solid, although 

 probably with a sub-crust layer underlying large areas of the solid 

 crust, and separating it from the solid nucleus. When first solidified 

 the earth was doubtless a regular oblate sjiheroid, and, when sufficiently 

 cool to allow condensation of aqueous vapor, covered with a universal 

 ocean. By continued cooling it gradually contracted, and if the rate 

 of cooling and contraction had been equal in all parts of the surface 

 it would have retained its regular spheroid form. But, without perfect 

 homogeneity of composition and equality of conductivity and of co- 

 efficient of contraction in all parts (which is extremely improbable), 

 such equality of cooling and contraction would be impossible. Some 

 parts, therefore, cooled and contracted toward the center more rapidly 

 than others. These more rapidly contracting areas would form hollows 

 and the less rapidly contracting areas protuberances. The waters would 

 be gathered in the hollows and form oceans, while the protuberances 

 would become continents. In other words, oceanic basin and land- 

 masses are the result of slight distortion of the regular spheroid by un- 

 equal radial contraction. This is evidently a true cause ; and, when 

 we consider the smallness of these inequalities in comparison with the 

 size of the earth, it will seem a sufficient cause. The mean inequality of 

 the kind we are now considering is about two and a half miles. This 



* It has been shown by G. H. Darwin that the great inequalities of the earth's sur- 

 face could not be sustained unless the earth be as rigid as granite for a depth of 1,000 

 miles. — Proceedings of the Royal Society, June, 1881. 



