. Elements of geology : a text-book for colleges and for the general reader. Geology. 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 ineq
. Elements of geology : a text-book for colleges and for the general reader. Geology. 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, t
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