Page:The Journal of geology (IA journalofgeology11893univers).pdf/128

increase very notably in density in the act of solidification, so that a solid crust would undoubtedly break up and sink in a liquid of the same material. But how then are we to explain gravitative equilibrium in the case of a rigidly solid globe. I answer, by two suppositions. 1. That the earth, though rigid as glass or even steel, to rapidly acting force, yet yields viscously to heavy pressure over large areas and acting for a long time. A solid globe of glass six feet in diameter will very perceptibly change form under its own weight. How much more the earth under its own gravity. This completely explains the oblateness of the earth even if solid throughout and had never been liquid at all. The earth, though rigid, behaves like a very stiffly viscous body; like, for example, the ice of glaciers though very much more stiffly viscous. This viscosity would not at all interfere with its rigidity under the tide-generating influences of the sun and moon—for these are far too rapidly acting.

2. The second supposition necessary is, that the earth is not absolutely homogeneous either in density, or in conductivity for heat, that in secular cooling and contraction the denser and more conductive areas, cooling and contracting faster, went down and became the ocean basins, while the lighter and less conductive areas were left as the more prominent land surfaces. And thus to-day the ocean basins are in gravitative equilibrium with the continental areas, because in proportion as oceanic radii are shorter are the materials also denser; and in proportion as the continental radii are longer, are the materials also specifically lighter. This condition of gravitative equilibrium Dutton calls Isostasy.

Thus then the great inequalities of the earth, constituting ocean basins and continental surfaces, are the result of unequal radial descent of the earth's surface by contraction in its secular cooling. This is by far the most satisfactory theory of these greatest inequalities.

In thus following the phenomena of Isostasy to their logical conclusion, we seem to have gone beyond the limits of our subject, which is the theory of mountains: but the close connection which probably exists between the cause of continents and the