554 THE JOURNAL OF GEOLOGY. 



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. I. That the earth, though rigid 

 as glass or even steel, to rapidly acting force, yet yields viscously 

 to heavy pressuse 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 con- 

 tinental radii are lo?iger, 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 desce?it 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 sub- 

 ject, which is the theory of mountains : but the close connection 

 which probably exists between the cause of continents and the 



