It is a very difficult problem to determine how the vast basins which are filled by the great oceans, especially that of the Pacific, were first produced. When the earth's surface was still in a molten state, it would necessarily take the form of a true oblate spheroid, with a compression at the poles due to its speed of rotation, which is supposed to have been very great. The crust formed by the gradual cooling of such a globe would be of the same general form, and, being thin, would easily be fractured or bent so as to accommodate itself to any unequal stresses from the interior. As the crust thickened and the whole mass slowly cooled and contracted, fissures and crumpling would occur, the former serving as outlets for volcanic activities whose results are found throughout all geological ages; the latter producing mountain chains in which the rocks are almost always curved, folded, or even thrust over each other, indicating the mighty forces due to the adjustments of a solid crust upon a shrinking fluid or semi-fluid interior. But during this whole process there seem to be no forces at work that could lead to the production of such a feature as the Pacific, a vast depression covering nearly one-third of the whole surface of the globe. The Atlantic Ocean, being smaller and nearly opposite to the Pacific, but approximately of equal depth, may be looked upon as a complementary phenomenon which will be probably explained as a result of the same causes as the vaster cavity. So far as I am aware, there is only one suggested cause of the formation of these great oceans that seems adequate; and as that cause is to some extent supported by quite independent astronomical evidence, and also directly bears upon the main subject of the present volume, it must be briefly considered.