Formerly, men looked upon the earth as a unit in time, as the result of one creative act, with all its outlines established from the beginning. It has been the work of modern science to show that its inequalities are not contemporaneous or simultaneous, but successive, including a law of growth,--that heat and cold, and the consequent expansion and contraction of its crust, have produced wrinkles and folds upon the surface, while constant oscillations, changes of level which are even now going on, have modified its conformation, and moulded its general outline through successive ages. In thinking of the formation of the globe, we must at once free ourselves from the erroneous impression that the crust of the earth is a solid, steadfast foundation. So far from being immovable, it has been constantly heaving and falling; and if we are not impressed by its oscillations, it is because they are not so regular or so evident to our senses as the rise and fall of the sea. The disturbances of the ocean, and the periodical advance and retreat of its tides, are known to our daily experience; we have seen it tossed into great billows by storms, or placid as a lake when undisturbed. But the crust of the earth also has had its storms, to which the tempests of the sea are as nothing,--which have thrown up mountain waves twenty thousand feet high, and fixed them where they stand, perpetual memorials of the convulsions that upheaved them. Conceive an ocean wave that should roll up for twenty thousand feet, and be petrified at its greatest height: the mountains are but the gigantic waves raised on the surface of the land by the geological tempests of past times. Besides these sudden storms of the earth's surface, there have been its gradual upheavals and depressions, going on now as steadily as ever, and which may be compared to the regular action of the tides. These, also, have had their share in determining the outlines of the continents, the height of the lands, and the depth of the seas. Leaving aside the more general phenomena, let us look now at the formation of mountains especially. I have stated in a previous article that the relative position of the stratified and unstratified rocks gives us the key to their comparative age. To explain this I must enter into some details respecting the arrangement of stratified deposits on mountain-slopes and in mountain-chains, taking merely theoretical cases, however, to illustrate phenomena which we shall meet with repeatedly in actual facts, when studying special geological formations. [Illustration: Fig. 1.]