It may be thought, however, that the igneous fluidity of the internal part of the globe is too mighty and improbable a conclusion to be based upon the increase of temperature, observed only to the depth of two or three thousand feet. But this is not the only evidence of such a condition of the earth's interior. Three hundred active volcanoes, and still more numerous extinct ones, have opened their mouths and poured forth their molten contents from a great depth, to bear witness to the existence of vast masses of melted rock beneath the earth's crust. The globe, too, is flattened at the poles, just to the amount it would be by rotation on its axis, had it been a liquid mass; and, therefore, there is every probability that it was once liquid; and if so once, its interior is probably still so, because the period for cooling it, when once surrounded by a solid crust, must be incalculably long. That this solid crust has once been liquid from heat, is most obvious to all who carefully examine it. For the unstratified rocks have certainly once been melted, and most of the stratified series were derived from the unstratified. Again, the organic remains dug out from the deep-seated strata prove that, when they were alive, the surface, even in high latitudes, must have been subject to a tropical, or even an ultra-tropical heat; thus showing us that the temperature of the globe has gradually diminished, as we should expect from the theory of original igneous fluidity. And, finally, no other hypothesis but the gradual cooling of the earth's crust, and the powerful volcanic agency that must from time to time have torn and ridged up that crust, will account for the present fractured and overturned condition of the strata, and the elevation of our continent from the ocean's bed. But this supposition does most satisfactorily explain all these phenomena, and also those of earthquakes and volcanoes.