The fact that masses of molten materials are seen at many points of the earth's surface to issue from figures in the crust of our globe, seems at first sight to find a simple explanation if we suppose our planet to consist of a fluid central mass surrounded by a solid crust. Hence we find that among those who first thought upon this subject, this hypothesis of a liquid centre and a solid crust was almost universally accepted. This hypothesis was supposed to find further support in the fact that, as we penetrate into the earth's crust by mines or boring operations, the temperature is found to continually increase. It was imagined, too, that this condition of our planet would best agree with the requirements of the nebular hypothesis of Laplace, which explains the formations and movements of the bodies of the solar system by the cooling down of a nebulous mass. But a more careful and critical examination of the question has led many geologists and astronomers to reject the hypothesis that the earth consists of a great fluid mass surrounded by a comparatively thin shell of solid materials. Volcanic outbursts and earthquake tremors, though so terrible and destructive to man and his works, are but slight and inconsiderable disturbances in a globe of such vast dimensions as that on which we live. The condition of the crust of the globe is, in spite of volcanic and earthquake manifestations, one of general stability; and this general stability has certainly been maintained during the vast periods covered by the geological record. Such a state of things seems quite irreconcilable with the supposition that, at no great depth from the surface, the whole mass of the globe is in a liquid condition. If, on the other hand, it be supposed that the solid crust of the globe is several hundreds of miles in thickness, it is difficult to understand how the local centres of volcanic activity could be supplied from such deep-seated sources. There are other facts which seem equally irreconcilable with the hypothesis of a fluid centre and a thin solid crust in our globe. If all igneous products were derived from one central reservoir, we might fairly expect to find a much greater uniformity of character among those products than really exists. But in some cases, materials of totally different composition are ejected at the same time from closely adjoining volcanic districts. Thus in Hungary and Bohemia, as we have seen, lavas of totally different character were being extruded during the Miocene period. In the island of Hawaii, as Professor Dana has pointed out, igneous ejections have taken place at a crater 14,000 feet above the sea-level, while a closely adjoining open vent at a level 10,000 feet lower exhibited no kind of sympathy with the disturbance. Whatever may be the cause of volcanic action, it seems clear that it does not originate in a universal mass of liquefied material situated at no great depth from the earth's surface.