§ 80. If we inquire the origin of those forces which have wrought the surface of our planet into its present shape, we find them traceable to the same primordial source as that just assigned. Assuming the solar system to have been evolved, then geologic changes are either direct or indirect results of the unexpended heat caused by nebular condensation. These changes are commonly divided into igneous and aqueous:—heads under which we may most conveniently consider them. All those periodic disturbances which we call earthquakes, all those elevations and subsidences which they severally produce, all those accumulated effects of many such elevations and subsidences exhibited in ocean-basins, islands, continents, table-lands, mountain-chains, and all those formations which are distinguished as volcanic, geologists now regard as modifications of the Earth’s crust produced by the still-molten matter occupying its interior. However untenable may be the details of M. Elie de Beaumont’s theory, there is good reason to accept the general proposition that the disruptions and variations of level which take place at intervals on the terrestrial surface, are due to the progressive collapse of the Earth’s solid envelope upon its cooling and contracting nucleus. Even supposing that volcanic eruptions, extrusions of igneous rock, and upheaved mountain-chains, could be otherwise satisfactorily accounted for, which they cannot; it would be impossible otherwise to account for those wide-spread elevations and depressions whence continents and oceans result. The conclusion to be drawn is, then, that the forces displayed in these so-called igneous changes, are derived positively or negatively from the unexpended heat of the Earth’s interior. Such phenomena as the fusion or agglutination of sedimentary deposits, the warming of springs, the sublimation of metals into the fissures where we find them as ores, may be regarded as positive results of this residuary heat; while fractures of strata and alterations of level are its negative results, since they ensue on its escape. The original cause of all these effects is still, however, as it has been from the first, the gravitating movement of the Earth’s matter towards the Earth’s centre; seeing that to this is due both the internal heat itself and the collapse which takes place as it is radiated into space.