Among other agencies of less importance, that have always operated geologically, is gravity. Its chief effect, at present is to bring the earth's surface nearer and nearer to a level, by causing the materials, which other agencies have loosened from its salient parts, to subside into its cavities and valleys. It also condenses many substances from a gaseous to a liquid or solid state, especially those deep in the earth's crust, and thus brings the particles more within the reach of cohesive attraction and chemical affinity, often changing the constitution, and always the solidity, of bodies. And in the position of the ancient mechanical rocks, occupying as they do the former basins of the surface, and in the superior consolidation of the earlier strata, we find proof of the action of gravity in all past geological time. Electricity too, in the form of galvanism, has never been idle. We have reason to think that it operates at this moment in accumulating metallic ores in veins; and this segregation appears to have operated in all ages, not only in filling veins, but also, probably, in giving a laminated character and jointed structure to mountains of slate, as well as a concretionary and prismatic form to others. Last, though not least, we may reckon among the agents of geological change the forces of cohesion and affinity. When water and heat, gravity and galvanism, have brought the atoms of bodies into a proper state, these agents are always ready to change their form and constitution; and they have ever been at hand to operate by the same laws, and we witness their effects in the oldest as well as the newest rocks found in the earth's crust. This point, however, has been sufficiently considered, when treating of the unvarying uniformity of the laws of chemistry and crystallography.