The study of the subterranean, or igneous, agencies has proved to be very unsatisfactory in the way of explaining the phenomena and referring them to the operation of understood physical agents, because so little is really known and so much remains to be discovered. Nevertheless, we have learned much that is of great importance in geological reasoning. We have seen that the earth contains within itself a great store of energy, and that its interior, in whatever physical state that may be, is highly heated, and possesses great quantities of material which is either actually or potentially molten, and is permeated with superheated steam and other gases. • This molten material is often forced upward, and is either poured out at the surface, or fills up fissures and cavities in the rocks, or pushes its way between them. Cooling under various circumstances, the molten masses consolidate into a great variety of characteristic rocks, frothy, glassy, or crystalline. Explosive discharges of steam blow the melted rock into fragments of all grades of fineness, and these fragments likewise accumulate either on the land or under water, and form rocks, the nature and origin of which may be readily recognized.

We have further seen that the operation of these subterranean forces produces shocks and jars in the interior, which are propagated to the surface as earthquakes, and there bring about permanent changes, associated with the Assuring and dislocation of the rocks, landslips, alteration in the course of rivers, formation of lakes, and the like. The frequency of earthquakes, their wide geographical range, and the constant tremor of the ground detected by delicate instruments, led us to infer that the crust of the earth is decidedly unstable.

This conclusion we found strengthened by the oscillations of level between land and sea, which, though extremely slow, are seen to be still in progress. Historical geology will show us that these changes of level have, in the course of ages, been effected on the grandest scale. Almost all the great continents are composed of rocks which, for the most part, were laid down in the sea and still contain the fossils of marine animals, and this shows that these continents have been under the sea. Not that all parts of any continent were submerged at the same time, but now one part and now another was overflowed and again emerged, until nearly all have been covered in their turn.

In brief, the principal geological functions of the subterranean agencies are two: (1) they bring up from below and form at the surface, and at all depths beneath it, certain characteristic kinds of rocks; and (2) they tend to increase the inequalities of the earth's surface, and thus to counteract the agencies which are cutting down the land and steadily tending to reduce it to the level of the sea.