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6
PRECIOUS STONES.
—more or less detached, or in the state of continu­ous rocks. The least examination shows further, that this earth itself is composed to a great extent of stones gradually decreasing in size; so that we easily arrive at the well-established conclusion that the earth and the stones have the same origin.
If then, in thought, we remove from the surface of the land the earth, whose thickness indeed is very inconsiderable, we perceive that the solid part of our globe consists exclusively of rocks.
These rocks are divided into two great classes: one formed of melted materials, like the lavas of our modern volcanoes; the other produced by seas, rivers, and lakes of ancient periods, in the same manner as we see deposits accumulated by the waters in our own time. The first are called igne­ous rocks; the second, sedimentary.
The Igneous Rocks, pushed from the interior of the earth in a plastic state, lift themselves above the surface of the soil in irregular shapes, in pre­cipitous peaks, or vast cones, and sometimes in those basaltic columns whose aspect is so striking and impressive.
The accompanying cut exhibits a good example of these basaltic rocks, but we may remark that columnar basalt is comparatively rare, and that igneous rocks in general possess no regularity of structure. Very frequently they are intersected by