(_a_) _Mechanical action._—The direct action of plants in forcing their roots into the crevices of rocks and minerals and thus both widening them by wedging, and by exposing new surfaces to weathering, has already been alluded to. That the mechanical force exerted by root growth is very great, may readily be judged from their effects in forcing apart, even to rupture, the walls of rock crevices; but actual measurement has shown the force with which the root, _e. g._, of the garden pea penetrates, to be equal to from seven to ten atmospheres, say from 200 to over 300 pounds per square inch. Such a force, exerted under the protection of the corky layer protecting the root tips, often produces surprising effects. (_b_) _Chemical action._—Vegetation takes a most important part, from a chemical point of view, both in the first formation of soils and in their subsequent relations to vegetable life. The lower forms of vegetation are usually the first to take possession of rock surfaces; foremost among these are the lichens. In humid climates we find these crust-like plants incrusting more or less all exposed rock surfaces, sometimes with a solid mantle that can be peeled off in wet weather, showing the corroded rock-surface, and the beginnings of soil clustering amid the root-fibrils beneath. A microscopic examination of the substance of these lichens often shows as a prominent ingredient, crystals of oxalate of lime, the lime having of course been derived from the rock, while the oxalic acid has been formed by the plant and used in the corrosion of the rock minerals. When it is remembered that this acid is comparable in strength to hydrochloric and nitric acids, the energy of the attack of the lichens is explained. Its progress can often be traced, even beyond the visible root fibers, by a change in the color of the rock; _e. g._, from rust-color to brick red.