Geology, as will be seen from the preceding details, is among the most comprehensive of the sciences. It invades the province of every one of them, and lays them all under contribution while following out its own peculiar researches. A dry description of rocks, in their simple mineral qualities, does not limit or exhaust its ample resources. Botany, zoology, meteorology—a part, in short, of every branch of natural history, as well as chemistry, physics, and astronomy, are severally enlisted in its service, and all give interest and importance to its discursive investigations. And thus, receiving gifts from every walk of science, geology gives back in turn, and imparts to each, illustrations new and rare, from its own wonderful storehouse. Geology, considered in itself, may be pursued in three different ways, as it resolves into three great leading branches of investigation. Observing the arrangement and superposition of rocks, as exhibited in the crust of the earth, along with their mineral distinctions and fossil contents, we embrace all the objects included in _descriptive_ or _phenomenal_ geology. The exposition, again, of the general principles by which such phenomena can be produced, constitutes what has been termed _geological dynamics_,—by which are traced the laws of action of known causes, and their relation to such changes as those which geology considers. The last branch leads to a consideration of the causes in which the phenomena have originated and the doctrines deducible from them. This has been called _physical geology_, and embraces all that is theoretic in the science. These three branches, while thus definitively distinct in themselves, are yet frequently combined in the works of writers on the subject; nor is it easy, or even possible, in practice, to separate them, as few will be content to describe without attempting also to explain. It has been no part of our vocation in these investigations to inquire into the _origin_ of a material universe;—what was its pre-existent state, and by what process this globe at first was brought into an earthy concrete form. Astronomy has tried various solutions. But whether by the splintering of other worlds, or the evolution of matter from a Saturnian ring, or by the condensation of gaseous star-dust diffused through infinite space, no astronomical hypothesis has proved satisfactory. Geology is better employed when she assumes a beginning to her researches upon the visible crust of the globe. The mystery of _creation_ is not within the range of her legitimate territory; and, while the investigation of laws and of the influence of secondary causes falls within the province of both, it may be safely admitted that neither astronomy nor geology are, of themselves, capable of giving us any real or precise account of the origin of the universe, or of any of its parts.