2. Having ascertained that the rocks of the earth have thus been produced by secondary causes, we next affirm, on the evidence of geology, that a distinct order of succession of these deposits can be ascertained; and though there are innumerable local variations in the nature of the rocks formed at the same period, yet there is, on the great scale, a regular sequence of formations over the whole earth. This succession is of the greatest importance in the case of aqueous rocks, or those formed in water; and it is evident that in the case of beds of sand, clay, etc., deposited in this way, the upper must be the more recent of any two layers. This simple principle, complicated in various ways by the fractures and disturbances to which the beds have been subjected, forms the basis of the succession of "formations" in geology as deduced from stratigraphical evidence. 3. This regular series of formations would be of little value as a history of the earth were it not that nearly all the aqueous rocks contain remains of the contemporary animals and plants. Ever since the earth began to be tenanted by organized beings, the various accumulations formed in the bottoms of seas and at the mouths of rivers have entombed remains of marine animals, more especially their harder parts, as shells, corals, and bones, and also fragments or entire specimens of land animals and plants. Hence, in any rock of aqueous formation, we may find fossil remains of the living creatures that existed in the waters in which that rock was accumulated or on the neighboring land. If in the process of building up the continents, the same locality constituted in succession a part of the bottom of the ocean, of an inland sea, of an estuary, and a lake, we should find in the fossil remains entombed in the deposits of that place evidences of these various conditions; and thus a somewhat curious history of local changes might be obtained. Geology affords more extensive disclosures of this nature. It shows that as we descend into the older formations we gradually lose sight of the existing animals and plants, and find the remains of others not now existing; and these, in turn, themselves disappear, and were preceded by others; so that the whole living population of the earth appears to have been several times renewed prior to the beginning of the present order of things. This seems farther to have occurred in a slow and gradual manner, not by successive great cataclysms or clearances of the surface of the earth, followed by wholesale renewal. This doctrine of geological uniformity is, however, to be understood as limited by the equally certain fact that there has been progress and advance, both in the inorganic arrangements of the earth's surface and in its organized inhabitants, and that there have, in geological as in historical times, been local cataclysms and convulsions, as those of earthquakes and volcanoes, often on a very extensive scale. Farther, there are good reasons to