Through the study of fossils, as is well known, geologists have been led to the all-important generalization that the vast series of fossiliferous or sedimentary rocks may be separated into a number of definite groups or formations, each of which being characterized by its own organic remains, but not properly and strictly, it must be understood, by the occurrence therein of any one particular fossil. However, a formation may contain some particular fossil or fossils not occurring outside of that formation, thus enabling an observer to identify a given group with tolerable certainty; or, as very often happens, some particular stratum or subgroup of a series, may contain peculiar fossils, whereby its existence may be determined with considerable readiness in divers localities. Each great formation, let it be said, is properly characterized by the association of certain fossils, the predominance of certain families or orders, or by an assemblage of fossil remains that represent the life of the period during which the formation was deposited. Fossils, then, not only enable us to determine the age of the deposits in which they are found, but they also further enable us to arrive at some very important conclusions respecting the manner in which the fossiliferous bed was deposited, and, consequently, to the condition of the particular region occupied by the bed at the period of its formation. Beds that contain the remains of animals, such as now inhabit rivers, we know to be fluviatile in their origin, and that at one time they must have either constituted actual river-beds, or been deposited by the overflowing of ancient streams. But if the beds contain the remains of mollusks, minute crustaceans or fish, such as are found to-day in lakes, then we conclude that they are lacustrine, and were deposited beneath the waters of former lakes. And, lastly, if the remains of animals such as now people the oceans are to be met with in the beds, then we know that they are marine in origin, and that they are fragments of an old sea-bottom. On the whole, the conditions under which a bed was deposited, whether in a shallow sea, in the immediate vicinity of a coast-line, or in deep water, can often be determined with considerable accuracy from the nature of the relics of the organisms which they contain. But we have thus far been dealing with the remains of aquatic animals. When, however, we consider the remains of aerial and terrestrial animals, or of plants, the determination of the conditions of deposition is not made out with such an absolute certainty. Remains of land-animals would, of course, occur in sub-aerial deposits, that is, in beds, like blown sand, accumulated upon the land, but the most of such remains of such animals are found in deposits which have been laid down in water, and hence their present position is due to the fact that their former owners were either drowned in rivers or lakes, or borne out to sea by water-channels.