§ 1. In the early days of geological science, the substances which composed the crust of the earth, as far as it could be examined, were supposed to be referable to three distinct classes: the first consisting of rocks which not only supported all the rest, but from which all the rest were derived, therefore called “Primary”; the second class consisting of rock formed of the broken fragments or altered substance of the primary ones, therefore called “Secondary”; and, thirdly, rocks or earthy deposits formed by the ruins and detritus of both primary and secondary rocks, called therefore “Tertiary.” This classification was always, in some degree, uncertain; and has been lately superseded by more complicated systems, founded on the character of the fossils contained in the various deposits, and on the circumstances of position, by which their relative ages are more accurately ascertainable. But the original rude classification, though of little, if any, use for scientific purposes, was based on certain broad and conspicuous phenomena, which it brought clearly before the popular mind. In this way it may still be serviceable, and ought, I think, to be permitted to retain its place, as an introduction to systems more defined and authoritative.