Sec. 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 rocks 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 This classification was always, in some degree, uncertain; and has been lately superseded by more complicated systems, founded on the character of the fossils contained in the various deposits, and on the circumstances of position, by which their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. Sec. 2. For the fact is, that in approaching any large mountain range, the ground over which the spectator passes, if he examine it with any intelligence, will almost always arrange itself in his mind under three great heads. There will be, first, the ground of the plains or valleys he is about to quit, composed of sand, clay, gravel, rolled stones, and variously mingled soils; which, if he has any opportunity,--at the banks of a stream, or the sides of a railway cutting,--to examine to any depth, he will find arranged in beds exactly resembling those of modern sand-banks or sea-beaches, and appearing to have been formed under such natural laws as are in operation daily around us. At the outskirts of the hill district, he may, perhaps, find considerable eminences, formed of these beds of loose gravel and sand; but, as he enters into it farther, he will soon discover the hills to be composed of some harder substance, properly deserving the name of rock, sustaining itself in picturesque forms, and appearing, at first, to owe both its hardness and its outlines to the action of laws such as do not hold at the present day. He can easily explain the nature, and account for the distribution, of the banks which overhang the lowland road, or of the dark earthy deposits which enrich the lowland pasture; but he cannot so distinctly imagine how the limestone hills of Derbyshire and Yorkshire were hardened into their stubborn whiteness, or raised into their cavernous cliffs. Still, if he carefully examines the substance of these more noble rocks, he will, in nine cases out of ten, discover them to be composed of fine calcareous dust, or closely united particles of sand;