I. “Il suffit de parcourir des parties très circonstrites de la surface du globe, pour voir qu'elle est composée de matériaux très-variés.” The most striking of these differences were, of course, noticed even in the least civilized times; and, as observation advanced, and experience accumulated, the minor and less obvious variations of constitution and texture were gradually reduced to order, and classified into the science of Mineralogy.
II. So far our science concerned itself merely with what we should now call a correct description of the mineral character of rocks, without entering into the question of their genesis or mode of origin. But at the dawn of geology—that is, when men first conceived that the various rocks and strata at the surface of our earth were formed at different, and often immeasurably remote periods—then the study of rocks entered on a new phase. The age and succession of these formations were judged, in the first place, from the order of their superposition; and when it was found, as a general rule, that each formation, or stage of this succession, was marked by rocks of a specific mineral character, it was hastily concluded that this mineral character was typical of their age, and that the latter could be deduced from the former: nor was this generalization devoid of broad principles of truth; the ancient and more recent formations are each undoubtedly composed of rocks of very different mineral character—the former being made up such of rocks as granite and the so-called primary limestones and clay-slates, while the latter consists of clays, chalk, or slightly consolidated sandstones.