The Palæozoic System comprises the earliest fossil-bearing strata. These have collectively a great thickness, and are of various mineral characters, being argillaceous, arenaceous, and calcareous, generally indurated, and occasionally metamorphosed.

As a general rule, these strata appear in the western and north-western districts of Britain, and, to a considerable extent, in the mari-time districts of Ireland. The rocks of this system contain the chief metaliferous workings, as in Cornwall, North Wales, Cumberland, Scotland, &c. The upper portion, namely, the Carboniferous, affords the chief source of our lead-ores which occur in the limestone of Derbyshire, Flintshire, and Cumberland; and the associated shales and sandstones, forming the Coal-Measures, contain the well-known extensive deposits of fossil fuel and iron-ore.

The palæozoic strata are numerous enough and sufficiently distinct among themselves to form several series. The first or lower of these comprises the Cambrian and Silurian; the middle is the Devonian; and the uppermost includes the Carboniferous and Permian.

The lower portion of the palæozoic system has two great groups of rocks; the lower one is named Cambrian by the Rev. Professor Sedgwick,—from its great extent in Wales;—the other is called Silurian by Sir Roderick Murchison, on account of the typical strata of this group having been worked out by him in the country once inhabited by the Silures,—the border-counties of England and Wales, where the group is extensively developed.