_S. W. England, S. Wales, Derbyshire and Yorkshire._ In these districts the Carboniferous limestone reaches a considerable thickness; in the Mendips it has a thickness of 3000 feet, and in the Pennine chain of 4000 feet. At the base of this limestone series there occurs in the southern districts the so-called lower limestone shale, consisting of clays, shales and sandy beds. Above the limestone we have the Millstone grit and Coal-Measures; but in the Pennine district there is a series of rocks consisting of impure limestones and shales, intercalated between the Millstone grit and Carboniferous limestone; for this group of rocks the term _Yoredale series_ has been proposed. In the Isle of Man and Derbyshire sheets of lava are interbedded with the calcareous sediments, affording clear proof of submarine volcanic eruptions. _N. England and Scotland._ In the Carboniferous rocks of Northumberland we have distinct indications of a shallower sea. The regular succession of limestone strata in West Yorkshire and other districts, gives place to a series of thinner beds of limestones, interstratified with shales and impure calcareous rocks. We have come within the range of land detritus which was spread out on the floor of a shallow sea. The lowest portion of the Mountain limestone is here represented by about 200 feet of shales and other rocks grouped together in the _Tuedian series_. The Upper Carboniferous limestone and Yoredale rocks of Yorkshire are represented by sandstones, carbonaceous limestones and some seams of coal, included in the _Bernician series_. Further north, again, another classification has been proposed for the still more aberrant succession of rocks; the lowest being spoken of as the _Calciferous sandstone_, and the upper as the _Carboniferous limestone_. The calciferous sandstone may be compared with the lower limestone shale and part of the Carboniferous limestone of England. The Carboniferous limestone of Scotland probably represents the upper part of the limestone of England and the Yoredale rocks of the Pennine and other areas. Turning to the upper members of the Carboniferous system—in the Coal-Measures, as they were called in 1817 by William Smith,—we have a series of coal seams, sandstones, shales, and ironstones occurring for the most part in basin-shaped areas. As a general rule, each seam of coal, which varies in thickness from one inch to thirty feet, rests on a characteristic unstratified argillaceous rock known as Underclay. The accompanying diagram (Fig. 4) illustrates the frequent intercalation of small bands of argillaceous and sandy rocks associated with the seams of coal.