Clays are generally the mud of rivers; sandstones, the detritus of old crystalline rocks; while limestones are organically or chemically formed. If, therefore, there is a great clay, it bespeaks a great river draining lands that were; though small and local clay-bands may be but the ruins of denuded sea-cliffs. The sandstone is the evidence of coast and reef, where and wave disintegrated plutonic rocks, or constructed over again the palæozoic or older sand-rocks. And limestones have been formed either from the denudation of calcareous cliffs in shallow seas.

There is not in Great Britain any coal-field so characeristic as that of South Wales; nor one which in outward appearance so little agrees with the general notion as to what a coal-field should be like. Instead of the barren and monotonous surface that we usually find in Durham, Staffordshire, Lancashire, or Scotland, we find scenery of a high order,—lofty hills, romantic dales, broken scaurs, and woods feathering down to the banks of the streams that run brawling to the Bristol Channel. It is a wonder indeed that tourists do not oftener explore these gems of South-West landscape, particularly as every valley is now accessible by railway. Nor is it merely in scenic interest that the basin is peculiar; for the very physical arrangement which gives the hill and dale enables much of the coal to be won by level, instead of pit, thus forming a marked feature in the economy of the working. It is with regard to this physical geography that I would say a few words, as viewed in relation to the geology of the basin.