The mining industry of the kingdom is divided into two distinct branches—that of the coal and iron mines, and that of the mines of tin, copper, lead, and zinc. The "coal measures," as the geological formations comprising the strata of coal are designated, are variously dispersed in the middle, northern, and western portions of South Britain, and in a broad belt of country which traverses the centre of Scotland, from the shores of Ayrshire to those of the Firth of Forth. There are, also, some coal-tracts in Ireland, but they are of comparatively small importance. In all these districts, the coal is found in beds, interstratified for the most part with various qualities of gritstone and shale, in which, in some of the districts, occur layers of ironstone, generally thin, but sometimes forming large masses, as in the Forest of Dean. When the surface of the coal country is mountainous and intersected by deep ravines, as in South Wales, the mineral deposites are approached by holes driven into the sides of the hills; but the common access to them is by vertical shafts, or well-holes, from the bottoms of which horizontal roadways are extended in long and confined passages through the coal strata, to bring all that is hewn to the "pit's eye," or bottom of the shaft, for winding up. It is requisite to have more than one shaft in the same workings; but where the coal lies so deep that the sinking of a distinct shaft requires an enormous outlay of capital, only one large shaft is sunk; and this is divided by wooden partitions, or brattices, into several distinct channels. There must always be one shaft or channel, called the "downcast pit," for the air to descend; and another, called the "upcast pit," for the return draught to ascend. The apparatus for lowering and drawing up is generally in the upcast shaft. This is either a steam-engine, a horse-gin, or a hand-crank. The thickness of the seams that are wrought varies from the eighteen-inch seams of the Lancashire and Yorkshire hills, to the ten-yard coal of South Staffordshire. But two, three, and four feet are the more common thicknesses of the beds that are wrought. When there is a good roof, or hard rock immediately over the coal, with a tolerably solid floor beneath it, thin coal-seams can be worked with advantage, because the outlay of capital for propping is then very limited; but the very hardness of the contiguous strata would require an outlay almost as great to make the roadways of a proper height for human beings of any age to work in.