There are three immense coalfields in England. The first lies north of the Trent, and occupies an area of 360 square miles; and although the quantity of coal annually raised in Northumberland and Durham amounts to a million and a half of tons, there is enough to last 1000 years. London is chiefly supplied from it. The second or central coalfield, which includes Leicester, Worcester, Stafford, and Shropshire, has an area of 1495 square miles, and supplies the manufactories round it, and the midland counties south and east of Derbyshire. The third or western coalfield includes South Wales, Gloucestershire, and Somersetshire. The coalfield of South Wales alone is 100 miles long, and 18 or 20 broad. The Workington and Whitehaven coal-mines go a mile under the sea; several shafts in the latter are 100 fathoms deep; and it is one of the finest in England for extent and thickness of strata, some of the seams being nine feet thick. The Scotch coal-measures occupy the great central low-land of Scotland, lying between the southern high lands and the Highland mountains; the whole of that rude tract is occupied by them, besides which there are other coalfields of less extent. Coal has been found in seventeen counties in Ireland, but the island contains only four principal coal districts—Leinster, Munster, Connaught, and Ulster. Thus, there is coal enough in the British islands to last some thousands of years; and were it exhausted, our friends across the Atlantic have enough to supply the world for ages uncountable. Moreover, if science continues to advance at the rate it has lately done, a substitute for coal will probably be discovered before our own mines are worked out.[107] The carboniferous strata are enormously developed in the States of North America. The Appalachian coalfield extends, without interruption, 720 miles, with a maximum breadth of 280 miles, from the northern border of Pennsylvania to near Huntsville, in Alabama, occupying an area of 63,000 square miles. It is intersected by three great navigable rivers—the Monogahela, the Alleghany, and the Ohio—which expose to view the seams of coal on their banks. The Pittsburgh seam, 10 feet thick, exposed on the banks of the Monogahela, extends, horizontally, 225 miles in length and 100 in breadth, and covers an area of 14,000 square miles, so that this seam of coal may be worked for ages almost on the surface, and in many places literally so. Indeed, the facility is so great, that it is more profitable to convey the coal by water to New Orleans, 1100 miles distant, than to cut down the trees with which the country is covered for fuel, and which may be had for the expense of felling. The coal is bituminous, similar to the greater part of the British coal; forty miles to the east, however, among the ridges of the Appalachian chain, there is an extensive outlying member of the great coalfield, which yields anthracite, a species of coal which has the advantage of burning without smoke.