Here, again, it is extremely probable that the coal-fields which remain, in spite of their gigantic existing areas, are but the remnants of one tremendous area of deposit, bounded only on the east by the Atlantic, and on the west by a line running from the great lakes to the frontiers of Mexico. The whole area has been subjected to forces which have produced foldings and flexures in the Carboniferous strata after deposition. These undulations are greatest near the Alleghanies, and between these mountains and the Atlantic, whilst the flexures gradually dying out westward, cause the strata there to remain fairly horizontal. In the troughs of the foldings thus formed the coal-measures rest, those portions which had been thrown up as anticlines having suffered loss by denudation. Where the foldings are greatest there the coal has been naturally most altered; bituminous and caking-coals are characteristic of the broad flat areas west of the mountains, whilst, where the contortions are greatest, the coal becomes a pure anthracite. It must not be thought that in this huge area the coal is all uniformly good. It varies greatly in quality, and in some districts it occurs in such thin seams as to be worthless, except as fuel for consumption by the actual coal-getters. There are, too, areas of many square miles in extent, where there are now no coals at all, the formation having been denuded right down to the palaeozoic back-bone of the country. Amongst the actual coal-fields, that of Pennsylvania stands pre-eminent. The anthracite here is in inexhaustible quantity, its output exceeding that of the ordinary bituminous coal. The great field of which this is a portion, extends in an unbroken length for 875 miles N.E. and S.W., and includes the basins of Ohio, Maryland, Virginia, Kentucky, and Tennessee. The workable seams of anthracite about Pottsville measure in the aggregate from 70 to 207 feet. Some of the lower seams individually attain an exceptional thickness, that at Lehigh Summit mine containing a seam, or rather a bed, of 30 feet of good coal. A remarkable seam of coal has given the town of Pittsburg its name. This is 8 feet thick at its outcrop near the town, and although its thickness varies considerably, Professor Rogers estimates that the sheet of coal measures superficially about 14,000 square miles. What a forest there must have existed to produce so widespread a bed! Even as it is, it has at a former epoch suffered great denudation, if certain detached basins should be considered as indicating its former extent. The principal seam in the anthracite district of central Pennsylvania, which extends for about 650 miles along the left bank of the Susquehanna, is known as the "Mammoth" vein, and is 29-1/2 feet thick at Wilkesbarre, whilst at other places it attains to, and even exceeds, 60 feet.