I believe, first promulgated by Mr. Featherstonehaugh in 1831, at a time when many geologists were disposed to assign a higher antiquity to the anthracite than to the bituminous coal-measures of the United States. The recent surveys have now established this fact beyond all question, and hence it becomes a subject of great interest to inquire how these two kinds of fuel, originating as they did from precisely the same species of plants, and formed at the same period, should have become so very different in their chemical composition. In the first place, I may mention that the anthracite coal-measures above alluded to, occurring in the eastern or most disturbed part of the Appalachian chain, are fragments or outliers of the great continuous coal-field of Pennsylvania, Virginia, and Ohio, which occurs about forty miles to the westward. This coal-field is remarkable for its vast area, for it is described by Professor H. D. Rogers as extending continuously from N. E. to S. W., for a distance of 720 miles, its greatest width being about 180 miles. On a moderate estimate its superficial area amounts to 63,000 square miles. It extends from the northern border of Pennsylvania as far south as near Huntsville in Alabama.

This coal formation, before its original limits were reduced by denudation, must have measured, at a reasonable calculation, 900 miles in length, and in some places more than 200 miles in breadth. By reference to the section (fig 5., p. 74.), it will be seen that the strata of coal are horizontal to the westward of the mountain in the region D, E, and become more and more inclined and folded as we proceed eastward. Now it is invariably found, as Professor H. D. Rogers has shown by chemical analysis, that the coal is most

72

DEBITUMINIZATION OF COAL.

CHAP. IV.

bituminous towards its western limit, where it remains level and unbroken, and that it becomes progressively debituminized as we travel south-eastward towards the more bent and distorted rocks. Thus, on the Ohio, the proportion of hydrogen, oxygen, and other volatile. matters, ranges from forty to fifty per cent. Eastward of this line, on the Monongahela, it still approaches forty per cent., where the strata begin to experience some gentle flexures. On entering the Alleghany Mountains, where the distinct anticlinal axes begin to show themselves, but before the dislocations are considerable, the volatile matter is generally in the proportion of eighteen or twenty per cent. At length, when we arrive at some insulated coal-fields (5', fig. 5.) associated with the boldest flexures of the Appalachian chain, where the strata have been actually turned over, as near Pottsville, we find the coal to contain only from six to twelve per cent. of bitumen, thus becoming a genuine anthracite. (Trans. of Ass. of Amer. Geol., p. 470.)

It appears from the researches of Liebig and other eminent chemists, that when wood and vegetable matter are buried in the earth, exposed to moisture, and partially or entirely excluded from the air, they decompose slowly and evolve carbonic acid gas, thus parting with a portion of their original oxygen. By this means, they become gradually converted into lignite or wood-coal, which contains a larger proportion of hydrogen than wood does. A continuance of decomposition changes this lignite into common or bituminous coal, chiefly by the discharge of carburetted hydrogen, or the gas by which we illumine our streets and houses. According to Bischoff, the inflammable gases which

are always escaping from mineral coal, and are so often the cause of fatal accidents in mines, always contain carbonic acid, carburetted hydrogen, nitrogen, and olifiant gas. The disengagement of all these gradually transforms ordinary or bituminous coal into anthracite, to which the various names of splint coal, glance coal, culm, and many others, have been given.

We have seen that, in the Appalachian coal-field, there is an intimate connection between the extent to which the coal has parted with its gaseous contents, and the amount of disturbance which the strata have undergone. The coincidence of these phenomena may be attributed partly to the greater facility afforded for the escape of volatile matter where the fracturing of the rocks had produced an infinite number of cracks and crevices, and also to the heat of the gases and water penetrating these cracks, when the great movements took place, which have rent and folded the Appalachian strata. It is well known that, at the present period, thermal waters and hot vapours burst out from the earth during earthquakes, and these would not fail to promote the disengagement of volatile matter from the carboniferous rocks.

STRUCTURE AND ORIGIN OF THE APPALACHIAN CHAIN.

The subjects discussed in the preceding pages, lead me naturally to say something respecting the structure of the Appalachian chain, and its geological relations to the less elevated regions east and west of it. The annexed ideal section (fig. 5.), to which I shall have frequently occasion to refer in the sequel, will give some notion of the principal phenomena, omitting a great

Appalachian Coal Field.

Fig. 5.

Ideal geological section of the country between the Atlantic and the Mississippi.

Length from E. to W. 850 miles.

Alleghanies.

i Anthracite.

C, D. Alleghanies or Appalachian chain.

D, E. Appalachian coal field west of the mountains.

E, F. Dome-shaped out-erop of strata on the Ohio, older than the coal.

4. Red sandstone with ornithicnites (new red or trias ?) usually much Note. The dotted lines at i and k express portions of rock removed by

number of details. Starting from the shores of the Atlantic, on the eastern side of the Continent, we first come to a low region (A, B), which was called the alluvial plain by the first geographers. It is occupied by tertiary and cretaceous strata nearly horizontal, and containing in general no hard and solid rocks, and is usually not more than from 50 to 100 feet high, from New Jersey to Virginia. In these states this zone is not many leagues in breadth, but it acquires a breadth of 100 and 150 miles in the Southern States, and a height of several hundred feet towards its western limits. The next belt, from в to c, consists of granitic rocks (hypogene), chiefly gneiss and micaschist, covered occasionally with unconformable red sandstone, No. 4 (New Red ?), remarkable for its ornithicnites. Sometimes also this sandstone rests on the edges of the disturbed paleozoic rocks (as seen in the Section). The region (B, C), sometimes called the "Atlantic Slope," corresponds nearly in average width with the low and flat plain (A, B), and is characterised by hills of moderate height, contrasting strongly, in their rounded shape and altitude, with the long, steep, and lofty parallel ridges of the Alleghany mountains. The out-crop of the strata in these ridges, like the two belts of hypogene and newer rocks (A, B, and B, C), above alluded to, when laid down on a geological map, exhibit long stripes of different colours, running in a N. E. and S. W. direction, in the same way as the lias, chalk, and other secondary formations in the middle and eastern half of England.

The narrow and parallel zones of the Appalachians here mentioned consist of strata, folded into a succession of convex and concave flexures, subsequently laid