there are proofs that the ancient land lay in that direction. The conglomerate which forms the general base of the coal-measures is 1500 feet thick in the Sharp Mountain, where I saw it, near Pottsville; whereas it has only a thickness of 500 feet, about thirty miles to the north-west, and dwindles gradually away when followed still farther in the same direction, till its thickness is reduced to thirty feet. (Rogers. Trans. Assoc. Amer. Geol., 1840-42, p. 440.) The limestones, on the other hand, of the coal-measures, augment as we trace them westward. Similar observations have been made in regard to the Silurian and Devonian formations in New York; the sandstones and all the mechanically-formed rocks thinning out as they go westward, and the limestones thickening, as it were, at their expense. It is, therefore, clear that the ancient land was to the east; the deep sea, with its banks of coral and shells, to the west.

I at first supposed that some deception might have arisen respecting the alleged thickness of the older fossiliferous rocks of the Appalachians, owing to the dislocations and inverted position of the beds, but I was soon convinced that due regard had been paid to the apparent repetitions caused by these disturbances, and I have little doubt that those Silurian and Devonian strata, which do not exceed in their aggregate thickness a mile and a half in the State of New York, acquire more than three times that thickness in the Pennsylvanian Alleghanies.

A few days' observation of the identity of the fossil plants, and the relative position of the anthracite, satisfied me that it was of the same age as the bituminous coal which I had seen at Blossberg. This opinion was,

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

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 of 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 bitussinous 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

A, B. Atlantic plain.

B, C. Atlantic slope.

C, D. Alleghanies or Appalachian chain.

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

E, F. Dome-shaped out-crop of strata on the Ohio, older than the coa!.

1. Miocene tertiary.

22. Eocene tertiary.

3. Cretaceous strata.

References to the

4. Red sandstone with ornithicnites (new red or trias ?) usually much invaded by trap.

5. Coal-measures (bituminous coal).

5. Anthracite coal-measures.

5. Carboniferous limestone of the Illinois coal field, wanting in the

Appalachian.

F, G. Illinois coal field.

h. Falls and rapids of the rivers at the junction the hypogene and newer formations.

i, k, l, m. Parallel folds of Appalachians becoming successively more open and flatter in going from E. to W.

different Formations.

6. Old red or Devonian, Olive slate, &c. 7. Primary fossiliferous or Silurian strata.

8. Hypogene strata, or gneiss, mica schist, &c.. with granite veins.

Note. The dotted lines at i and k express portions of rock removed by denudation, the amount of which may be estimated by supposing similar lines prolonged from other points where different strata end abruptly at the surface.

N. B. The lower section is a continuation of the upper cre.