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.

Appalachian Coal Field.

GEOLOGICAL SECTION.

B, C. Atlantic slope.

C, D. Alleghanies or Appalachian chain.

6

F

Silurian.

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

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

1. Miocene tertiary.

2. 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.

5

Coal.

h. Falls and rapids of the rivers at the junction of 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 one.

CHAP. IV.

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

76

STRUCTURE AND ORIGIN OF

CHAP. IV

open by denudation. The component rocks are of

great thickness, all referable to the Silurian, Devonian, and Carboniferous formations. There is no principal or central axis, as in the Pyrenees and many other chains-no nucleus to which all the minor ridges conform; but the chain consists of many nearly equal and parallel foldings, having what the geologists term an anticlinal and synclinal arrangement. This system of hills extends, geologically considered, from Vermont to Alabama, being more than 1000 miles long, from 50 to 150 miles broad, and varying in height from 2000 to 6000 feet. Sometimes the whole assemblage of ridges runs perfectly straight for a distance of more than 50 miles, after which all of them wheel round together, and take a new direction, at an angle of 20 or 30 degrees to the first.

Mr. R. C. Taylor had made considerable progress in unravelling the structure of certain portions of this chain, before the commencement of the State Surveys of Virginia and Pennsylvania, the former conducted by Professor W. B. Rogers, the latter by his brother, Professor H. D. Rogers, both aided by a numerous corps of assistants. To these elaborate and faithful surveys we owe the discovery of the clue to the general law of structure prevailing throughout this important range of mountains, which, however simple it may appear when once made out and clearly explained, might long have been overlooked, amidst so great a mass of complicated details. It appears that the bending and fracture of the beds is greatest on the south-eastern or Atlantic side of the chain, and the strata become less and less disturbed as we go westward, until at length they regain their original or horizontal position. By

reference to the section (fig. 5.), it will be seen that on the eastern side, or on the ridges and troughs nearest the Atlantic, the south-eastern dips predominate, in consequence of the beds having been folded back upon themselves, as in i, those on the north-western side of each arch having been inverted. The next set of arches (such as k) are more open, each having its western side steepest; the next (1) opens out still more widely, the next (m) still more, and this continues until we arrive at the low and level part of the Appalachian coal-field (D, E).

In nature, or in a true section, the number of bendings or parallel folds is so much greater that they could not be expressed in a diagram without confusion. It is also clear that large quantities of rock have been removed by aqueous action or denudation, as will appear if we attempt to complete all the curves in the manner indicated by the dotted lines at i and k.

The movements which imparted so uniform an order of arrangement to this vast system of rocks must have been contemporaneous, or belonging to one and the same series, depending on some common cause. Their geological date is unusually well defined. We may declare them to have taken place after the deposition. of the carboniferous strata (No. 5.), and before the formation of the red sandstone (No. 4.). The greatest disturbing and denuding forces have evidently been exerted on the south-eastern side of the chain, and it is here that igneous or plutonic rocks are observed to have invaded the strata, forming dykes, some of which run for miles in lines parallel to the main direction of the Appalachians, or N.N.E. and S.S.W. According to the theory of the Professors Rogers, the