Part 50 (1/2)
_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 at ** joins on to the upper one at *.]
Starting from the sh.o.r.es 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, before described (pp. 171. 206. and 224.), which are nearly horizontal. The next belt, from B to C, consists of granitic rocks (hypogene), chiefly gneiss and mica-schist, covered occasionally with unconformable red sandstone, No. 4. (New Red or Trias?), remarkable for its ornithichnites (see p. 327.). 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 characterized by hills of moderate height, contrasting strongly, in their rounded shape and alt.i.tude, with the long, steep, and lofty parallel ridges of the Alleghany mountains.
The outcrop 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 open by denudation. The component rocks are of great thickness, all referable to the Silurian, Devonian, and Carboniferous formations. There is no princ.i.p.al 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 is termed an anticlinal and synclinal arrangement (see above, p. 48.). 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 a.s.semblage 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.
We are indebted to the state surveyors of Virginia and Pennsylvania, Prof.
W. B. Rogers and his brother Prof. H. D. Rogers, for the important discovery of a clue to the general law of structure prevailing throughout this 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 ma.s.s 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. 379.), it will be seen that on the eastern side, or in the ridges and troughs nearest the Atlantic, 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 (_l_) 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 quant.i.ties 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, if not contemporaneous, at least parts of one and the same series, depending on some common cause. Their geological date is well defined, at least within certain limits, for they must 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 d.y.k.es, some of which run for miles in lines parallel to the main direction of the Appalachians, or N.N.E. and S.S.W.
The thickness of the carboniferous rocks in the region C is very great, and diminishes rapidly as we proceed to the westward. The surveys of Pennsylvania and Virginia show that the south-east was the quarter whence the coa.r.s.er materials of these strata were derived, so 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 (at C) 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 30 feet.[329-A] 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, where the Atlantic now is; the deep sea, with its banks of coral and sh.e.l.ls to the west, or where the hydrographical basin of the Mississippi is now situated.
In that region, near Pottsville, where the thickness of the coal-measures is greatest, there are thirteen seams of anthracitic coal, several of them more than 2 yards thick. Some of the lowest of these alternate with beds of white grit and conglomerate of coa.r.s.er grain than I ever saw elsewhere, a.s.sociated with pure coal. The pebbles of quartz are often of the size of a hen's egg. On following these pudding-stones and grits for several miles from Pottsville, by Tamaqua, to the Lehigh Summit Mine, in company with Mr.
H. D. Rogers, in 1841, he pointed out to me that the coa.r.s.e-grained strata and their accompanying shales gradually thin out, until seven seams of coal, at first widely separated, are brought nearer and nearer together, until they successively unite; so that at last they form one ma.s.s, between 40 and 50 feet thick. I saw this enormous bed of anthracitic coal quarried in the open air at Mauch Chunk (or the Bear Mountain), the overlying sandstone, 40 feet thick, having been removed bodily from the top of the hill, which, to use the miner's expression, had been ”scalped.” The acc.u.mulation of vegetable matter now const.i.tuting this vast bed of anthracite, may perhaps, before it was condensed by pressure and the discharge of its hydrogen, oxygen, and other volatile ingredients, have been between 200 and 300 feet thick. The origin of such a vast thickness of vegetable remains, so unmixed with earthy ingredients, can, I think, be accounted for in no other way, than by the growth, during thousands of years, of trees and ferns, in the manner of peat,--a theory which the presence of the Stigmaria _in situ_ under each of the seven layers of anthracite, fully bears out. The rival hypothesis, of the drifting of plants into a sea or estuary, leaves the absence of sediment, or, in this case, of sand and pebbles, wholly unexplained.
[Ill.u.s.tration: Fig. 380. Cross section.]
[Ill.u.s.tration: Fig. 381. Cross section.]
But the student will naturally ask, what can have caused so many seams of coal, after they had been persistent for miles, to come together and blend into one single seam, and that one equal in the aggregate to the thickness of the several separate seams? Often had the same question been put by English miners before a satisfactory answer was given to it by the late Mr.
Bowman. The following is his solution of the problem. Let _a a'_, fig.
380., be a ma.s.s of vegetable matter, capable, when condensed, of forming a 3-foot seam of coal. It rests on the underclay _b b'_, filled with roots of trees _in situ_, and it supports a growing forest (C D). Suppose that part of the same forest D E had become submerged by the ground sinking down 25 feet, so that the trees have been partly thrown down and partly remain erect in water, slowly decaying, their stumps and the lower parts of their trunks being enveloped in layers of sand and mud, which are gradually filling up the lake D F. When this lake or lagoon has at length been entirely silted up and converted into land, say, in the course of a century, the forest C D will extend once more continuously over the whole area C F, as in fig. 381., and another ma.s.s of vegetable matter (_g g'_), forming 3 feet more of coal, may acc.u.mulate from C to F. We then find in the region F, two seams of coal (_a'_ and _g'_) each 3 feet thick, and separated by 25 feet of sandstone and shale, with erect trees based upon the lower coal, while, between D and C, we find these two seams united into a 2-yard coal. It may be objected that the uninterrupted growth of plants during the interval of a century will have caused the vegetable matter in the region C D to be thicker than the two distinct seams _a'_ and _g'_ at F; and no doubt there would actually be a slight excess representing one generation of trees with the remains of other plants, forming half an inch or an inch of coal; but this would not prevent the miner from affirming that the seam _a g_, throughout the area C D, was equal to the two seams _a'_ and _g'_ at F.
The reader has seen, by reference to the section (fig. 379. p. 327.), that the strata of the Appalachian coal-field a.s.sume an horizontal position west of the mountains. In that less elevated country, the coal-measures are intersected by three great navigable rivers, and are capable of supplying for ages, to the inhabitants of a densely peopled region, an inexhaustible supply of fuel. These rivers are the Monongahela, the Alleghany, and the Ohio, all of which lay open on their banks the level seams of coal. Looking down the first of these at Brownsville, we have a fine view of the main seam of bituminous coal 10 feet thick, commonly called the Pittsburg seam, breaking out in the steep cliff at the water's edge; and I made the accompanying sketch of its appearance from the bridge over the river (see fig. 382.). Here the coal, 10 feet thick, is covered by carbonaceous shale (_b_), and this again by micaceous sandstone (_c_). Horizontal galleries may be driven everywhere at very slight expense, and so worked as to drain themselves, while the cars, laden with coal and attached to each other, glide down on a railway, so as to deliver their burden into barges moored to the river's bank. The same seam is seen at a distance, on the right bank (at _a_), and may be followed the whole way to Pittsburg, fifty miles distant. As it is nearly horizontal, while the river descends it crops out at a continually increasing, but never at an inconvenient, height above the Monongahela. Below the great bed of coal at Brownsville is a fire-clay 18 inches thick, and below this, several beds of limestone, below which again are other coal seams. I have also shown in my sketch another layer of workable coal (at _d d_), which breaks out on the slope of the hills at a greater height. Here almost every proprietor can open a coal-pit on his own land, and the stratification being very regular, he may calculate with precision the depth at which coal may be won.
The Appalachian coal-field, of which these strata form a part (from C to E, section, fig. 379., p. 327.), is remarkable for its vast area; for, according to Professor H. D. Rogers, it stretches 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.
[Ill.u.s.tration: Fig. 382. View of the great Coal Seam on the Monongahela at Brownsville, Pennsylvania, U. S.
_a._ Ten-foot seam of coal.
_b._ Black bituminous or carbonaceous shale, 10 feet thick.
_c._ Micaceous sandstone.
_d d._ Upper seam of coal, 6 feet thick.]
This coal formation, before its original limits were reduced by denudation, must have measured 900 miles in length, and in some places more than 200 miles in breadth. By again referring to the section (fig. 379., p.
327.), it will be seen that the strata of coal are horizontal to the westward of the mountains 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 a.n.a.lysis, 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. 379.) a.s.sociated 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.[333-A]
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 illuminate 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 olefiant 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.