14

CUMBERLAND COAL FIELD.

CHAP. XIV.

as great in the coal afterwards examined by me farther west at Pomeroy on the Ohio, confirming the theory first advanced by Mr. H. D. Rogers, of the progrossive debituminization of coal as we advance from west to east, or from the horizontal coal fields in the plains of the Ohio to the anthracite of Pennsylvania. (See above, pp. 89 and 249, vol. i.)

The coal measures of this part of Maryland are usually called the Cumberland coal field, from Fort Cumberland, famous for the wars of the English with the French and Indians, in which General Washing, ton took part, before the American Revolution. The carboniferous strata consisting, as usual, of shale, grit, sandstone, limestone, argillaceous iron ore, and coal, are arranged geologically in a trough, about twentyfive miles long, from north to south, and from three to four miles broad. Professor Silliman and his son, who surveyed them, have aptly compared the shape of the successive beds to a great number of canoes placed one within another. The entire thickness of the coal measures is about 1500 feet, including the fundamental quartzose sandstone, called by the miners here, as in England, the millstone grit, which is about forty feet thick, and contains small pebbles, sometimes as big as nuts. These pebbles, therefore, are very diminutive in comparison with those before mentioned as occurring in the same rock in the anthracite basins of north-eastern Pennsylvania, where some of them were stated to be as large as a hen's egg. The conglomerate of that region, it will be remembered, was 1500 feet in thickness, instead of forty feet as at Frostburg, showing the reduction of size in

the formations of mechanical origin as we proceed westward. (See above, pp. 84 and 86, vol. i.)

The seams of coal at Frostburg are numerous, there being three workable, besides nine or ten smaller beds. Under several of these, I found clays with Stigmaria, usually, as elsewhere, unaccompanied by any other fossil plants. At one spot, however, on the north-eastern confines of the coal basin, about fifty feet above the millstone grit, I saw a bed of coal, four feet thick, resting on a blue clay containing Stigmaria. This clay was twenty feet thick, and as usual without slaty texture, and the rootlets, commonly called leaves, radiated in all directions from the stems of the Stigmaria. Dispersed plentifully through the same clay, I found the leaves of two species of Pecopteris, and an Asterophyllite, the only instance, in several hundreds which I examined in the United States, where ferns and other coal plants were associated with the Stigmaria, imbedded in its natural position, and not having been drifted.

Higher in the series, but still 300 feet below the principal coal seam, an interesting example occurs of a black shale full of marine shells, resting on a seam of coal about three feet thick. When we have once embraced the doctrine of the origin of pure coal from terrestrial plants, which grew like peat in the spots where we now find them, the contact of an incumbent regular bed of black bituminous slate, ten or twelve feet thick, abounding in sea shells perfectly preserved, is highly interesting. Captain George Green, superintendent of the mines here, kindly presented me with a collection of these shells, which are referable to no less than seventeen species. Some are identical with,

16

WIDE RANGE OF

CHAP. XIV.

and almost all the rest have a near affinity to, species found in the Glasgow and other British coal measures. Among the rest is Bellerophon Urii, and two others of the same genus; Euomphalus carbonarius, several species of Nucula, one of Loxonema, and a Producta, allied to P. scabricula.

Among the plants occurring usually in the shaly roof or ceiling of the coal, are many identical with European species, such as Calamites dubius and C.nodosus, Pecopteris arborescens, and two other species in ironstone shale, both in fructification; also Lepidodendron tetragonum, L. aculeatum, Neuropteris cordata, N. gigantea, Sigillaria reniformis, Caulopteris, Stigmaria, Asterophyllites tuberculata, A. foliosa, and many others.

I have alluded to two species of ferns (Pecopteris) in fructification. One of these, abundant in the Jack Porter mine, appears to agree with the European Hemitelites Trevirani of Göppert. It agrees in its venation and the position of its sori with the recent subgenus Goniopteris. When we consider how rapidly the fructification decays on the back of the leaves of ferns, it is wonderful to see them thus petrified. The resemblance, moreover, of some of the common American and European coal plants, such as Pecopteris lonchitica, and P. Serlii, to ferns now living, such as Pteris caudata, and P. aquilina, is well worthy of notice. The leaves would be undistinguishable if the veins in the fossil species were not finer, closer together, and more perpendicular to the mid-rib, than in the recent ferns.

The specific agreement of so many of the American coal plants with European fossils implies a

greater uniformity in the carboniferous flora, throughout a large part of the globe, than appears to have prevailed in the co-existing conchological fauna, so far as it is known at present. Those English naturalists who assisted me in naming my American plants, came to the opinion that two thirds of them are the same as species well known in the coal measures on the other side of the Atlantic. M. Adolphe Brongniart informs me that he has arrived at the same result, the general accuracy of which cannot, I think, be impugned by questioning the botanical determinations arrived at from such characters, as the venations of fern leaves, or the markings left by the attachment of fronds on the bark of such trees as Sigillaria and Lepidodendron. If the prevailing vegetation of two distant parts of the globe were now to become fossil, the more common species would nowhere present so uniform a character, if we confined our comparison simply to corresponding organs, namely, the leaves, bark, fruits, the internal woody fibre, whether cellular or vascular, and the roots, if, indeed, the Stigmariæ be of that nature. As to the ferns, it should not be forgotten, that, although in the existing state of the globe, they are less cosmopolite than lichens and mosses, there are some of them, nevertheless, which have an extremely wide range, such as Didymochlæna sinuosa, common to Brazil, Java, and Manilla; and Polypodium incanum, to Brazil and the Cape of Good Hope. The recent ferns of North America, according to Pursh's Flora, are sixtynine in number, of which fifteen, according to the same authority, are natives of Europe. It is also worth remarking, that very few of the genera of liv

18

DENUDATION.

CHAP. XIV.

ing ferns are confined to one particular country, or even to one continent. The larger genera appear to have species in nearly all the regions of the world, except the colder latitudes. The mere generic resemblance, therefore, of the fossil ferns of North America and Europe, would not have been remarkable, as indicating a different geographical distribution from that now prevailing.

While at Frostburg, I rode one day on horseback, with Captain Green, superintendent of some of the mines there, and followed the course of Jenning's Run, returning by Cumberland. In this route, we saw a fine section of the coal measures, the underlying grit or conglomerate, and a great thickness (5000 or 6000 feet) of still older Devonian and upper Silurian strata. In those ridges, along the crest of which the yellow and white quartzose carboniferous grit crops out, the monotonous outline was occasionally broken by outliers of the rock twenty feet and upwards in height, remaining in situ with their perpendicular sides and sharp angles (see fig. 7), and show