all intents and purposes the same in all regions and in all periods, from the Devonian coals of Bear Island in the Arctic to the Tertiary coals of Wyoming or Trinidad; but the varying descriptions and explanations presented by students make equally certain that one cannot ascertain what the essential conditions are, if his investigation be confined to areas embracing a score or even several hundreds of square miles. The investigation must cover a great area, in which merely local features do not obscure those which are general and which actually bear upon the problem in hand. Such an area is the Appalachian Basin of the eastern United States, where one finds the Pennsylvanian or Coal Measures divided into.

The Appalachian coal field, now embracing approximately 70,000 square miles of almost continuous deposits, occupies only a part of the original area. The deep synclinal basins of anthracite in eastern Pennsylvania are separated by 50 to 100 miles from the great bituminous region at the west, while southwardly one finds insignificant fragments along the eastern side until he comes to Georgia and Alabama. The greatest extent of the area of deposit was probably at the close of the Pottsville, when it reached from southern New York in west southwest direction to beyond central Alabama, more than 800 miles; at the north, it spread from the old Appalachian land, at the east, westward to beyond Newark in

J. J. Stevenson, "Carboniferous of the Appalachian Basin," Bull. Geol. Soc. Amer., Vol. 18, 1907, p. 178. The Pottsville is subdivided in this paper into Beaver and Rockcastle. I. C. White, in West Virginia Geol. Survey, Vol. Ia, 1908, p. 13, has suggested that Rockcastle be replaced by New River and Pocahontas; this should be accepted, as Stevenson did not assign proper significance to Pocahontas, regarding it as merely a subordinate stage.

Ohio, while at the south it reached the western boundary of Alabama. The area of deposit at that time embraced not less than 200,000 square miles. The present outcrop approaches the western. border at a few localities in Ohio and Kentucky as well as in Alabama, but for the most part it is two score or more miles east from the original limit. The eastern border is approached in the southern anthracite field of Pennsylvania and apparently it was not far eastward from the Pocahontas outcrop in Virginia; in Alabama, the eastern outcrop is not more than 25 miles from the original border on that side. But, in most of the space north from Alabama, the present continuous outcrop is from 30 to 100 miles west from that border as it probably existed at the close of the Pottsville. The Appalachian field included a small part of New York, more than two thirds of Pennsylvania, the western third of Maryland. nearly the whole of West Virginia, the eastern third of Ohio and Kentucky, with southwestern Virginia, eastern Tennessee, the northern half of Alabama as well as northwestern Georgia. Here then is an area of sufficient extent to provide ample illustration of purely local features and their relations to the effects of widely acting agents.

THE APPALACHIAN BASIN.

The Appalachian basin, from its origin to the close of the Palæozoic, was the scene of frequent changes in the relations of land and water. Schuchert and Ulrich have shown that such changes were merely commonplaces in the earlier periods. Those students are not in agreement respecting several matters, which have much interest from a philosophical standpoint, but they are in full agreement respecting all matters which concern the questions at issue here. As Schuchert has shown, the Appalachian basin originally was continuous with the broad Mississippi region and much of it was covered with sea. Toward the close of the Ordovician the Taconic revolution began, which, at the east, widened

3

C. Schuchert, "Palæography of North America," Bull. Geol. Soc. Amer., Vol. 20, 1910, pp. 427-606; E. O. Ulrich, "Revision of the Paleozoic Systems," ibid., Vol. 22, 1911, pp. 281-680.

Appalachia by additions along the western side from New England through Virginia, thus giving a great area on which erosion could. work and did work, as evidenced by clastic sediments in the northern part of the basin. The Cincinnati uplift of authors, occurring at the same time, led to the elevation of Cincinnatia (of Schuchert), the western boundary of the basin. Contemporaneously, as indicated by Schuchert, a less conspicuous land area, Alleghania, appeared within the basin, rudely parallel to Appalachia and extending southwardly across western Pennsylvania and eastern Ohio. It was separated from Cincinnatia by the Ohio basin. During the Silurian, there was occasional communication across Cincinnatia with the Mississippi region beyond and the faunas indicate that the basin opened northwardly to the Atlantic ocean. The same type of evidence shows that the basin was divided by a land area in southern Virginia, so that there was a northern sea extending into New York, as well as a southern sea in Tennessee and Alabama.

Studies by many geologists make clear that the southern portion. of Appalachia was unstable. Southward from central Pennsylvania, the early Devonian rocks are wanting along the eastern side, while in southern Virginia and thence southward only the lower beds of the Middle Devonian are found. In Alabama the Devonian, more than 8,000 feet thick in central Pennsylvania, is represented by only dark shale rarely exceeding 100 feet and, near its southern limit, varying from 3 to 30 feet.

The area embraced in Schuchert's Alleghania was of decided instability. It received deposits during the Chemung, for that formation with its characteristic conglomerates crossed the area, though with reduced thickness; but all the principal elements of the section observed in central Pennsylvania are present. The intervals decrease toward this area and there is notable thinning above the upper conglomerate. The Catskill beds, following the Chemung, thin out against Alleghania, showing that once more it was above water. That formation, as defined by Vanuxem, the first to assign a definite meaning to the term, is 3,900 feet thick in Fulton county of Pennsylvania; 3,000, in the eastern portion of Bedford; 1,980 in western Bedford; it is concealed in Somerset,

except on the western edge, where it is brought up by a great anticline and is 10 to 15 feet thick, while, at 4 or 5 miles farther west in Fayette county, it has disappeared and the Upper Pocono beds of the Mississippian rest on the Chemung. The rate of decrease under Somerset is very nearly the same as that in Fulton and Bedford. This thinning is shown on the western side of the Catskill deposits. from New York to New River in Virginia, beyond which southwardly Chemung and Catskill both disappear.*

The Carboniferous was opened by subsidence in the basin. The northern portion still received the greatest deposits along the eastern side in the old valley or "trough of sedimentation," but the area widened westwardly so that the later Pocono rocks of Pennsylvania. overlap the Catskill of the Devonian and rest without apparent nonconformity on the Chemung rocks of Alleghania, as they do beyond in the Ohio basin. There was distinct widening eastwardly in Virginia and southward. Campbell showed that phenomena in Virginia, which had puzzled earlier observers, were due to overlap; that the coal-bearing Mississippian deposits rest there on Ordovician rocks, which in all probability had been upraised during the Taconian revolution. Still farther south, the oldest rocks of the Mississippian overlap the Catskill, the Chemung and, at length, even the thin Chattanooga shales, the last Devonian representative toward the south. But during succeeding stages of the Mississippian, there was distinct contraction of the area of deposit on the western side, for the Maxville lies within the Logan and the Shenango within the Maxville; at times, there may have been dry land in the Alleghania region. But at the south there was continued depression,

J. J. Stevenson, "Bedford and Fulton Counties," Second Geol. Surv. of Penn., 1882, pp. 73-75, 81; "The Upper Devonian Rocks of Southwest Pennsylvania," Amer. Jour. Sci., III., Vol. XV., 1878, pp. 423-430; "On the Use of the Name Catskill," ibid., Vol. XLVI., 1893, pp. 330–337.

M. R. Campbell, Palæozoic Overlaps in Montgomery and Pulaski Counties, Virginia,” Bull. Geol. Soc. Amer., Vol. 5, 1894, p. 182.

J. J. Stevenson, "Lower Carboniferous of the Appalachian Basin," Bull. Geol. Soc. Amer., Vol. 14, 1903, p. 85. This paper, on pp. 89-96, contains a discussion of the varying geographical conditions during the Mississippian.

increasing southwardly, so that in that direction the thickness of the deposits increases until eastern Alabama is reached, where one finds progressive overlap and each deposit has its attenuated outcrop beyond that of its predecessor. Toward the close of the Mississippian, Alleghania was becoming better defined; all of the formations are present in most of it but they are very thin, nowhere more than 400 feet thick at the north, less than one fifth as much as in the anthracite region. Whether or not the Shenango shales of western Pennsylvania are synchronous with the upper red beds of the Mauch Chunk region cannot be determined, as a gap of 60 miles exists, from which the beds have been removed. Fossils are rare and insufficient for correlation; they make evident, however, that marine conditions prevailed in Alleghania, for the individuals obtained in southwestern Pennsylvania are large, well developed and thoroughly characteristic. The water was probably too shallow and too variable in distribution to permit abundant life; the sun cracks, ripple marks and other features indicate that these fine muds were spread out on mud flats, with constantly shifting areas of tidal waters. It is certain that withdrawal of the sea was continuous on the western side, so that before the close of Mississippian, the Ohio basin had become dry land and Alleghania had become once more a distinct ridge, dividing the basin longitudinally from New York to central Tennessee. But the withdrawal affected almost the whole of the Appalachian basin; and this withdrawal may have been caused by extensive deformation of the surface. While Alleghania was raised at the west, there was rejuvenation of Appalachia at the east. In the later Mississippian, the streams had reached base level along the borders, for only fine muds were carried into the basin; but the Pottsville opens with coarse deposits from Pennsylvania to Alabama.

The distribution and character of the Pottsville deposits' seem to place beyond doubt the assertion that at the beginning of the Pennsylvanian the whole basin, excepting at the southwest corner,

'D. White, "Deposition of the Appalachian Pottsville," Bull. Geol. Soc. Amer., Vol. 15, 1905, pp. 267-282; J. J. Stevenson, Carboniferous of the Appalachian Basin," ibid., Vol. 18, 1907, pp. 142–150.