From west end of Poteau Mountain, Indian Territory, 24 miles northwest of Coaldale, Ark.

Lophophyllum profundum.

Crinoidal fragments.

Stenopora? sp.

Fistulipora sp.

Rhombopora lepidodendroides.
Septopora biserialis.

Lingulidiscina aff. L. convexa.
Derbya crassa.

Productus cora.

Productus cf. P. nebraskensis.
Marginifera ingrata.

Spirifer rockymontanus.
Spirifer cameratus.

Spiriferina kentuckyensis.

Composita subtilita.

Hustedia mormoni.

Pugnax osagensis.

Dielasma bovidens?.

Aviculipecten aff. A. coxanus.

Aviculipecten germanus.

Lima retifera?.

Pteria ohioensis?.

Pseudomonotis ? sp.

Leda bellistriata.

Myalina aff. M. perniformis.

Solenomya sp.

Schizodus affinis?.

Allerisma terminale?.
Parallelodon tenuistriatus.
Cypricardinia carbonaria.

Astartella concentrica?.

Astartella varica.

Edmondia mortonana?.

Pleurophorus immaturus?.

Pleurophorus tropidophorus.

Euconospira n. sp.

Pleurotomaria aff. P. tenuicincta.

Pleurotomaria aff. P. subconstricta.

Pleurotomaria aff. P. giffordi.

Patellostium marcouanum.

Bellerophon harrodi?.

Aclis stevensana.

Sphærodoma sp.

Orthoceras cribrosum?.

Nautilus sp.

Phillipsia cliftonensis?.

From west end of Poteau Mountain, Indian Territory, 24 miles northwest of Coaldale, Ark., at point 150 feet above shale fossils.

Deltopecten occidentalis?.

Myalina perattenuata.

Myalina aviculoides.

Aviculipinna americana.

Allerisma terminale.
Schizodus compressus.
Pleurophorus sp.

All these fossils are of Pennsylvanian age, and all show closely related faunal facies. I regard them as rather low in the Pennsylvanian. Very little discrimination can be made between these collections on the strength of their invertebrate faunas. Those from the black shale associated with coal beds have to a certain extent an individual aspect, owing to their limited variety, a fact which I ascribe to impure or brackish water conditions, but even these are related to the more varied and more obviously marine assemblages of species.

The lot collected near Sixmile post-office, in sec. 25, T. 8 N., R. 28 W., which represents, I believe, the highest horizon in the collection from Arkansas, has to a limited extent peculiarities that differentiate it from some of the other collections coming from lower beds. One fact in point is the relatively restricted representation of brachiopods, which group is more abundant in some of the other lots. The pelecypods are largely the same. It is very doubtful, however, whether this difference (the brachiopod representation) should be given any stratigraphic significance. It may be local or accidental. Other

collections from the same horizon at somewhat separated points would be necessary to determine this fact.

Lithologically the collections from shale on the west end of Poteau Mountain in Indian Territory are distinct from the others; and though a good many species are common to both series of collections, I believe the Poteau Mountain material to represent a younger horizon. This is perhaps especially true of the collection obtained from 150 feet above the shale fauna. The latter can possibly be correlated with the Flint Ridge horizon of Ohio.

STRUCTURE.

GENERAL FEATURES.

The coal field of Arkansas lies in a long, comparatively narrow synclinal trough extending eastward along Arkansas River valley from the Indian Territory line. North and south of the field the coalbearing strata have been eroded, leaving only barren rocks of the Atoka and Winslow formations exposed. This trough lies between the Boston Mountains on the north and the Ouachita Mountains on the south, two elevated land masses which have very different geologic structures. The Ouachita region is characterized by long, sharp ridges formed by the erosion of alternating hard and soft beds dipping at high angles. They are in fact strike ridges of upturned rocks. The various strata comprising this mass have been subjected to violent strains and compressed in a north-south direction, throwing them into folds whose axes extend approximately east and west. Subsequent to this crumpling the upper parts of the folds have been eroded, leaving the edges of the harder beds projecting in ridges as we now find them. In the process of folding the strata were here and there broken and in many places the beds slipped out of position so that their outcrops are not now continuous for very great distances. Such fractures are called faults by geologists. As a general rule, the faults of the Ouachita Mountain province are of the kind that is produced by compression. In such faults the fracture plane is, as a rule, inclined, and the beds on the upper side have slipped over those on the lower side-in fact, are thrust over them-and for this reason such displacements are called thrust faults. In general the faults of the Ouachita Mountain region extend approximately east and west parallel with the trend of the ridges."

b

The rocks of the Boston Mountain province are not so intensely folded as those of the Ouachita province, but are, as a rule, only slightly distorted from their original horizontal positions. They are

a For a description of the geologic structure of the Ouachita Mountains, see Taff, Joseph A., Atoka folio (No. 79), pp. 6 and 7; Coalgate folio (No. 74), p. 5; and manuscripts of McAlester, Windingstair, and Tuskahoma folios, not yet published.

Purdue, A. H., Winslow folio. (In preparation.)

characterized by low dips, seldom exceeding 4 or 5°. This mass appears to have been raised up like a great block of the earth's crust, allowing the lowlands of Arkansas Valley to break away from it. Where one part of the earth's crust is raised up higher than an adjoining part, the beds at the two levels must, either bend or break. A flexure produced in this way is described as a monocline or monoclinal fold. Fractures or faults thus produced differ in type from those found in the Ouachita Mountains in that they occur in beds not crumpled by lateral pressure and otherwise little disturbed from their original positions. Usually in such faults the plane of fracture does not stand exactly vertical, and the beds on the upper or hanging-wall side slip down, while those on the lower or foot-wall side rise. Faults of this kind are described as normal, drop, or tension faults. They are characteristic of the geologic structure of the Boston Mountains, and a series of them, extending approximately east and west, occurs along the north side of Arkansas Valley, and in many places separates the Boston Mountain province from the coal field.

FIG. 8.-Map showing structural features of Arkansas coal field.

It will be seen from this description that the Arkansas Valley syncline, which contains the coal field, is peculiar in that on its south side the underlying rocks rise by pronounced folds and thrust faults, while on the north side the underlying rocks are brought to the surface by a simple monoclinal uplift, accompanied at some places by normal faulting.

The structures within the coal field partake of the nature of both these types, since the rocks are definitely folded at many places along the southern edge of the field, while near the northern edge the dips are gentle, the beds lie nearly flat, and normal faults are of frequent occurrence. The principal structural features of the field are shown in fig. 8.

ANTICLINES.

Although the coal field in general has been described as a synclinal trough, it is folded into a number of minor synclines and anticlines. The determination of these features is of great importance to the miner and coal prospector, since without a knowledge of this structure it is impossible to determine the best methods of prospecting and working the coal. Generally the largest areas of coal land are in the synclines, and where the structure is of this type the bed is at its greatest depth in the axis of the fold. The coal bed is most easily prospected around the margins of such basins, but in mining it is usually most economical to locate the main shaft at the deepest place, so that the loaded cars can be moved to it by gravity. Where the coal beds lie deep, however, the value to the prospector of recognizing the anticlinal structure is of almost equal importance, for it is in the anticlines that he must expect to find the coal for which he is searching nearest to the surface. In general the minor anticlines and synclines of the Arkansas coal field have approximately east and west axes only a few miles in length.

Backbone anticline.-Probably the best known anticline in the whole district is that in Backbone Ridge, between Bonanza and Hackett, which extends from the State line eastward for about 7 miles. The coal-bearing beds that outcrop on the north and south sides of this ridge are tilted at high angles, and in its axis rocks lying far below the coal horizons are exposed. (See fig. 9.) This anticline is an unsymmetrical fold resembling the Ouachita Mountain type, and represents a thrust of the rocks from the south over those on the north side of the ridge. The strata are broken along its axis and a well-marked thrust fault, which at the Indian Territory boundary has a displacement of over 2,000 feet, has been traced nearly to Greenwood, 7 miles to the east. The area of marked folding also

ends here.

Biswell Hill anticline.-Biswell Hill, an elliptical domelike eminence northeast of Greenwood, which is mantled over by the Hartshorne sandstone, is the topographic expression of a broad anticline whose axis is parallel with and a short distance northeast of the east end of the Backbone Ridge anticline. The fold is slightly unsymmetrical in that the dips to the north are higher than those to the south. Like the Backbone Ridge anticline, it marks a barren

area surrounded by the outcrops of the Hartshorne coal, which dips away from it to the north and south. The structure, however, is apparently favorable for the accummulation of natural gas.

Massard Prairie anticline.-Another anticline, which is of considerable economic importance, since it has been successfully exploited for natural gas, is that at Massard Prairie, 5 miles southeast of Fort Smith. The surface indications here suggest a broad elliptical uplift with low dips to both the north and the south. The many drill holes in the axis of this fold indicate a barren area surrounded by the outcrop of the Hartshorne coal.

Montreal anticline.-Another anticline of some importance in coal mining is one which extends west from Montreal, in the southern part of Sebastian County, to the State line. This is a very broad fold, but it serves to bring the Hartshorne coal horizon nearer the surface than it is in the region north and south of it.

Hartford anticline.-The Hartford anticline, a broad fold whose axis extends for several miles in a northeast-southwest direction, is of great economic importance, since it brings the coal horizon within easy reach from the surface. At Coops Prairie, northeast of Hartford, this fold assumes a domelike structure almost ideal for the accumulation of natural gas. The wells located here supply the gas used at Huntington and at the Mansfield brick factories.

Mill Creek Ridge anticline.-East of Greenwood the coal field narrows down to a width not exceeding 15 miles, and in this strip there is at many places only one simple syncline from the south side of the coal field to the north. It comprises, however, a few minor anticlines which are worthy of mention. One of these is the Mill Creek Ridge, which extends from Arbuckle Island, in Sebastian County, to Roseville, in Franklin County. This ridge belongs rather to the northern edge of the coal field, from which it is not wholly disconnected. It is a broad, open fold characterized by low-dipping rocks. Since the rock which mantles this fold is for the most part Hartshorne sandstone, it can not be regarded as coal land, though its southern boundary marks the edge of the coal fields.

Game Hill anticline. There is also a small, short, elliptical anticline in Logan County, extending westward from Caulksville to Branch, which is worthy of examination, since it brings the Hartshorne horizon within a few hundred feet of the surface. There is also some evidence here of a fault, which will be described later.

Prairie View anticline.-The Prairie View anticline, about 10 miles east of Paris, appears to be a broad, open fold, but of sufficient magnitude to bring up the Hartshorne sandstone and Atoka shale, leaving a barren area in the center. The folding here is rather pronounced, and there is an east and west fault along the axis. It is not known whether it is of the thrust or normal type, though its