continuity along the north side of the field is broken at a number of places and its recognition here depended on stratigraphic evidences other than continuous exposures. Nevertheless, the formation has been identified with a considerable degree of certainty in several disconnected areas within the field, along its northern border, and its character over the whole field is fairly well known. The McAlester shale formation extends from Indian Territory into Arkansas, where it comprises the whole of the coal-bearing series. Some of the lower beds of the Savanna formation are probably also present in Arkansas, though it has not been possible to identify them with certainty at any point. The McAlester formation as represented in Arkansas contains a group of sandstone beds near its middle portion. These were traced over a large part of the field and their stratigraphic position determined. On the geologic map herewith presented this group of sandstones is differentiated from the more shaly parts of the McAlester formation, subdividing it into three members called Spadra shale, Fort Smith formation, and Paris shale, which can be recognized throughout the field. The economic value of such a subdivision will not fail to be appreciated, since the boundaries of the formations mapped will serve as contour lines indicating approximately the depth of the Hartshorne coal horizon. The rocks that underlie the Hartshorne formation in Arkansas have received little attention. They comprise a succession of shale and sandstone known to exceed 5,000 feet in thickness near the southern edge, but they are probably not more than 2,000 feet thick near the northern boundary of the field. In the southern part this group consists mainly of shale, but along the northern boundary the sandstone beds are much more important, making up possibly one-third of the section. The subdivisions of the coal-bearing rocks of Arkansas, as differentiated and described in this report, are briefly tabulated as follows: The general relations and lithologic features of these subdivisions are set forth in Pl. I. ATOKA FORMATION. The rocks belonging to this formation outcrop in the surrounding regions both north and south of the coal field and also occur in a number of limited areas included in it, the best known of which is that in the Backbone Ridge between Hackett and Bonanza. They have also been penetrated at a number of places by deep drill holes through the overlying rocks. South of the coal field the rocks of the Atoka formation are tilted at high angles, and extensive sections of them can be measured, but on the north their attitude is more commonly horizontal and only the upper members are exposed. In the latter region these rocks correspond in stratigraphic position. with the Winslow formation as mapped by Purdue in the Winslow quadrangle. In general the Atoka formation consists of shales with occasional sandstone beds, which in the northern part make up possibly as much as one-third of the section, while farther south the proportion of sandstone is very much less. The thickness of these rocks north FIG. 3.-Graphic sections of the upper part of the Atoka formation. A, north of Van Buren; B, Massard Prairie; C, Jenny Lind; D, south of Huntington; E, Denning; F, Dardanelle. of the coal field has been estimated to be approximately 1,500 feet, but near Mansfield, in the southern part, over 3,000 feet are exposed and 2,000 more have been pierced by deep drilling. The shales vary from fine-grained, black or blue clay stones to gray argillaceous sandstones. Generally the more sandy beds are harder, and where they outcrop form ridges and other prominent topographic features. Although the rocks of this formation have not been investigated in close detail, the observations made and the records of deep wells drilled through them show that the sandstone members are lenticular bodies, having no general distribution over the field. In many places where they have been examined the sandstones are very irregularly bedded, and in some places finger out into shale beds. A characteristic exposure of one of these beds is shown in Pl. IV, A. The variation in the succession of sandstones and shales making up the Atoka formation is graphically shown in the sections presented in fig. 3. Lentils of coal, most of which are too thin to be of value, occur at rather infrequent intervals. Such coals seem to be more common in the eastern part of the field than in the western, a condition which may be partly due to the better exposures in the eastern portion. Some of these beds have been mined locally, and attempts to exploit them at various places are still in progress, though nearly all the old openings are deserted. Though fossil plants have been found associated with the coal beds at several points, no satisfactory collections of them have been made. Several collections of marine invertebrates, which are described under the heading "Paleontology" (pp. 31-35), were obtained from some of the upper beds near Van Buren. These rocks have been prospected successfully at a number of places in the search for oil and gas. The natural gas used at Fort Smith is obtained from sandstone lenses 1,000 to 2,000 feet below the top of the formation, while that at Mansfield and Huntington comes from sands 4,000 feet stratigraphically below the top of the formation. HARTSHORNE SANDSTONE. The Hartshorne sandstone is economically the most important formation, outside of the coal beds themselves, to be described in the district, since its outcrop indicates the position of the most important coal horizon. It had been identified at Bonanza and Hackett, on the Arkansas-Indian Territory boundary, previous to the beginning of this investigation. From these points its outcrop has been traced eastward around the end of the Backbone Ridge anticline, and its identity with the sandstone of Devils Backbone Ridge, which underlies the coal bed at Montreal and Burma, is established by drill holes at short intervals across the Greenwood syncline. From the latter localities it has been traced by continuous exposure eastward through the whole length of the coal field. The outcrop of this sandstone marks the northern boundary of the coal field through most of its length, and the rock also occurs in a number of isolated areas within the coal district, as is shown on the accompanying geologic map. This formation is essentially sandstone, but does not differ materially from many of the heavier sandstone beds contained in the underlying rocks, except for its more general distribution over the field. Usually it can be identified by the presence of one or more coal beds in the rocks immediately above it. In its typical exposures it consists of massive, irregularly bedded rock, at most places more than 100 feet thick, and where the dips are moderately high it almost |