about 800 feet deep were made at Buffalo Gap; the one in town stopped in shale; the other, near the foot of the hills, reached the Dakota sandstone.

CENTRAL SOUTH DAKOTA. ·

Central South Dakota is underlain by a thick body of Pierre shale, below which is

the usual succession of beds of Niobrara and Benton age, Dakota and associated sandstones, and possibly the attenuated eastern margins of some of the lower sedimentary rocks exposed in the Black Hills. To the west there may be 3,000 to 4,000 feet of strata; to the east near Missouri River the depth to granite or quartzite is between 1,200 and 1,500 feet. In the eastern area the prospects for valuable quantities of oil or gas are not favorable. Figure 10 shows the general relations.

The strata lie very nearly level under a wide area in the great syncline between the Black Hills uplift and the Sioux quartzite ridge. Apparently, as shown by contour lines in Plate I, the general dip is to the north, toward the deep basin in Perkins County. Doubtless there are many subordinate undulations of the strata with domes in which gas or even oil might be found. Small volumes of gas occur mixed with artesian water at Pierre and under a district extending a few miles north of that place. Borings for artesian water have been made at intervals along the Chicago & Northwestern Railway and the Chicago, Milwaukee & St. Paul Railway extensions to Rapid City, but they have continued only to the Dakota sandstone, as has also a 2,600-foot boring made several years ago 20 miles northeast of the Rosebud Agency. Data regarding these borings are given in the table on page 16.

FIGURE 10.-Section across central South Dakota from the Sioux quartzite region to the Black Hills.

Deep borings in South Dakota west of Missouri River.

The underground geology of eastern South Dakota has been determined by more than a thousand borings for artesian wells. Many of these have reached granite or quartzite bedrock, which lies from 50 to 1,600 feet below the surface, and none of them have found oil. Gas, however, has been obtained in small amount, mingled with the artesian water at Pierre and in a district of modern extent north and northeast of that place. The strata in this region are shown in figure 8. They are probably too thin to present prospects for any commercial supplies of oil or gas. The structure of the Dakota and associated sandstones, as determined by wells, is shown by the contours of the map (Pl. I). North of latitude 44° 30' there is a general rise of the beds to the east; but the monocline bears a low dome, with its crest at Orient, and a shallow basin passes north and south through the center of Brown and Spink counties. Near latitude 44° the beds rise southward to a broad dome in which, however, the underlying floor of quartzite rises more steeply into an underground ridge. This ridge is not overlapped by the Dakota sandstone, but the beds of Benton and Niobrara age extend some distance up its slope. The quartzite is at or near the surface in a zone that passes through Alexandria and Sioux Falls.

Descriptions of the geology of eastern South Dakota are given in the Aberdeen-Rédfield, Alexandria, DeSmet, Elk Point, Huron, Mitchell, Olivet, and Parker folios,1 and a summary of the salient facts is given in my report on the geology and underground waters of South Dakota.2

NORTH-CENTRAL SOUTH DAKOTA.

The geology of the Cheyenne River and Standing Rock Indian. reservations has been studied by Calvert, Beekly, Barnett, and Pishel. These observers confirm the representation of structure shown on the map (Pl. I), stating that the beds of the Lance formation, Fox Hills sandstone, and Pierre shale are nearly horizontal, having a low dip to the northwest. They found several local faults which displace the beds a few feet, but in "some places the throw is as much as 100 feet.”

NORTHWESTERN SOUTH DAKOTA.

The strata on the northeast slope of the Black Hills uplift comprise a continuous succession from the Deadwood formation to the Lance formation and have a general dip to the east and northeast, into a deep basin whose axis is at longitude 102° 30′ on the North Dakota-South Dakota boundary line. In the eastern part of Meade County the dips swing around to the north and northwest on the Southeast slope of this basin. The structure of the part of the basin which contains lignite in Perkins and Harding counties has been worked out in detail by Winchester and his associates, and in figure 11 is given a reproduction of their structure map with the contours renumbered to show the relation of the top of the Dakota sandstone. The original map shows the structure of the lower part of the Lance formation, which is about 3,700 feet above the Dakota sandstone according to my determinations of thickness in regions to the north and far to the southwest. It is stated that the coal field is in a very shallow basin, which deepens to the northeast, and whose center lies near or northeast of Lemmon. Along the western margin the beds show the steepest dip, notably in an area 20 miles southeast of Buffalo, where the direction is east and the rate about 65 feet to the mile. At the south end of Slim Buttes the dip is north. and the rate about 25 feet to the mile. Near Meadow it is N. 10° W.

1 Todd, J. E., and Hall, C. E., U. S. Geol. Survey Geol. Atlas, folios 165, 100, 114, 156, 113, 99, 96, and 97.

2 Darton, N. H., U. S. Geol. Survey Water-Supply Paper 227, 156 pp., 15 pls., 1909. Calvert, W. R., Beekly, A. L., Barnett, V. H., and Pishel, M. A., Geology of the Standing Rock and Cheyenne River Indian reservations, North and South Dakota: U. S. Geol. Survey Bull. 575, 49 pp., 8 pls., 1914.

Winchester, D. E., Hares, C. J., Lloyd, E. R., and Parks, E. M., The lignite field of northwestern South Dakota: U. S. Geol. Survey Bull. 627, 169 pp., 11 pls., 1916.