about 15 feet to the mile, and a few miles north of Bison N. 60° E. about 7 feet to the mile. Near Little Missouri River west of Cave Hills and Slim Buttes there are low anticlines, one of them a northern extension of the Glendive anticline of Montana. A second arch a short distance to the west brings up the Pierre shale in a small area south of the west Short Pine Hills, 15 miles south-southeast of Camp Crook. Very little faulting has occurred in this area, and the faults found had displacement of only 3 feet.

FIGURE 11.-Diagram showing structure under parts of Harding and Perkins counties, S. Dak. (After Winchester and others.) The contour lines are drawn at 100-foot intervals and show the approximate position of the top of the Dakota sandstone below sea level.

COLORADO.

NORTHEASTERN COLORADO.

The main structural feature in northeastern Colorado is the wide, deep basin whose lowest portion is a few miles northwest of Denver. Here the Dakota sandstone is somewhat below sea level, or more than 5,000 feet below the surface. Exposures of the Cretaceous rocks that occupy the basin are not very satisfactory, but apparently the beds have a low but continuous slope to the west or northwest. The steepest grade is probably in Kit Carson County and the northern part of Lincoln County, where the dip averages 40 to 50 feet to the mile, or about half a degree. Farther north the rate is much less, and possibly there is local doming. Near the Rocky Mountain front the strata are upturned steeply, but the monocline is traversed diagonally by anticlines and synclines of northwesterly trend, pitching down to the southeast. Some of these in the Boulder region have been de

scribed by Fenneman1 Oil occurs in sandy members of the Pierre shale in this area, as in the Florence Basin, and it is not unlikely that similar conditions may be found farther north and east in northeastern Colorado.

Many deep holes have been sunk at Denver and in its vicinity for artesian water, which is obtained from the Arapahoe and Denver formations. Several deep holes have been bored in the vicinity of Boulder for oil, some of them reaching the Dakota sandstone. A 1,700-foot hole at Castle Rock, a 1,440-foot hole at Sedalia, a 2,465foot hole at Loveland, a 1,225-foot hole at Stout, a 1,155-foot hole at Akin, and a 2,400-foot hole at Otis all failed to reach the Dakota sandstone and therefore did not fully test the underground resources. Several artesian wells at Greeley and vicinity, from 1,165 to 2,260 feet deep, draw water from the Dakota or overlying sandstones.

East of Fort Collins there is a low anticline which extends for some distance north and south, parallel to the Rocky Mountain front. According to W. T. Lee, it is defined mainly by a group of sandstones in the Pierre shale, one member of which farther south is known as the Hygiene sandstone. The strata are arched up several hundred feet, presenting a condition favorable for oil or gas accumulation if these materials are present in the area. A boring was made 3,900 feet deep through 2,500 feet of alternating sandstone and shale of the Pierre formation and ended in shale believed to represent the shaly division (Apishapa shale) of the Niobrara. Doubtless this shale was near the base of that division, and the boring would have reached sandstone in the upper part of the Benton within the next 500 feet and the Dakota and underlying sandstones 1,500 feet or more farther down.

SOUTHEASTERN COLORADO.

The structure of southeastern Colorado is complex compared with that of other portions of the central Great Plains. The most striking features are a high dome centering on the Colorado-New Mexico State line in southeastern Las Animas County; a deep basin with its axis a few miles west of Trinidad; an arch passing west of Pueblo and rising into the front range of the Rocky Mountains to the north; a deep basin at Florence; a deep basin with bottom a few miles northwest of Denver; the steep eastward-dipping monocline along the foot of the mountains, interrupted by irregularities in northern Huerfano and western Pueblo counties; the dome and anticline in the northeast corner of Huerfano County; and a sharp local uplift due to igneous intrusion at Two Buttes.

1 Fenneman, N. M., Geology of the Boulder district, Colo.: U. S. Geol. Survey Bull. 265, 101 pp., map, pls., 1905.

FIGURE 12.-Section along Arkansas Valley from Pueblo to Canon City, Colo.

The relations of these features are shown by the contours of Plate I, and further details are given in the Pikes Peak, Colorado Springs, Pueblo, Nepesta, Walsenburg, Apishapa, Spanish Peaks, and El Moro folios,' and in my report on the Arkansas Valley."

Some general features are shown in the cross sections, figures 12, 13, and 14 and Plate IV, and details of structure in the Apishapa

FIGURE 14.-Diagram showing structure in the Apishapa quadrangle, Las Animas, Huerfano, Pueblo, and Otero counties, Colo. The lines show the configuration of the Dakota sandstone, with its altitude above sea level. (After Gilbert and Stose.)

quadrangle are shown in figure 14, reduced from the structure-section sheet in the Apishapa folio. The succession and character of the formations are shown in the table on page 22.

1U. S. Geol. Survey Geol. Atlas, Folios 7, 203, 36, 135, 68, 186, 71, and 58. Darton, N. H., Geology and underground waters of the Arkansas Valley in eastern Colorado U. S. Geol. Survey Prof. Paper 52, 90 pp., 28 pls., 1906.

Geologic formations of southeastern Colorado, south of Denver Basin.

W. T. Lee has found that on the west slope of the great dome on the Colorado-New Mexico State line there is a small dike of cellular basalt which carries considerable petroleum. It crops out about 2 miles west of the Mexican village of Trinchera and doubtless derives the oil from some deep-seated sandstone. A hole said to be about 1,000 feet deep was sunk near this dike to the Dakota sandstone, but water came into the hole and boring was abandoned.

An old bore hole southwest of Barela, of unknown depth, yields considerable gas, which is piped to a near-by ranch for use.

Although numerous deep borings have been made in southeastern Colorado, most of them have penetrated no farther than the Dakota sandstone and have not tested the underlying beds. A few of the deeper holes, however, have been extended into the underlying red