that direction. As is shown by the structure contours, the top of the dome is considerably lower than that of the Big Coulee-Hailstone dome.

FAULTS.

The most striking feature of the structure of the Lake Basin field. is the long, narrow belt of shearing that crosses the field from the northwest corner southeastward about 8 miles north of Billings. The most intense shearing occurred along the steeply dipping south flank of the Big Coulee-Hailstone dome and around the southeast side of the Broadview dome. From the Broadview dome to Acton very few faults were observed, but from Acton to Rattlesnake Butte the stresses produced by the forces of deformation again appear to have been relieved by the development of a considerable number of parallel faults. In order to facilitate description the principal faults are indicated on Plate XVIII by letters (A-A', B-B', etc.).

FAULTING FROM THE VICINITY OF GIBSON TO HAILSTONE BASIN.

Extending from Gibson eastward along Six Shooter Creek for several miles and thence southeastward toward Battle Butte is an area where the strata have been slightly uplifted, as is indicated by the exposures of the Judith River formation. Between this minor. uplift and the Big Coulee-Hailstone dome is a small syncline, whose axis is shown by the patches of the Lance formation, which has been folded down upon the underlying beds. The torsional stresses affecting the rocks of this field were apparently relieved by the development of a great number of shear planes. These shear planes cut not only the rocks involved in the synclinal structure but also those forming the steeply dipping south flank of the Big CouleeHailstone dome. Between Hailstone Basin and the west edge of the field the principal movement has taken place along faults A-A', C-C', E-E', and F-F', but interspersed among these are a large number of smaller faults. In most of the faults the formations on the southeast side of the fault plane have been dropped with reference to those on the northwest side. Along the principal faults the deformation has resulted in a combined downward and northeastward movement of the beds on the southeast side. Where the syncline was faulted and the principal movement was the dropping of the beds on the southeast side of the fault plane, the Lance area is much wider on the southeast side of the fault but practically continuous with the area on the opposite side. Where the downward movement was the principal one there is obviously very little horizontal offset in the axis of the syncline. Furthermore, where the principal movement was downward, the beds on the southeast side of the fault are dropped with reference to those on the opposite side through practically the entire length of the fault, as along fault C-C', where the synclina

axis appears to have been moved northeastward but very little and at the extreme southwest end of the fault the beds on the northwest side have been dropped only about 20 feet.

Along most of the principal faults, however, the conditions are very different. For example, along faults A-A' and F-F' the downthrow is to the southeast along the northeast end and to the northwest along the southwest end. These conditions suggest either a scissors movement or a dominant movement more nearly horizontal than the dip of the beds on the southwest limb of the syncline. In the block between faults E-E' and F-F' the beds appear to have been twisted considerably, but the principal axis of the syncline in all probability passes from a point near the northwest corner of sec. 9 to the southeast corner of sec. 11. In the fault block immediately to the south the synclinal axis almost coincides with the center line of the area of Lennep sandstone in sec. 18. Northeast of this axis the beds are downthrown on the southeast side of the fault F-F', and toward the west they are dropped on the north side. This fact, together with the relation of the synclinal axes, indicates that the beds southeast of the fault F-F' were not only dropped considerably but were carried eastward with reference to those on the opposite side of the fault plane. The block between faults E-E' and F-F' was probably thrust eastward somewhat with reference to the beds northwest of the fault plane E-E', but it seems probable that the main direction of movement was along a line slightly steeper than the angle of dip of the south limb of the syncline, thereby producing but a small offset in the Lennep ridge at the southwest end of the fault, a small offset in the synclinal axis, and a large offset in the beds along the northeast end of the fault.

Very little can be said with absolute certainty regarding the inclination of the fault planes. Twenty-five observations of the strike and dip of well-defined shearing planes west of Hailstone Basin were recorded. The directions of these planes ranged from N. 12° E. to east, and 14 of them were found to trend between N. 35° E. and N. 65° E. Eight of the planes dip toward the northwest, and seventeen toward the southeast. The angles of dip ranged from 10° to 80°, and half of the planes were found to be inclined between 30° and 60°. The field notes also record considerable evidence of shearing along bedding planes and planes inclined to the bedding at a very small angle.

FAULTING FROM THE WEST SIDE OF HAILSTONE BASIN TO THE EAST SIDE OF BROADVIEW DOME.

The steeply dipping south flank of the main fold from the west side of Hailstone Basin to the east side of the Broadview dome has been cut by a large number of faults along some of which the dis

21544°-19-Bull. 691-10

placement is considerable, as is shown by the structure contours. Most of the faults, however, are of small extent horizontally and do not represent much displacement. The principal movement has taken place along faults F-F', G-G', K-K', and P-P'. On the map the unbroken lines represent the faults so far as it was possible to trace them with certainty, and the broken lines represent the portions that were merely inferred. For most of the small faults the unbroken line represents the extent to which the beds were found to be offset. The large fault G-G' may or may not be continuous with the small one shown in the bed of Six Shooter Creek on the west line of sec. 30, T. 3 N., R. 20 E. If it is, the downthrow is on the opposite side of the fault plane, for at the township line the fault plane is inclined about 45° SE., and the beds on the southeast side of the fault are dropped 30 feet. Between faults G-G' and K-K' is a portion of the rim of Hailstone Basin which is more or less of a unit in itself. It is uplifted with reference to the beds on the southeast and the northwest. It has been cut by a series of small faults, the beds along the west sides of which have been dropped with reference to those on the east sides. In the north central part of sec. 15 the Eagle sandstone ends abruptly, and the valley to the east is apparently occupied by the Claggett shale. There is a line of displacement along the small fault cutting through the Claggett shale area, and the sandstone immediately west of it appears to be continuous for more than half a mile. The Eagle sandstone is cut off abruptly by fault J-J', the plane of which is inclined 45° NW. From the fault plane the sandstones of the Eagle circle around toward the north, as indicated on the map, and dip west at an angle of about 8°. One of these prominent Eagle sandstones has been cut by numerous small parallel faults, one of which, in the northwest corner of sec. 14, is represented on Plate XVI. As a rule the beds on the north side of the fault have been dropped from 6 to 8 feet and offset toward the east. At one of the northernmost faults the sandstone is completely sliced by shearing planes and the beds on the north side of the line of disturbance are dropped from 50 to 75 feet and offset toward the east about 300 feet. Plate XXI, A, shows a view taken from the point on the Eagle sandstone in the northeast corner of sec. 9, T. 3 N., R. 20 E., looking southeast. The view shows the ridges formed by the lowest-ledge of the Eagle sandstone and the upper ledge of sandstone in the Colorado shale as they are offset along the different fault planes. Plate XXI, B, gives a view taken from the Eagle sandstone on the west side of fault I-, looking south at the point on the north slope where the fault cuts through into Hailstone Basin. The fault plane, which is inclined about 25° NW., passes up the little gully shown in the center background of the picture. On the left is the massive Eagle sand