are also reported to have shown signs of oil. A rumor was current that a drilled well 100 feet deep near the northwest corner of sec. 26, T. 4 N., R. 24 E., supplies water which has a very pronounced odor of petroleum, but apparently the report was not verified. Oil is also reported to have been seen on the water in secs. 13 and 14, T. 4 N., R. 20 E. Pockets of oil encountered in digging wells have been reported from several localities in the northern part of T. 2 N., Rs. 26 and 27 E. Evidently this oil must come from the Bearpaw shale.

EXISTENCE OF FAVORABLE STRUCTURE.

A careful examination of the structure of the rocks and its relation to concentrations of oil and gas in many parts of the world has given rise to the structural or anticlinal theory. The conditions that control the accumulation of oil and gas, according to this theory, are briefly as follows:

(a) A reservoir rock. This is commonly known as an oil sand, although it may be a very sandy shale, a fractured rock of some kind, a loose conglomerate sufficiently porous to allow the accumulation of oil or gas, or a limestone composed largely of interlocking crystals of calcite.

(b) An impervious cap rock to seal over the reservoir rock and prevent the upward escape of the oil and gas.

(c) Folds in the rock favoring the accumulation of oil and gas in certain localities, these substances migrating from more extensive areas of adjoining beds less favorably situated for their retention.

(d) Saturation of the rocks by ground water, on which the oil and gas will move on account of their lower specific gravity and be forced into the upper parts of the folds. According to the anticlinal theory, if a porous rock containing gas, oil, and water is folded between other rocks which are nonporous, these substances, under the influence of gravity, separate and arrange themselves according to density. Gas, being the lightest, rises to the crest of an anticline, the oil separates out below, and the water seeks the deeper portions of the beds. Detailed field observations have shown not only that many of the concentrations of oil and gas are intimately related to anticlines and domes, but also that gas, oil, and water are related in the manner indicated. Although the recognition of these facts has caused most geologists to accept the anticlinal theory in its broader aspects, many geologists are willing to accept it only in a modified sense, as recent study has shown that accumulations of oil and gas occur not only in the crowns of the arches but also in many places on the flanks of the folds where the dips are interrupted for some distance, the interruptions forming structural terraces. Recent studies indicate also that the conditions of accumulation are entirely

different in saturated and unsaturated rocks-that in thoroughly saturated rocks the oil and gas are borne upward on the sheet of underground water and are caught in the crowns of the arches, whereas in dry rocks the principal point of accumulation of oil is in the bottoms of the synclines or at any point where the slope of the rock is not sufficient to overcome friction.

The close relation which has been observed between anticlinal structure and accumulations of oil and gas suggests that two folds in the Lake Basin field should be carefully tested with the drill. These are the Broadview dome and the Big Coulee-Hailstone dome. The latter seems by far the more promising, for the reason that the Broadview dome is considerably lower than the crest of the Big Coulee-Hailstone dome and is virtually a part of it, being separated from it only by a small syncline. Even if sands are present beneath the Broadview dome and oil and gas have been formed in the shale of the surrounding region, the collecting area of this dome is very small compared with that of the Big Coulee-Hailstone dome. In the detailed description of the structure of the Hailstone dome (p. 135) attention is called to the fact that beyond the upper flat portion of the Big Coulee-Hailstone dome the beds are inclined toward the east, northeast, north, and northwest at the average rate of about 175 feet per mile (equivalent to a dip of 2°) for a distance ranging from 5 to 20 miles, and that toward the north and northwest the 2° dip continues from 15 to 20 miles and is finally terminated by the steeply upturned beds along the southwest side of the Woman's Pocket anticline and the southeast side of the Shawmut anticline. The real efficacy of an extensive collecting area depends on the presence and continuity of porous sands. Even if the conditions are favorable for the formation of oil and gas along the flanks of the dome, these substances can not eventually accumulate in the crest unless there are more or less continuous porous sands present through which the oil and gas can migrate. It is highly probable that if oil or gas has accumulated in the crest of either of these domes it is sealed in by the upper portion of the Colorado shale, for any sands that may be present in the lower part of the Colorado do not reach the surface in either dome. There seems to be no evidence of oil or gas having escaped along any of the fault planes, but that fact does not necessarily prove that no oil or gas has accumulated in the lower sands of the Colorado shale, for it is difficult to ascertain to what extent the lower sands have been faulted. It is the writer's belief that in all except a few of the larger faults the forces which effected a displacement of the harder sandstones resulted merely in distortion of the great body of soft shale. It is somewhat doubtful to what extent even the larger fault fissures remain open in the shale for the free transmission of fluids. The faults that cut the Eagle sandstone in

the Broadview dome exhibit small displacement and probably do not extend far into the Colorado shale.

Considering the size of the Broadview dome, its relation to the Big Coulee-Hailstone dome, and the location and depth of the test well in the SE. sec. 13, on the Broadview dome, the writer would hesitate to recommend further drilling of the smaller dome until the Big Coulee-Hailstone dome had been thoroughly tested. In view of the size of the Big Coulee-Hailstone dome and the location of the 79 Oil Co.'s well (p. 144) and the well in sec. 17, T. 3 N., R. 21 E., in relation to the crest of the dome, together with the fact that many dry holes. are found even in productive fields, it would seem that further drilling of the Big Coulee-Hailstone dome is justified. If that dome proves barren the possibilities of oil and gas in commercial quantities in the Lake Basin field are believed to be very slight.

OIL AND GAS GEOLOGY OF THE BIRCH CREEK-SUN RIVER AREA, NORTHWESTERN MONTANA.

The area described in this paper lies adjacent to the front range of the Rocky Mountains in northwestern Montana and is part of a large region in the northern Great Plains of the United States and Canada which seems to deserve consideration as prospective territory for oil and gas development. The thick bodies of Upper Cretaceous shale, the sands of which have yielded the greater part of the oil produced in Wyoming and Colorado and large quantities of gas in Alberta, underlie much of this region, in many places at depths that can be reached by the drill. The outcrops of these Cretaceous formations afford unquestionable evidence of being petroliferous, so that drilling in places of favorable structure, such as on well-developed domes or anticlines, seems to offer a reasonable chance of success. It is certain that the plains of northwestern Montana can not be classed as barren of oil until a considerable number of the favorable localities have been carefully drilled. Random drilling regardless of the lay of the rocks, such as has been undertaken up to the present time, has been uniformly unsuccessful. Moreover, even though the region as a whole should eventually prove to contain fair amounts of oil, the area actually underlain by commercial pools might comprise much less than 1 per cent of the total area underlain by the Cretaceous formations, so that the chances of success by random locations are very slight.

Predictions of oil pools from a formation some hundreds of miles beyond localities where it has already proved productive, on the basis of identical geologic age and kind of rock and undoubted indications that it is petrolíferous, have often proved unwarranted in the Rocky Mountain region, especially in parts. of southern Wyoming and western Colorado, and also in other parts of the United States. Nevertheless under these conditions, if favorable anticlines or other structural features are present, the geologist seems forced into an attitude of persistent optimism until fair tests with the drill have been made on a reasonable number of the more pronounced folds. He can arrive at the general decision that a given area does or does not offer possibilities for production in certain anticlines or structural features, but a further refinement of his conclusions, based on a critical consideration of the specific