and studied.

These sections have been examined by Albert Johannsen, of the Geological Survey, whose preliminary report is as follows:

Tentative name: Andesite porphyry with latite affinities.

Megascopic: A dark-gray porphyritic rock, weathering a dirty yellow. The black phenocrysts, up to one-eighth inch in diameter, become very pronounced on the weathered surface.

Microscopic: Porphyritic; about one-fifth phenocrysts. The groundmass has an intersertal texture in which the irregular areas are of a dirty-brown serpentine. The phenocrysts, which are chiefly augite, are generally in broad, lath-shaped sections. A few of the feldspar crystals of the groundmass are larger than the remainder and may be classed with the phenocrysts. The groundmass consists of dirty yellowish-brown serpentine, less augite, about the same amount of magnetite, much less orthoclase, and some pseudomorphs, now serpentine, which have the form of olivine. The feldspar consists of plagioclase and orthoclase; the plagioclase varies in composition from andesine to andesine labradorite. Apparently none is more basic than Abs, AnãoThe index of refraction is ±551.

METAMORPHIC ROCKS.

Very few metamorphic rocks are found in the plains portion of the Great Falls region, although in the mountainous districts surrounding the field the sedimentary rocks have been highly metamorphosed by intrusive dikes, sheets, and laccoliths. The Highwood Mountains bordering this field on the north have been caused by igneous intrusion in Cretaceous rocks, which were metamorphosed to such an extent that they have resisted subsequent erosion and now stand out from the surrounding plains as a cluster of high peaks. Intrusions in the form of stocks and laccoliths are more or less common along the base of the adjoining mountain ranges. From some of these intruded rock masses the overlying sediments have been removed, exposing a central core of igneous rock, around which contact-metamorphic phenomena are well exhibited. A good illustration of these conditions is found on the east side of Little Otter Creek, about 3 miles south of Mann, outside of the area here discussed.

As previously stated, intrusive rock in this district is most commonly found in the form of dikes. In most localities these have metamorphosed the sediments into which they were intruded for some distance back from the contact, converting sandstone into quartzite and shale into slate. Phenomena of this character were observed at several places, notably on the north side of the Big Belt Mountains, about 7 miles southwest of Orr, in the vicinity of Rocky Ridge, and throughout that portion of the field which lies east of Stanford. No places were observed where the intrusives had cut the workable coals and thereby altered them by metamorphism along the contact. For this reason no special study was made of the character of the contact-metamorphic rocks of the field. It is highly probable that the intrusives which cut the sediments on the northeast side of Belt Butte have had some effect on the Kootenai coals of that

district, providing they extend so far east, but as there are no exposures of the coal beds the phenomena could not be observed. There are also, on the upper part of Hazlett Creek, dikes which in their northeast extension may cut and alter by metamorphism workable coals, but these dikes could not be traced on the surface into the coal area. The same is true of the dike forming Rocky Ridge, which extends southward from Highwood Mountains, but disappears at the northern edge of the Otter Creek coal area.

STRUCTURE.

PLAINS PROVINCE.

GENERAL CONDITIONS.

Throughout the plains portion of the region described the structure is relatively simple. The rocks as a rule lie nearly horizontal, dipping with a small angle to the north and east, away from the mountains, but in the mountainous portion the structure is more complex. Although low dips of 3° to 5° prevail throughout the plains province, and the district is one of little disturbance, the rocks on closer examination are found to be gently folded into a series of shallow synclines and low anticlines. This structural feature is scarcely perceptible to the casual observer, being revealed only by a careful examination of the beds exposed along the sides of the larger streams, such as Otter and Belt creeks and Smith River and its principal tributaries. The major axes of these folds appear to be roughly parallel to the Little Belt Mountains uplift, but the folds are only of slight magnitude and the individual warps are broad. The largest and most perceptible of the synclinal depressions crosses Otter Creek between the mouth of Williams Creek and the Nollar mine. Its effect is to carry the coal-bearing rocks of the Otter Creek area about 100 feet below Otter Creek valley for a distance of 3 or 4 miles.

The slight deformation of the coal-bearing rocks has had an important bearing on the development of the coal beds of this field. Wherever stream valleys cross the coal-bearing areas and cut sufficiently deep to expose the coal, they produce favorable conditions for mining. Owing to the horizontal position of the beds, entries can be driven for long distances nearly at right angles to the direction of the dip, which is in general to the north, without producing an appreciable lift in the haulage of the coal. The gentle northward dip of the coal-bearing rocks can be turned to advantage in mining by driving the main entry at an angle greater than 90° with the direction of the dip, thus causing the entry to extend up the dip sufficiently to produce natural drainage of the workings. Though in general the rocks lie nearly horizontal throughout the Great Falls field, there are minor undulations in the strata which are too local to be observed on the surface, but which

are shown in the maps of the mine workings. Some of these cause more or less difficulty in mine haulage, making it necessary to use special appliances to fit the topographic conditions in the mine.

DOMES.

Local doming of the strata, due to laccolithic intrusion of igneous rock, is more or less common along the north side of the Little Belt Mountains and in the vicinity of the Judith Mountains, farther east. Skull Butte, in the plains province at the east end of the district, is without doubt a domal uplift of this character. It is nearly circular in outline, its greatest diameter being about 14 miles, and its quaquaversal dips ranging from 20° to 30°. Erosion of its center has not advanced sufficiently to uncover igneous rocks. This uplift exposes the coal in the steeply dipping beds about its base.

In the vicinity of Stockett local uplift and erosion occurred prior to the deposition of Jurassic sediments, as is shown by the unconformable relations of Jurassic sandstone and Madison limestone. Exposures of the limestone occur in which the strata are tilted and eroded, with Jurassic sandstone deposited across the upturned ends. The strongest dip of the limestone beds seen was about 10°. The various formations in this vicinity are perceptibly thinner than elsewhere, a stratigraphic feature probably due to the presence of the dome during the deposition of these sediments. The unconformable relations of the Carboniferous and Jurassic formations, and the moderately steep dips of the latter as exhibited in Sand Coulee, about 2 miles east of Stockett, are shown in Pl. VI. This doming of the Carboniferous rocks in the Stockett region is probably not of wide extent, but its exact limits can not be ascertained owing to the lack of exposures. Its northsouth dimension, as shown by outcrops along Sand Coulee, is about 41⁄2 miles, but its east and west boundaries are not known. At Stockett and along Sand Coulee valley, owing to the thinning of the Jurassic and Lower Cretaceous formations, also to the absence of the Quadrant in the vicinity of the dome, the coal horizon occurs only about 150 feet above the Madison limestone, which is exposed in the bottom of the valley. This feature might be misleading to prospectors who are not familiar with the local conditions about Stockett, for in other parts of the Great Falls coal field, especially along Belt Creek and in the Otter and Sage Creek areas, the coal bed is separated from the Madison limestone by about 650 feet of rock.

At the head of Ming Coulee, where coal of workable thickness is exposed, the beds dip steeply to the northwest. These local dips are due to a large dome farther south, outside of the area treated in this report. The Quadrant formation is also absent, causing the coal bed to occur about 250 feet above the top of the Madison.

On Boston Coulee, about 2 miles west of Eden, a local dome of the strata exposes the coal-bearing bed along Boston Coulee for about 11 miles, and also to the southward up a small tributary of that coulee for an equal distance. This small uplift, which causes the coal outcrop to take a T-shaped form, is shown on the coal map (Pl. II).

FAULTS.

No large faults occur within the area here discussed, but minor faults are not uncommon, especially in the vicinity of Belt and Stockett. The throw of these faults ranges from 5 to 20 feet, and their presence is usually difficult to detect on the surface. They are generally first encountered by miners who are working the coal bed, and in some places their presence has caused considerable difficulty in mining operations. At Belt, on the west side of Belt Creek, such a fault extends nearly west for about 1 miles, displacing the coal bed a few feet and causing difficulty in operations along the north side of the underground workings of the Anaconda Copper Mining Company's mine. In Armington Coulee, about half a mile above the mouth, a sharp fold in the beds trends northward toward Belt Butte. The beds may possibly be more or less fractured along its axis, but exposures at this place were inadequate for positive determination of this point. Other small displacements have been reported from some of the smaller mines along the east side of Belt Creek in the vicinity of Armington and Belt, notably in the Richardson mine and to a less degree in the Smauch and Millard mines, but these faults appear not to cause any appreciable displacement of the sediments at the surface. On the north side of Stockett a small fault in the Madison limestone has a throw of about 15 feet, extending east and west. The Cottonwood Coal Company reports that a small north-south fault in the coal-bearing rocks was encountered in mining about three-fourths of a mile east of the town; but no evidence of this fault was observed on the surface. The minor faults throughout the Great Falls coal field are not shown on the geologic map. It is possible that many such small faults are scattered over the field and will be discovered on more extensive development of the coal deposits, but it is difficult if not impossible to locate them, owing to the fact that their throw is generally insufficient to be perceptible on the surface.

LITTLE BELT MOUNTAINS.

The general structure of the Little Belt Mountains, which border this area on the south, is that of an anticlinal uplift with sharply dipping sides and a flat summit. In the central portion of the range the stratified rocks lie nearly horizontal, but along the northern flank of

54937-Bull. 356-09- 4

the uplift, as found on the head of Geyser Creek, the limestone dips at an angle of 15° to 20° toward the lower plains country. As previously stated, the simple structure of the northern part of the uplift has been considerably modified by the intrusion of igneous rocks in the form of laccoliths, which have caused local doming of the strata in many places. Only one of these laccolithic domes lies within the area described, but there are others, such as those east of Kibby and in the head of Dry Wolf Creek, whose marginal structure extends into the district. In the vicinity of the larger intruded masses of igneous rock the dips are in many places steep and variable, but in that portion of the mountain front where local intrusions have not disturbed the strata, they dip away normally from the uplift at angles of 6° to 12°, lessening gradually toward the lower plains country.

HIGHWOOD MOUNTAINS.

The Highwood Mountains, which border on the north the east end of the area described in this report, are structurally of a different type from the Little Belt Range. They consist of a group of isolated peaks, which were formed by igneous intrusions in Cretaceous rocks that were horizontally bedded or slightly inclined toward the east. Subsequent to this intrusion stream erosion carved out this cluster of peaks from the surrounding plains.

ECONOMIC GEOLOGY.

GENERAL STATEMENT.

The mineral resources of the area treated in this report are somewhat varied, but the principal one at present is coal. Fire clay of a superior quality is found in beds of workable thickness along Belt Creek and its tributaries, and at many places throughout the district raw materials suitable for the manufacture of Portland cement can be obtained. Gypsum deposits occur at different horizons in the Quadrant formation near Riceville, and at Goodman this mineral has been mined in a small way for a number of years. Building stones of different varieties, also limestone, are common in many parts of the field. Sand and gravel can usually be obtained locally. Iron pyrite is mined as a by-product with the coal and shipped to Great Falls, where it is used in the process of pyritic smelting.

COAL.

GEOLOGIC OCCURRENCE.

Throughout the Great Falls coal field the coal occurs in the lower part of the Kootenai, or Lower Cretaceous rocks, mainly at a horizon about 60 feet above the base of the formation. Coal of workable thickness is not continuous, however, at this horizon, but varies locally.