SEDIMENTARY ROCKS 473 



buttes — laccoliths, sills, and dikes — are within the Colorado beds. The 

 main West Butte laccolith lies on lower Colorado strata and domes up 

 upper Colorado; Middle Butte is formed of interbedded sills and lower 

 Colorado shales; the outlying laccoliths of East Butte are between mem- 

 bers of the Colorado. Such gas and oil as have been revealed by the 

 drill are in the lower part of the Colorado formation. 



In the Sweet Grass Hills, as generally, the basal member of the Colo- 

 rado is a sandstone. Here it is about 75 feet thick and varies from 

 "hard gray sandstone" to "dark sandy shale." Such a sandstone, resting 

 on Kootenai red shales' and succeeded by marine shales, is found in most 

 of the Montana sections. It is absent only in a central belt running 

 northwestward through Billings, Lewistown, and Great Falls. East of 

 this belt the "parting" sandstone is present in the Bighorn Mountains, 

 where it is the top of the Cleverly of Barton's reports, and in the Cat 

 Creek oil field, where it is the "Cat Creek sand." To the west a sand- 

 stone between the red and black shales is found at Big Coulee, Shawmut, 

 Sun Eiver, Collins, and the Sweet Grass Hills. This sand is sometimes 

 called Kootenai, sometimes Dakota, and sometimes Colorado — never, 

 however, on fossil evidence, which is absent. It would seem logical to 

 call the Cat Creek and Bighorn sandstone "Dakota," as it is almost 

 certainly continuous with typical Dakota; to recognize that Dakota is 

 absent in the central belt, and to tentatively call the lower sand in the 

 western area "Lower Colorado." Since the Colorado Sea came in from 

 the south and crept northward over the Kootenai land surface, its ])a3al 

 sandstone should be slightly more recent to the northwest. 



Above the basal sandstone, in the Sweet Grass Hills, is a series of blue, 

 brown, and black shales with numerous sandstone bands. This series is 

 about 400 feet thick, above which the beds are more calcareous, but still 

 have much sand, to a total thickness of about 800 feet, above the base of 

 the Colorado. The top of this sandy phase is the horizon which, north 

 and east of Great Falls, can be definitely correlated with the top of the 

 Mowry member. 



This means that the lower 800 feet of Colorado in the Sweet Grass 

 Hills is the equivalent of the Mowry and Thermopolis formations of 

 southern Montana and Wyoming. Their character has, however, changed 

 in the long distance from the type localities. The Mowry has become 

 more sandy ; the shale of the Thermopolis is less bituminous and more 

 sandy, and many thin sandstone layers have appeared. A glance at the 

 general sections will show that this is the normal behavior of these forma- 

 tions to the north and west of their type localities in the Bighorn Basin 

 of Wyoming. Sections G (Big Horn), on figure 14, and 5 (Yellowstone 



