MINING GEOLOGY 
93 
in commercial quantities in one of the minor domes on the Sweet- 
grass arch. The significant geological feature of the occurrence 
lies in the fact that the oil occupies one of the so-called “flat struc¬ 
tures,” which heretofore have not yielded commercial quantities 
of oil in the Northwest. 
So uniformly disappointing have been the results of oil drilling 
throughout the Northwest, especially in Wyoming and Montana, 
in folds whose limbs do not dip at least in places at angles of more 
than ten to fifteen degrees, that geologists have been decidedly 
guarded, if not actually discouraging, in their reports on such 
structures, even where other conditions have been favorable. Such 
were the conditions in the Rocky Ridge dome in the Sweetgrass 
arch, where oil was discovered by the Gordon Campbell-Kevin 
Syndicate, on March 14, 1922. 
The well is located in section 16, township 35 N., range 3 W., 
on the flanks of the Sweetgrass arch, which is a huge, northerly 
plunging, flat-topped anticline, 130 miles long by eighty miles wide. 
The well is only a mile from the surrounding escarpment of the 
Cretacic Virgelle standstone, and was started in the Coloradan 
shales which are exposed, throughout the arch, inside of the 
bounding escarpment. After the drill had penetrated both the 
Coloradan and Comanchan (Kootenai) formations, the latter 
furnishing enough gas for the boiler and other camp require¬ 
ments, oil was obtained at a depth of 1,770 feet in a sandstone, 
presumably the Ellis (Jurassic) formation, although some have 
considered it, because of the depth and the presence of limestone, 
to be the Madison (Mississippian) formation. From surface ob¬ 
servations, there does not appear to be a dip greater than two 
degrees near the well, although dips as high as five degrees are 
found in places on the flanks of the arch. 
The amount of oil, which is a paraffine base type of 32.2° 
Baume, has not been determined, but one pumping test of six 
hours resulted in 100 barrels. It can not be supposed, however, 
that a production of even 100 barrels a day can necessarily be 
maintained for any considerable length of time. Nevertheless, 
the well has clearly demonstrated that oil has migrated up the 
very low dips of the Sweetgrass arch and has accumulated in com¬ 
mercial amounts in one of the “flat structures” of the northwest. 
If further development proves that oil occurs throughout the 
