218 KEMPS ORE DEPOSITS. 



schists and slates. These are lenticular masses of limited extent, 

 horizontally and below, somewhat like a magnetite lens (Example 

 12) in shape, and carrying a small amount of gold with little or 

 no pyrites. 1 



MONTANA. 



2.10.04. Geology. The eastern part of the State belongs to 

 the Prairie region, which is, however, in portions greatly scarred 

 by erosion, forming the so-called Bad Lands. The approaches to 

 the Rocky Mountains are not abrupt and sudden as in Colorado, 

 but are marked by numbers of outlying ranges of both eruptive 

 and sedimentary rocks. The chain of the Rockies takes a north- 

 westerly trend in Wyoming, and so continues across Montana. It 

 is rather the prolongation of the Wasatch than of the Colorado 

 Mountains, whose strike is for the Black Hills. The character of 

 the ranges is also very different. They are less elevated and have 

 broad and well-watered valleys between, that admit of considera- 

 ble agriculture. Geologically the country is in marked contrast 

 with Colorado. "While in the latter the Paleozoic is feebly de- 

 veloped, in the former it reaches great thickness. In the eastern 

 ranges W. M. Davis gives Lower Cambrian 10,000 to 15,000 feet; 

 Silurian and Devonian, not yet recognized ; Carboniferous lime- 

 stones, 3500 feet; Trias, not definitely recognized; Jurassic 

 and Cretaceous sandstones, shales, and thin limestones, 15,000 

 feet. This more closely resembles the Wasatch and Great Basin 

 sections (see 2.08.29, and 2.11.01). Much granite of a basic or 

 dioritic character is present (Example 17), and great develop- 

 ments of eruptive rocks of extremely interesting character. No 

 more interesting field for geological work awaits the investigator. 2 



1 A. J. Bowie, "Notes on Gold Mill Construction," M. E., X. 1881. 

 W. B. Devereux, "The Occurrence of Gold in the Potsdam Formation," 

 M. E., 465 ; Engineering and Mining Journal, Dec. 23, 1882, p. 334. H. 

 O. Hofman, "Gold Mining in the Black Hills," M. E., XVII. 498 ; also in 

 preliminary report cited under Carpenter, under Geology. 



2 S. Calvin, "Iron Butte : Some Preliminary Notes," Amer. Geol., 

 IV. 95. G. E. Culver, "A Little Known Region of Northwestern Mon- 

 tana," Wis. Acad., Dec. 30, 1891. W. M. Davis, "The Relation of the 

 Coal of Montana to the Older Rocks," Tenth Census, Vol. XV., p. 697. 

 Rec. J. Eccles, " On the Mode of Occurrence of Some of the Volcanic 

 Rocks of Montana," Quar. Jour. Geol. Sci., XXXVII. 399. G. H. El- 

 dridge, "Montana Coal Fields," Tenth Census, Vol. XV., p. 739. S. F. 

 Emmons, Tenth Census, Vol. XIII., 97. Rec. Hayden's Survey, Ann. 



