DISTRIBUTION AND GEOLOGIC AGE 609 



sures is such, that by one sufficiently acquainted with its peculiarities the 

 outcrops may be recognized miles away. 



DISTRIBUTION 



In the type locality, on the east slope of the Bighorn Eange, the Big- 

 horn "limestone" is a massive light-colored dolomite about 300 feet thick. 

 To the east, in the Black Hills of South Dakota, the Whitewood "lime- 

 stone," first described by Jaggar 3 and afterward by Darton and others, 

 has the typical characteristics of the Bighorn formation and contains very 

 similar fossils. Westward, as noted by Darton, 4 the dolomite has been 

 recognized in the Owl Creek, Wind Eiver, Absaroka, Gros Ventre, and 

 Teton ranges. In most of these exposures it is about 300 feet thick, 

 although, in the vicinity of the Wind Eiver basin it dwindles to 150 and 

 locally even to less than 100 feet. It disappears entirely in southeastern 

 Wyoming. To the southwest, however, I have recognized the formation 

 by its distinctly lithologic character in Labarge Mountain, 40 miles north 

 of Kern merer, Wyoming. It is there at least 150 feet thick and perhaps 

 more. In the northern part of the Wasatch Eange near Brigham, Utah, 

 an extremely massive white dolomite occupies approximately the position 

 of the Bighorn and has many of its characteristics. It is several hundred 

 feet thick, but has not yet been adequately studied. Published descrip- 

 tions 5 of the rocks in southwestern Montana and Yellowstone Park 

 strongly suggest that the Bighorn dolomite is represented there as a part 

 of the Jefferson limestone of Peale, and that it may be essentially the 

 equivalent of the Jefferson limestone of Hague, Weed, and others. Far- 

 ther extensions into Colorado on the south are discussed by Darton, 6 and 

 others into central Utah, and even Nevada on the west might be sug- 

 gested if correlation were the primary purpose of this paper. 



3 T. A. Jaggar, Jr. : Economic resources of the northern Black Hills. U. S. Geological 

 Survey, Professional Paper No. 26, 1904, p. 16. 



4 N. H. Darton : Fish remains in Ordovician rocks in Bighorn Mountains, Wyoming, 

 with a resume" of Ordovician geology of the Northwest. Bull. Geol. Soc. Am., vol. 17, 

 1906, pp. 554-556. 



6 Hague, Iddings, and Weed : Descriptive geology of Yellowstone Park. U. S. Geo- 

 logical Survey, Monograph XXXII, pt. 2, 1899 ; Absaroka Folio, Geological Atlas of the 

 United States, No. 52, 1899; Yellowstone Park Folio, Geological Atlas of the United 

 States, No. 30, 1896. 



A. C. Peale: Paleozoic section at Three Forks, Montana. '. S. Geological Survey, 

 Bull. 110, 1893. 



W. H. Weed and L. V. Plrsson : Geology of Castle Mountain Mining District U. S. 

 Geological Survey, Bull. 139, 1896; Geology of Little Rocky Mountains, Journal of 

 Geology, vol. iv, 1890, pp. 399-428. 



W. H. Weed: Geology of Little Belt Mountains, Montana. U. B. Geological Survey. 

 20th Ann. Rept., pt. iii, 1900, pp. 287-289. 

 Loc. cit. 



