PALEOZOIC STRATIGRAPHY OF ROCKY MOUNTAINS 383 



species found in the Nevada limestone in the vicinity of Eureka — 

 some in the lower, some in the upper, fossiliferous horizon there. 



THE DEVONIAN SYSTEM: THE THREE FORKS FORMATION 



Type sections. — As defined by Peale 1 in the type sections in the 

 Three Forks quadrangle, Montana, this formation includes all 

 strata between the top of Member 18 of the Jefferson dolomite and 

 the base of the Madison limestone (Mississippian) . Raymond 2 

 has collected and studied large faunas from several horizons in the 

 type locality opposite Logan. Haynes 3 has carried on Raymond's 

 work, and studied in detail the stratigraphy of the formation in all 

 exposures within about twenty miles of Logan. 



Members ig and 20. — The chief fossiliferous zone of the Three 

 Forks, Member 20 of the Devonian system (Members 4 and 5 of 

 Haynes) has not been identified in Montana east of the Three 

 Forks quadrangle, nor anywhere in Wyoming. It is quite possible 

 that outliers of it may yet be found in those areas, however, as 

 Member 19 is widely distributed there. The top of the Jefferson 

 dolomite is clearly marked in nearly every section in western 

 Wyoming and southwestern Montana by the massive, breccia ted 

 Member 18. Were it not for this, the beds of Member 19 might 

 easily be confused with those of Member 17, to which they bear 

 much likeness. In the Dead Indian Creek section, where Member 

 18 was not recognized, certain strata were assigned to Member 19 

 because their relation to overlying strata corresponds to the rela- 

 tion of Member 19 to overlying beds in the Crandall Creek section, 

 where Member 18 is present. 



Beds of doubtful age in the Bighorn Range, and near Cody. — In 

 the Goose Creek Ridge and Rattlesnake Mountain sections there 

 is a belt of platy buff and yellow dolomites and calcareous shales 



1 A. C. Peale, "The Paleozoic Section in the Vicinity of Three Forks, Montana," 

 U.S. Geol. Survey, Bull, no (1893). 



2 P. E. Raymond, "On the Occurrence in the Rocky Mountains of an Upper 

 Devonian Fauna with Clymenia," Amer. Jour. Sci., XXIII (1907), 116-22; "The 

 Fauna of the Upper Devonian of Montana," Annals of the Carnegie Museum, V (1909), 

 141-58; "The Clymenia Fauna in the American Devonian," Proc. Seventh Intern. 

 Zool. Cong. (Boston, 1907). 



3 W. P. Haynes, op. cit., pp. 13-54. 



