132 C. W. TOM LIN SON 



d) Where Members 20B and 21 are absent, the thickness of Member 19 is 

 variable, though not extraordinarily so. 



e) Member 21 consists of clastic sediment, chiefly sandstone and black 

 or deeply stained shale. 



/) Member 21 contains "a fauna which is different in most of its forms 

 from that of the lower members, and is more like that of the Madison lime- 

 stone" 1 which overlies it. 



7. Between Members 18 and 19 of the Devonian system 

 (between the Jefferson limestone and the Three Forks formation) : 



a, Sharp lithologic change at this horizon. 



b, Member 18 is moderately variable in thickness. 



c, In the Teton River section the base of Member 19 contains nodules 

 of limonite. In the Crandall Creek section the same horizon is very deeply 

 iron-stained and carries small geodes of amorphous hematite. 



It will be noted that this evidence is entirely circumstantial. 

 6. Between Members 14 and 15 of the Devonian system 

 (between the main and upper divisions of the Jefferson dolomite) : 



a) There is a 16-foot sandstone at this horizon in the Labarge Mountain 

 section, and much sandstone in Member 15 in the Teton River section. 



b) Members n-14, with a maximum aggregate thickness of 550 feet, are 

 absent in the Teton River, Antler Peak, and Crandall Creek sections. 



c) Members 12-14 are not known north of Labarge Mountain. 



d) The Nevada limestone of eastern Nevada includes a lower and an upper 

 fossiliferous zone, separated by from 2,000 to 4,000 feet of barren beds. The 

 Jefferson fauna includes elements of both of the fossiliferous zones of the 

 Nevada, suggesting that the great thickness of the Nevada is due to the presence 

 of medial barren members which are not found in the Jefferson. 2 



e) In the Snowy Mountain section in Yellowstone Park, Weeds describes 

 a 25-foot belt of limestone conglomerate at what may be this horizon. 



5. Between Members 3 and 4 of the Devonian system (between 

 the basal and main divisions of the Jefferson dolomite) : 



a) There is a 10-foot bed of sandstone (Member 4) at this horizon in the 

 Labarge Mountain section, and a thinner bed of extremely sandy dolomite in 



1 W. P. Haynes, "The Fauna of the Upper Devonian of Montana, Part 2, The 

 Stratigraphy and the Brachiopoda," Annals of the Carnegie Museum, X (1916), 27. 

 It is to be noted that Haynes reached the conclusion that, nevertheless, "there is no 

 sharp break in the record here" {ibid., p. 20). 



2 See discussion of "The Jefferson Dolomite." 



3 W. H. Weed, "Geology of the Southern End of the Snowy Range," U.S. Geol. 

 Survey, Monographs, XXXII, Part 2 (1899), chap, vi, p. 206. 



