376 C. W. TOM LIN SON 



chiefly because of the persistance of Halysites catenulatus into the 

 Jefferson dolomite, favors the idea of continuous sedimentation in 

 this locality from Silurian into Devonian. Elsewhere in the Rocky 

 Mountains, however, where the Silurian is absent, there was cer- 

 tainly an emergent interval immediately preceding the inauguration 

 of Devonian sedimentation. The fact that no physical evidence 

 of hiatus at that time has been noted as yet in northern Utah means 

 little. The recurrence of Halysites in the Devonian certainly 

 means no more, as regards continuity of submergence, than its 

 persistence from the Richmond into the Silurian; yet the discon- 

 formity between the Richmond and the Silurian on Blacksmith 

 Fork is well marked. 



How far the Silurian system originally extended over the Rocky 

 Mountain province can only be conjectured. Throughout Wyo- 

 ming, wherever the Devonian occurs, it is apparent that the Silurian 

 system, if ever represented, was completely removed by erosion 

 prior to the Devonian submergence. 



A small fauna collected by Blackwelder in the Gros Ventre 

 Range from beds just below the Leigh dolomite was referred by 

 Kindle and Weller to the Silurian, and by Ulrich to the Richmond. 1 

 The latter interpretation is probably correct, as the Leigh of the 

 Teton Range is confidently correlated with part of the Richmond 

 series in the Bighorn Range. Blackwelder's section in the Gros 

 Ventres does not include beds of the types which characterize the 

 Laketown dolomite in Utah. 



The Silurian is not represented in Hintze's 2 section in the central 

 Wasatch. The beginning of known Devonian deposition in that 

 region was certainly preceded by an interval of emergence. Neither 

 Silurian nor Devonian strata are known in the Uinta Range. 3 



THE DEVONIAN SYSTEM: THE JEFFERSON DOLOMITE 



The basal division of the Jefferson dolomite.- — The age of the 

 beds here called Members i and 2 of the Devonian system has 

 been discussed under the Silurian. Member 3, which follows 2 in 



1 Eliot Blackwelder, "Origin of the Bighorn Dolomite of Wyoming," Bull. Geol. 

 Soc. Amer., XXIV (1913)) 6l °- 



2 Op. cit. 3 F. B. Weeks, op. cit., XVIII (1907), 427-48. 



