﻿85 Outcrops 



lins, about 160 miles east cf Labarge Mountain. Here the Jef- 

 ferson limestone is absent. The interval between the Cambrian 

 quartzite and the Carbon ferous is occupied by a hard gray ncn- 

 magnesian limestone about 180 feet in thickness, of undetermined 

 age, but clearly a different formation from the Jefferson. From 

 the foregoing it appears that the eastern border of the area in 

 which the Jefferson limestone is known to occur would be de- 

 fined by a north-south line crossing the western half of the State 

 between the Absarokas and the Bighorn Mountains, in the 

 northern part, and the towns of Kemmerer and Rawlins, in the 

 southern part. Sections studied by the writer in the Wasatch 

 Mountains of northeastern Utah, near Logan, show that the 

 Jefferson is present there, with the same physical characteristics 

 which it shows in western Montana. The formation probably 

 did not extend eastward into Colorado ; at least not as far east as. 

 the Glen wood section, where the Devonian is represented by a 

 very different type of limestone which holds a fauna quite dif- 

 ferent from that of the Jefferson limestone. All the Colorado 

 sections which have been studied show the Devonian, where pre- 

 sent, to be represented by a formation which is distinctly dif- 

 ferent, both lithologically and faunally, from the Jefferson 

 limestone. 



The evidence now at hand indicates that the eastern edge of 

 the formation lies somewhere near the 109th parallel in the States 

 of Montana, Wyoming, and northern Utah. What its southern 

 and western limits may be remains to be determined. The forma- 

 tion has a known east -west extent of about 150 miles in Montana 

 and a north-south extent of about 425 miles. Its actual extent 

 is probably much greater. The paleontologic evidence of the 

 identity of this formation with the Nevada limestone of Nevada 

 is presented elsewhere in this paper. 



Sections in Montana. — Some of the stratigraphic evidence on 

 which the extent of the Jefferson limestone as outlined above is 

 based may be shown by consideration of a few typical sections 

 which include this formation and exhibit its relations. The 

 section at Threeforks, Montana, the type locality of the Jeffer- 

 son, as given by Peale, is as follows : 



