﻿Bulletin 20 82 



characterizing it. The first of these has been accomplished and 

 the data on which the place of the formation in the time scale 

 is based are presented in this paper. Only a beginning has been 

 made on the other two subjects of investigation, but the results 

 are believed to be of sufficient interest to geologists engaged with 

 the stratigraphic and faunal problems of the northern Rocky 

 Mountain region to justify bringing them together in this paper. 

 The observations of the writer show that the Jefferson limestone 

 extends some 250 miles south of the previously known area of its 

 development in Montana. 



The rather meager fauna which is described represents an area 

 and conditions which generally must have been very unfavorable 

 to life, the greater part of the beds in most sections being barren 

 of fossils. Most of the. fossils listed have been obtained at a few 

 localities where the conditions of life and of fossilization were 

 exceptionally good. The discovery of other such exceptional 

 sections in the future may be expected to add many species to the 

 fauna. 



Nomenclature 



The Jefferson limestone is well exposed near Threeforks, 

 Montana, a few miles above the mouth of the Jefferson River, 

 from which its name is derived. The formation, as defined by 

 A. C. Peale 1 , who introduced the name, comprises a series of 

 magnesian limestones 640 feet in thickness, resting upon a 

 pebbly limestone provisionally referred to the Cambrian, and 

 terminated above by the Threeforks shale 2 , which carries a rich 

 Devonian fauna. The Jefferson limestones are described by 

 Peale as "brown and blackish in color and microgranular in 

 structure, due to their being very crystalline, and occur in alter- 

 nations of rather massive beds of very dark limestones, from 



'Bull. U. S. Geol. Survey, No. no, 1893, p. 27. 



2 The Threeforks shale generally contains some calcareous beds. At 

 Logan near the type locality 10 feet of limestone occupies the middle of the 

 formation. Where the limestone element predominates over the shale 

 this formation has been called the Threeforks limestone as in Geol. Atlas 

 U. S., Folios Nos. 30 and 52. 



