320 INDEX TO THE STRATIGRAPHY OF NORTH AMERICA. 



The Threeforks shales, which rest conformably upon the Jefferson limestone, may be 

 divided into three parts — the lower or orange-colored shales, about 50 feet in thickness ; a band 

 of grayish-brown argillaceous limestone, 20 feet thick; and the upper green, black, and argil- 

 laceous shales, some 50 feet in thickness, which are crowded with Devonian fossils and capped 

 by a band of yellow laminated sandstone 25 feet thick. These Devonian fossils were found 

 both in the northern and in the southern portion of the district, and doubtless occur wherever 

 the beds are found. A coal-black slate is seen near the upper part of the shales in the vicinity 

 of the head of the Missouri River. In the Jefferson Canyon this has developed into a bed of 

 coal, in which are found limestone nodules containing the same Devonian forms that are else- 

 whei-p i«>und in the shales. The shales are near the border liae of the Devonian and Carbon- 

 iT^i ^ JniSiud their organic remains contain a minghng of forms of both, but the preponderance 

 of evidence is in favor of their Devonian age. 



On examining fossils collected in course of the survey of Yellowstone Park, 

 Girty ^^^° confirmed the determination of the Devonian age of the Threeforks shale 

 and noted the relation of the fauna to the Devonian of the White Pine district, 

 Nevada. Fuller collections have been made by Kindle *^^ in a special study of the 

 Jefferson limestone and by Raymond ^^^ from the Threeforks shale. Kindle con- 

 cludes that the Jefferson is "earlier than Upper Devonian" and probably includes 

 both Lower and Middle Devonian. Raymond describes the fauna of the Threeforks 

 shale as Upper Devonian. 



Kindle says, in part : 



The Jefferson limestone has a wide distribution in the northern Rocky Mountain region. 

 In Montana its distribution seems generally to coincide with that of the Madison limestone, 

 which immediately follows it in the sections where the highest formation of the Devonian system 

 is absent. Its distinctive composition and color have enabled geologists to identify the Jefferson 

 limestone over a rather wide area in Montana and northwestern Wyoming. The writer has 

 recognized the formation still farther south in western Wyoming and northeastern Utah. 

 [Details foUow.] 



The evidence now at hand indicates that the eastern edge of the formation lies somewhere 

 near the one hundred and ninth 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 mUes 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, Mont., the type locality of the Jefferson, as given by Peale, is as foUows: 



