﻿Bulletin 20 84 



its distribution seems generally to coincide with that of the Madi- 

 son 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 recog- 

 nized the formation still farther south in western Wyoming and 

 northeastern Utah. 



Our information concerning the areal limits of the formation 

 relates mainly to its northern and eastern extent. The most 

 westerly section which has been studied is in the Ehilipsburg 

 quadrangle. Here the formation has about its maximum thick- 

 ness, which would indicate that it probably extended consider- 

 ably farther westward. In northwestern Montana the forma- 

 tion is absent, according to Willis, who reports that the Carboni- 

 ferous limestone rests upon the Algonkian 5 . In the Fort Benton 

 quadrangle, in the north-central part of the State, the formation 

 has thinned to about .100 feet, according to Weed 6 . The greatly 

 reduced thickness here would indicate that the formation prob- 

 ably has no great extent northeast of Fort Benton. In Wyoming 

 the eastern boundary of the formation can be stated within rather 

 broad limits. Hague recognizes the formation in the Absaroka 

 quadrangle, but in the Bighorn Mountains, just east of this quad- 

 rangle, Darton finds the Carboniferous limestone resting upon the 

 Ordovician and the Jefferson formation absent. In the southern 

 part of Wyoming the writer has studied two sections of the 

 Paleozoic rocks, oriented in an east-west direction not far from 

 the forty-second parallel. The more westerly of these sections is 

 that at Labarge Mountain, about 35 miles east of the Idaho line. 

 The Jefferson limestone is represented by about 1,000 feet of drab 

 to black magnesian limestones in this section, resting upon rocks 

 of Cambrian or later age. The other section is located near Raw- 



s Bull. Geol. Soc. Am. ,vol. 13, 1902, pp. 316-325. 

 "Folio 1, U. S. Geol. Survey, 1899, p. 2. 



