﻿83 NOMENCLATUTE 



10 to 15 feet in thickness, with lighter-colored, more laminated lime 

 stones in layers of 2 feet to 5 or 6 feet each. 



This limestone shows a striking uniformity over a wide area, 

 not only in its dark chocolate-brown or almost black color, but 

 also in chemical composition, as shown by Peale. Weed's 

 characterization of the formation as it appears 7 5 miles north of 

 Threeforks, as comprising "chocolate-brown or steel-gray 

 crystalline limestones, generally having a distinctly granular or 

 saccharoidal texture, which is especially noticeable in weathered 

 surfaces," 3 would apply equally well to many sections exposed in 

 the vicinity of the type locality. 



The Threeforks shales, which separate the Jefferson from the 

 Madison limestone at Threeforks, are in many places absent from 

 the section. Where this is the case the line between the two 

 limestones is usually indicated by the contrast in color and com- 

 position, the Madison being much lighter in color than the Jef- 

 ferson and nonmagnesian. 



The comparative uniformity of the lithologic features of the 

 Jefferson limestone over a considerable area has enabled 

 geologists to discriminate the formation in each of the published 

 Montana folios that include the Devonian, as well as in the 

 Yellowstone Park and Absaroka folios. 



In the Little Belt Mountains quadrangle the Jefferson lime- 

 stone is "a distinct stratigraphic unit" according to Weed, but 

 for cartographic reasons it is united with the Threeforks forma- 

 tion and called the Monarch formation in the folio 4 . 



The f aunal contents of the Jefferson formation are generally 

 very meager. The collections which have been made heretofore 

 have been insufficient to determine with any degree of certainty 

 the age of the formation. 



Areal Distribution and Stratigraphic Details 



Extent of formation. — The Jefferson limestone has a wide dis- 

 tribution in the northern Rocky Mountain region. In Montana 



3 Twentieth Ann. Rept. U. S. Geol. Survey, pt. 3, 1900, p. 28! 

 4 Geologic Atlas U. S., folio 56, U. S. Geol. Survey, 1899, p. 2. 



