280 



WINTHROP P. HAYNES 



Devonian. — The strata of Devonian age in this region are 

 divided into two distinct formations, the Jefferson limestone and 

 the Three Forks formation. 



Jefferson limestone: The Jefferson limestone is well described 

 by Dr. Peale 1 as a massively bedded brown to dark-gray or black 

 crystalline magnesian limestone with the composition of a dolomite. 

 It is well exposed in the region under discussion in the form of 

 brown cliffs ioo to 200 feet high (Fig. 6). In a few of the ridges 



®f&k" ..*& 



Fig. 7. — Valley in Three Forks formation, near Rekap Station 



north of Three Forks the limestone is black in color, but shades of 

 brown are the customary colors. In this region the Jefferson 

 limestone has a thickness of about 500 feet, but it diminishes in 

 thickness to the north and northeast, ,as noted in the sections in 

 adjacent quadrangles. 



Three Forks formation: Lying upon the Jefferson limestone 

 is a series of shales and limestones which have been described by 

 Dr. Peale 2 and named the Three Forks shales. The writer has 

 made a careful study of this formation in all of this region, and has 

 measured numerous sections and made extensive collections of 



1 A. C. Peale, Bull. U.S. Gcol. Survey, No. no, 1893, pp. 27-2S. 



2 Ibid., pp. 29-30. 



