BLACKWELDER: new formations in WYOMING 421 



Jefferson limestone. The formation rests disconformably on the 

 Leigh dolomite member of the Bighorn, or in some places on the 

 massive member and is separated from the overlying Madison 

 limestone locally, if not generally, by another eroded surface. 

 Lithologically, it consists of a varied sequence of shales and 

 dolomites in many colors from white to gray, green, lavender, 

 buff, red, brown, and black. Somber colors predominate. Some 

 of the beds are massive, others thin and brittle. Fossils are rare, 

 but have been found in several localities. They indicate Devo- 

 nian age, but permit no greater refinement in the determination. 

 The name is derived from the canyon of Darby Creek, on the 

 west slope of the Teton Range, where the formation is well ex- 

 posed. It extends over most of northwestern Wyoming and has 

 been recognized in modified condition as far southwest as the 

 Wasatch Range. There are strata in the Bighorn Range^ which 

 resemble parts of the Darby formation, but it is not certain that 

 they actually represent it. 



A typical section of the Darby formation is exposed in the 

 east slope of Sheep Mountain near the head of Green River. 



Madison limestone 



Feet. 

 26. Limestone; very massive, hard, dense to crystalline; discon- 

 formity at the base indicated by irregular eroded surface 

 and sharp contact 44^ + 



Darhy formation 



25. Dolomite; gray, weathering brown, with deeply pitted ^rf ace; 



silicified corals and quartz geodes 80 



24. Shale and dolomite; interbedded buff-brown dolomite and 

 black to gray calcareous shale; stained reddish on surfaces, 

 and covered with Spirophyton markings 22^ 



23. Dolomite and shale; interbedded dense gray, brown-weather- 

 ing dolomite, and drab clay-shale, weathering green 41 



22. Shale; pale greenish-gray calcareous shale, with thin beds of 



shaly dolomite 19 



21. Dolomite and shale; Hght brown sandy dolomite with green- 

 ish-gray calcareous shale ; ripple marks 5^ 



20. Dolomite; dove-color to russet-oUve, thin-bedded and shaly 19^ 



19. Shale; sandy and calcareous, greenish to lavender 5 



* ToMLiNSON, C. W., Journ. Geol. 25: 47-49. 1917. 



