UPPER COAL MEASURES. 69 



black mud-shales or carbonaceous argillites, generally very thin and some- 

 times calcareous, passing into impure limestones. 



About the middle of the formation is a tolerably persistent develop- 

 ment of limestone of the usual blue-gray color and dolomitic in composi- 

 tion. Its thickness, however, varies very much according to locality. It 

 was best observed in Big Sacramento gulch, a short distance above the Lon- 

 don fault, where are two beds of limestone with associated shales, about 

 fifty feet apart and each about ten feet in thickness. 



Fossils. From the limestones in Big Sacramento gulch were obtained 

 the following forms: 



* 



Spiriferina Eentuckensis. 

 Athyris subtilita. 



Productus costatus. 



Productus muricatus. 

 Aviculopecten interlineatus. 

 MeeJcella stricecostata. 



From micaceous schists in the upper part of the formation between 

 Lamb and Sheep Mountains were obtained abundant casts of Equisetacece. 



upper coal Measures (h). Less favorable opportunities were offered for 

 studying this group than for either of the preceding, since its beds were 

 found only at the extreme limits of the map and in regions where continu- 

 ous outcrops are rare. It consists of alternating calcareous and silicious 

 beds, the latter not being distinguishable from those of the Weber Grits 

 at the base, but passing upward into reddish sandstones, which in their turn 

 are sometimes difficult to distinguish from the overlying red sandstones of 

 the Trias. Its lower limit is drawn at the base of the first important lime- 

 stone bed above the Weber Grits. This limestone, locally called the Robin- 

 son Limestone from the fact that it forms the ore-bearing horizon of an im- 

 portant mine of that name in the Ten-Mile district, is remarkable for being 

 the first true limestone observed among the calcareous beds of the region. 

 All below this horizon are practically dolomites of varying purity. As 

 developed in this mine, it is of drab color, conchoidal fracture, and of pecul- 

 iarly compact texture, resembling a lithographic stone. Its purity and 

 textural characteristics are apparently not persistent outside of the Ten-Mile 

 district. In the upper horizons of this district are found mud-shales, resem- 

 bling in lithological character the Permo-Carboniferous of the Wasatch. 

 Their fossil remains are found, however, to be distinctly Coal Measure forms. 



