the Muddy Mountains, Nevada, etc. 45 



correspond to the Middle Cambrian Tapeats sandstone 

 and Bright Angel shale of the Grand Canyon district. 



Eock formations younger than the Paleozoic are 

 exposed in St. Thomas Gap. They are identical with 

 formations in the Muddy Mountains. 



Rocks of the Muddy Mountains. 

 Paleozoic Formations. 



Devonian System. — Limestones of Devonian age are the 

 oldest rocks recognized in the Muddy Mountains. They 

 are in Callville jMountain, a great block which has been 

 thrust over Jurassic sandstone. All of the beds between 

 the overthrust plane and recognized Carboniferous are 

 here referred to the Devonian, but further work may 

 show that some of the lower strata belong to older 

 periods. Devonian fossils were found in a zone about 

 50 feet thick, more than 300 feet below the base of the 

 Mississippian. Below this f ossilif erous horizon there 

 are 900 feet of limestone in which no fossils were found 

 except a few algae. As the beds are of the same general 

 nature and as no unconformity could be found in the 

 series, the entire thickness of 1,200 feet is tentatively in- 

 cluded in one formation, which will here be called the 

 Muddy Peahs limestone because of its prominence in the 

 Muddy Peaks mass. 



The limestone is dense and hard, and many beds have 

 a siliceous appearance. Layers are regular and heavy, 

 ranging from 2 to 20 feet in thickness. In color they 

 are either very dark from included carbonaceous matter, 

 or decidedly light, beds or zones of the tw^o alternating. 

 Lenses and thin layers of sandstone occur at intervals, 

 especially near the top of the formation. At the base 

 brecciation and shattering are extreme through a thick- 

 ness of 100 to 500 feet. 



Most of the fossils collected from the formation have 

 been misplaced, and therefore a complete list is not avail- 

 able for publication in this paper. The following par- 

 tial list has been identified by Dr. E. 0. Ulrich : Helio- 

 phyllum sp. undet. (calyx less than 1/2 inch in diameter), 

 Zaphrentis sp. undet. (calyx 1 inch or less in diameter), 

 small Stromatoporoid of undetermined genus and species, 

 Atrypa aff. reticularis (differs from the typical form in 



