108 GEOLOGY OF THE BLACK HILLS. 



In the Rocky Mouiituin region the Carboniferous is the most persistent 

 antl important of the Paleozoic formations. It is found in ahuost every 

 •rreat section and is everywhere identified by fossils. Its characteristic 

 rocks are limestone and sandstone, and are so durable that in the progress 

 of deirradation thev have been laid bare over large areas. Nevertheless 

 the subdivisions known at the P^ast have here received no general recogni- 

 tion. The })revailing fossils are of Coal Measure types, but with these are 

 nearly always mingled Sub-Carboniferous forms, and it is but rarely that 

 an\- distinctive Sub-Carboniferous strata have been recognized A Permian 

 has been separated by King in the Uinta ]\Iountains, by Peale in Colorado, 

 and by other observers ftir at the south, but in many localities Coal 

 Measure fossils seem to extend to the top of the system. 



In the Black Hills we have established no subdivisions on paleonto- 

 logical grounds, and are unable to affirm or deny the presence either of 

 Sub Carboniferous or of Permian The rock series is definitely and con- 

 tinuously divided into several members, but the lowest and the highest of 

 these members alike failed to afford us distinctive fossils. It is to be 

 anticii)ated that future investigations will correlate them at least with the 

 several members of the system as known in the neighboring portions of 

 the Rock}' Mountains, but for the present they can only be designated 

 {IS Carboniferous. 



The local subdivisions are four in number, and they are distinguished 

 purel}' by lithological characters. Beginning with the lowest, they are as 

 follows : 



Feet. 



1. ISlialy limestone. — Tliiuly bedded, iini)ure, arenaceous limestone, pinkish 



in color, often tilled with eomininnted tiagiueuts of crinoidal coliunns; 

 contains also Productus, cyatho])liyHoid corals, etc 20-60 



2. Gruij limestone. — .Massive yray or whitish limestone, uniformly pure, con- 



taining Spiri/era Rocly-montana, Productus, etc 150-200 



3. Silici<nts limestone. — Wcatheiinjf usually in a i)eculiar brecciated manner; 



somewhat cavernous and ol'teu stained with pinkish and reddish streaks; 

 containinf,' much tlinty and silicious matter, often with a banded struc- 

 ture; with crystals of calcite lining the interior of weathered crevices 

 or cavities. Fossils are ionnd, but usually ill preserved and onl^^ ex- 

 posed and obtainable when the rock is much weathered. There have 

 been iccogni/.ed Athi/ris suhfilitaj Hpirifera Roely-montana, Productus. 

 corals, etc 150-175 



