108 GEOLOGY OF THE BLACK HILLS. 



In the Rocky Mountain region the Carboniferous is the most persistent 

 and important of the Paleozoic formations. It is found in almost every 

 great section and is everywhere identified by fossils. Its characteristic 

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

 of degradation they have been laid bare over large areas. Nevertheless 

 the subdivisions known at the East have here received no general recogni- 

 tion. The prevailing fossils are of Coal Measure types, but with these are 

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

 any distinctive Sub-Carboniferous strata have been recognized A Permian 

 has been separated by King in the Uinta Mountains, by Peale in Colorado, 

 and by other observers far 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 

 anticipated 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 

 as Carboniferous. 



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

 purely by lithological characters. Beginning with the lowest, they are as 

 follows : 



Feet. 



1. Slialy limestone. — Thinly bedded, impure, arenaceous limestone, pinkish 



in color, often filled with comminuted fragments of crinoidal columns; 

 contains also Productus, cyathophylloid corals, etc 20-60 



2. Gray limestone. — Massive gray or whitish limestone, uniformly pure, con- 



taining Sjpirifera Rocky-montana, Productus, etc 150-200 



3. Silicious limestone. — Weathering usually in a peculiar brecciated manner; 



somewhat cavernous and often staiued with pinkish and reddish streaks; 

 containing much flinty and silicious matter, often with a banded struc- 

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

 or cavities. Fossils are found, but usually ill preserved and only ex- 

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

 been recognized Athyris subtilita, Spirifera Roclcy-montana, Productus., 

 corals, etc 150-175 



