42 GEOLOGY OF THE BLACK HILLS. 



of Professor Whitfield, added several new species to the fauna of the period. 

 Of the remaining portions of the Silurian system, which in the eastern United 

 States reach a maximum thickness of over 20,000 feet, we have no deposits 

 in the Black Hills, though in other portions of the Rocky Mountain region 

 equivalent strata have been found. 



The Devonian system, as has been said, has no representatives in the 

 rocky strata of the Hills. Though the exposures are excellent and exami- 

 nations were carefully made at the proper horizon, no trace of it could be 

 found. The general thinning of the formation from east to west — from 

 14,000 or 15,000 feet in the Appalachian region to 1,000 feet in Ohio and 

 150 feet in Iowa — culminates in the Black Hills in complete disappearance. 



The Carboniferous system is the most prominent and frequently the 

 sole representative of the Paleozoic age in the Far West; and while in the 

 East it is divisible into two periods Sub-Carboniferous and Coal Measures, 

 the latter bearing the great deposits of coal, in the Far West it contains no 

 coal and is largely composed of calcareous deposits, in which the two periods 

 have not usually been distinguished from each other. In the Black Hills 

 we were not enabled to determine to what period or periods of the Carbon- 

 iferous age the strata were properly to be referred. Though they were 

 found highly fossiliferous, sufficiently characteristic forms were not obtained 

 to decide the question, and it was not possible to give to the study the 

 necessary careful attention. 



Above the beds of undoubted Carboniferous age were found a series 

 of pink, red, and white sandstones and limestones, with no well-marked 

 separation from the Carboniferous below, and these pass upward into a 

 very impure, concretionary or distortedly weathered, reddish sandstone, 

 above which is the red clay of the Red Beds. In the absence of good fos- 

 sils, it was difficult to determine to which system these beds should be 

 referred, whether to the Carboniferous below or to the Red Beds above, 

 but preference was finally given on lithological grounds to the former. 



The Trias (?), or Red Beds, the as yet blank leaf in the geology of 

 the Rocky Mountains, is finely exposed around the Hills, forming in the 

 Red Valley, or "Race Course," one of the most prominent features of the 

 topography. It consists of three parts — a red clay at base, separated by an 



