174 GEOLOGY OF TUE BLACK HILLS. 



Black Hills. — Exposed around the Hills with its characteristic structure 

 Well exposed on the west side of the Beaver Creek, where it contains great 

 (juantitios of Lioceramus prohhntaticus with adhering Ostrea congesta; on the 

 Belle Fourche near the Great Bend ; and near Bear Butte. 



Thickness, roughly estimated, 100 to 200 feet. 



No. 2. — Fort Benton groxip. 



Dark-jTray laminated days, soinctiuu'.s altcniatiiiji near the iii)i)er part with seams 

 and h»yers of soft gray and liglit-colored liine.stoues. Inoceramus ])robleinaticuSj 1. ten- 

 uico.status, I. latus, I.fraffilifi, Ostrea conyesta, Veniella Mortoni^ P/iohidonii/a papyracea, 

 Ammonites MvUauanus. Prionocyclus Woolyari, Mortoniceras tShoshonense, Scaj)hites 

 Warrcnaniis, *S'. larvaformis, S. rentricostis, S. rermiformis, Xaiitilus eleyans, etc. 



Lncalities. — Extensively developed near Fort Benton on the T7i)per ^Missouri ; also 

 along the latter, from ten miles above James Biver to Big Sioux Biver; and along the 

 eastern slope of the Bocky Mountains, as well as at the Black Hills. 



Thickness, 800 feet. 



') 



Four vertebrates are described by Professor Cope from the Fort 

 Benton clays. 



Black Hills. — This group, with its characteristic dark plastic clays, 

 which usually contain large quantities of alkaline salts and selenite, is a 

 very persistent feature in the geology of the outer rim of foothills. It is 

 found resting upon the ferruginious sandstones of the Dakota group, and 

 forms usually the outer slope to the series of foothills that border the encir- 

 cling Red Valley. In its upper portion are freqiiently found thin bedded 

 calcareous sandstones, containing numerous but fragmentary fossils, Inoce- 

 ramus, Ostrea congesta, fish teeth, etc. 



Thickness, 200 to 300 feet. 



No. 1. — Dakota groxip. 



Yellowish, reddish, and occasionally white sandstone, with at places alternations 

 of various colored clays and beds and seams of impure lignite; also silicifiedwood, and 

 great numbers of leaves of the higlier types of dicotyledonous trees, with casts of 

 PharcUa ? Dahotensis, TrUjonarca Sioiixensis, Cyrena arenarea, Margaritana Xehracen- 

 sisj etc. 



Localities. — Hills back of the town of Dakota ; also extensively developed in the 

 surrounding country in Dakota county below the mouth of Big Sioux Kiver ; and 

 thence extending soutliward into Xortheastern Kansas and bevond. 



Thickness. 100 feet. 



