174 GEOLOGY OF THE 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 

 quantities of Inoceramus problematicus 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 group. 



Dark-gray laminated clays, sometimes alternating near the upper part with seams 

 and layers of soft gray and light-colored limestones. Inoceramus problematicus, I. ten- 

 uicostatus, I. latus, I.fragilis, Ostrea congesta, Veniella Mortoni, PJiolaclomya papyracea, 

 Ammonites Mullananus, Prionocyclus Woolgari, Mortoniceras Shoshonense, Scaphites 

 Warrenanus, 8. larvceformis, 8. ventricosus, 8. vermiformis, Nautilus elegans, etc. 



Localities. — Extensively developed near Fort Benton on the Upper Missouri ; also 

 along the latter, from ten miles above James Biver to Big Sioux Elver ; 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 Eed Valley. In its upper portion are frequently found thin bedded 

 calcareous sandstones, containing numerous but fragmentary fossils, Inoce- 

 ramus, Ostrea congesta, fish teeth, etc. 



Thickness, 200 to 300 feet. 



No. 1. — Dalcota group. 



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

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

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

 Pharella ? Dalcotensis, Trigonarca Siouxensis, Cyrena arenarea, Margaritana Nebracen- 

 sis, 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 Biver ; and 

 thence extending southward into Northeastern Kansas and beyond. 



Thickness. iOO feet. 



