172 GEOLOGY OF THE BLACK HILLS. 



Black Hills. — Well exhibited on the western side of the Hills on the 

 Cheyenne River near (east of) Old Woman Fork, where it is represented 

 by ferruginous sandstones with Veniella liimiilis, Sphceriola transversa, Idon- 

 earca SJmmardi, etc. Not well distinguished on the eastern side ; appears 

 lithologically almost inseparable from No. 4. 



On the Cheyenne in the vicinity of Rapid Creek, below the base of 

 the Tertiaiy deposits, occurs a gray or brown clay with occasional varie- 

 gated strata, pink, reddish, or ash-colored, containing large quantities of 

 iron in irregular seams. This has a thickness of about 30 feet, and below 

 it was found the gray clay with calcareous concretions filled with forms 

 characteristic of No. 4, but with some also of those peculiar to No. 5. 

 Besides this deposit the highest well-marked strata of the Cretaceous ob- 

 served on the eastern side of the Hills are the plastic clays of No. 4, which 

 occupy by far the larger portion of the valleys of the Belle Fourche and 

 South Fork of the Cheyenne. North of the Cheyenne No. 5 is reported 

 to exhibit its usual characters. 



Paleontologically No. 4 and No. 5 are not well distinguished on the 

 eastern side of the Hills ; the fauna appears to show a blending of the 

 forms elsewhere found peculiar to each formation. 



Thickness, roughly estimated, 100 feet. 



No. 4. — Fort Pierre group. 



(a) Dark-gray and bluish plastic clays, containing, near tlie upper part, Nautilus 

 Delcayi, Placenticeras placenta, Baculites ovatus, B. compressus, ScapMtes nodosum, Denta- 

 lium gracile, Crassatella Evansi, Cuculkea Nebrascensis, Inoceramus Sagensis, I. Nebra- 

 scensis, I. Vanuxemi, boues of Mosasaurus Missouriensis, etc. 



(b) Middle zone, nearly barren of fossils. 



(c) Lower fossiliferous zone, containing Ammonites complexus, Baculites ovacus, B. 

 compressus, Heteroceras Mortoni, H. tortuni, H. umbilicatum, H. cochleatum, Ptychoceras 

 Mortoni, Odontobasis vinculum, Anisomyon borealis, Amauropsis paludiniformis, Inocera- 

 mus subhviris, 1. tenuilineatus, bones of Mosasaurus Missouriensis, etc. 



(d) Dark bed of very line unctuous clay, containing muck carbonaceous matter, 

 with veins and seams of gypsum, masses of sulphuret of iron, and numerous small 

 scales of fishes. Local ; filling depressions in the bed below. 



Localities. — («■) Sage Creek, Cheyenne River, and on White River above the Mau- 

 vaises Terres. (o) Fort Pierre and out to Bad Lands ; also down the Missouri on the 

 high country to Great Bend, (c) Great Bend of the Missouri below Fort Pierre, (ft") 

 Near Bijou Hill, on the Missouri. 



Thickness of the whole group, 700 feet. 



