172 GEOLOGY OF THE BLACK HILLS. 



Blade JliUs. — Well exhibited on the western side of the Hills on the 

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

 by ferrnginous sandstones with Veniella humiUs, Sphceriola transversa^ Idon- 

 earca Shumardi, 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 Tertiar}" 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 

 occu2)y 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 the upper part, Nautilus 

 Dclayi, Placenticcras jdaccnta, Baciditcs oratiis, li. comjjrc.sHUs, lSca2)hitcx )W(1osuis, Deiita- 

 liiun gracile, Crussatdla Eransi^ Cucidkca Xebrascensis, Inoccramun ISagcnsis, I. Xehra- 

 scensis, I. Vamwemi, bones of j}[o,sasaurus Missouriensis, etc. 



(b) Middle zone, nearly barren of fossils. 



(c) Lower Ibssiliferous zone, containing Ammonites coniplcjcus, Baeulites ovacus, B. 

 compressuH^ Jlctcrocenis Mortoui, II. torttmi, H. umbilicatvvi, H. cochleatunt, Ptuchoccras 

 Mortoni, Odontobasis rinviduiiuAnisomyon borcalis,Am<inroj)sis pidndiniformis^ Inocera- 

 wns subkcvis, I. tenuilineatus, bones of Mosasaurus Missouriensis, etc. 



{d) Dark bed of very tine unetuous clay, containing nuicli carbonaceous matter, 

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

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



Loaditics. — (a) 8age Creek, Clieyenne Eiver, and on White l\iver above the Mau- 

 vaises Tenes. (/>) Fort IMeire and out to Bad Lands; also down the Missouri on the 

 high country to Great Bend, (c) Great Bend of the IMissouri below Fort Pierre, (d) 

 Near ISijoii Hill, on the .Missouri. 



Thickness of tlie whole group, 700 feet. 



