THE UPPER CRETACEOUS GROUPS. . 185 



its fossils this region of the lower Cheyenne can scarcely be excelled by any 

 locality of Cretaceous fossils in the West. 



The concretions are not uniformly scattered through the mass of the 

 formation, but are found most abundant along two lines or horizons. One 

 of these is near the base and the other near the summit of the series, and 

 the intervening space is barren of fossils. On the South Cheyenne the 

 greater number of forms collected were from the upper zone, which is here 

 but a short distance below the base of the Tertiary. Some of the principal 

 forms are : Cyncyclonema rigida, Pteria linguiformis, P. JSfebrascensis, P.psen- 

 dopteria, Inoceramus sublevis, I. Sagensis, I Barabini, Nucidana bisulcata, N. 

 subequilatera, Lucina occidentalism L. ventricosa, L. subundata, Thracia suhgra- 

 cilis, Necera Moreauensis, Fasciolaria fusiformis, Fusus Cheyenensis, Aporrhais 

 Meeki, Anchura sublevis, Lunatia concinna, Amauropsis paludiriatformis, Akera 

 glans-oryza, Dtntalium gracile, Nautilus Dekayi, ScapJiites nodosus, and Ancy- 

 loceras tricostatus. 



Between Rapid and Spring Creeks and thence southward to French 

 Creek and bej^ond are a series of outlying areas of the White River Ter- 

 tiary, which rise in white bluffs above the plateau of Cretaceous. The 

 character of the Cretaceous at their point of junction is illustrated in the 

 following section observed near Spring Creek: 



Tertiary. 



Feet. 

 4. White plastic clays, with impure limestone at base and seams of chalcedony .150-200 

 3. Fine, white, quartz gravel, very loosely cemented 5-8 



Cretaceous. 



2. Plastic clay, gray or brown, with much iron oxide in seams; often variegated 



in color, pink, ash, red. and brown. (Fox Hills ?) 30 



1. Gray or drab clays, hardly distinguishable from the clays at the level of the 

 Cheyenne, 600 feet below ; containing within 100 feet of the top concretions 

 full of the characteristic fossils of the Fort Pierre group, but associated 

 with some indicative of the Fox Hills — 



The dip of the rocks along the Cheyenne in this vicinity is toward the 

 south and east, but, is low and barely perceptible. The surface of the 

 country in the vicinity of Rapid Creek is flat, and broken only by the dry 

 ravines that head back a short distance from the immediate bank of the 



