THE CRETACEOUS. 169 



* Tancredia cequilateralis. * Trigonia Conradi. 



* Tancredia bidbosa. Trigonia Montanaensis. 



* Tancredia corbuliformis. Trigonia quadrangular is. 



* Tancredia inornata. ■ *Unio nucalis. 



* Tancredia postica. TJnio Steivardi. 



* Tancredia Warrenana. *Valvata scarbrida. 

 *Thracia arcuata. *Viviparus Gilli. 

 *Thracia sublevis. Volsella formosq. 



* Trapezium Belief our chensis. Volsella isonema. 



* Trapezium subequalis. * Volsella pertenuis. 

 Trigonia Americana. 



S KCTION VIII. 

 THE CRETACEOUS,. 



The rocks of the Cretaceous system surround the Black Hills in an 

 annular rim of irregular width ; and this by means of a belt following- the 

 valley of the Cheyenne is connected with the great Cretaceous exposure of 

 the upper Missouri. Beyond the border of Cretaceous on the south and 

 southeast of the Hills stretch the White River Tertiary deposits which 

 occupy a large area ; while on the west and on the north lie the deposits of 

 the Lignitic or Fort Union Tertiary, extending in one direction to the Wind 

 River Mountains and in the other to our northern boundary and beyond. 



The Cretaceous of the Far West underlies a vast area east of the 

 mountains, reaching from beyond the Canadian line on the north into Mexico 

 on the south ; and though its deposits are found nowhere within the great 

 interior basin, they extend from the southern part of the Plains west- 

 ward into New Mexico and Arizona, and thence northward into Utah and 

 Colorado, following around the southern end of the main spurs of the 

 Rocky Mountains, as though in Cretaceous time the mountain chains had 

 already received their definition and the Cretaceous sea laved only their 

 outer slopes. A little north of the Black Hills the Cretaceous occupies 

 a broad synclinal slope, with a width, from the Rocky Mountains to 



