THE COLORADO FORMATION 



91 



have been obtained from some of the specimens of fossil wood. 

 Charcoal also occurs in the chalk as well as in the Benton lime- 

 stone. Nodules of pyrite are abundant in some outcrops. 



The lower Rudistes beds present a varied and extensive 

 invertebrate fauna. The Hesperonis beds contain fewer inver- 

 tebrates, but a vastly greater number of vertebrates than the 

 lower Rudistes beds. 



The upper 125 feet of the Timpas beds is probably the 

 equivalent of the Rudistes beds, while the Apishapa beds are the 

 equivalent of the Hesperonis beds in the Colorado area. In the 

 Black Hills area the position of the Pteranodon beds is occupied 

 by a bed of shales with a thin bed of limestone. The beds are 

 wanting in the Iowa-Nebraska and eastern Dakota areas. In the 

 following table, which is intended to show the relation of the 

 different subdivisions in the representative areas, the eastern 

 Dakota area is omitted, as it does not differ materially from the 

 Iowa-Nebraska. 



W. N. Logan. 



