1902.] DOUv'^LASS — CRETACEOUS AXD LOWER TERTIARY. 211 



tions and hard sandstone layers. In the latter there are, in some 

 places, plant impressions. The softer layers contain fossil wood, 

 bivalve moUusks, turtles and bones of Dinosaurs of the genus 

 Claosaurus. The bones are generally petrified and occur also in 

 the dark concretions which also contain plant remains. Though 

 they are, as a" rule, excellently preserved, yet sometimes there is 

 what seems to be a good portion of a Dinosaur broken into myriads 

 of little fragments. The beds are probably either fresh or brackish 

 water. 



This formation was observed in several places in this region, and 

 in all there were bone fragments ; but we found no other equally 

 good exposures. About twenty- five miles to the southeast, in the 

 Lake Basin north of Columbus, the formation lying immediately 

 below the Fort Pierre in one place has a considerable thickness of 

 sandstone containing petrified logs, but only one or two small 

 fragments of bone were found. Some of the plants of this forma- 

 tion are related to Sequoia. The bivalve shells were so fragile as 

 to crumble with the soft matrix in which they were imbedded. 



Lying over these beds is a series of shales and hard laminated 

 sandstones. Some fossil leaves were seen in the latter. A series of 

 dark shales, perhaps thirty feet thick, was carefully examined. The 

 shales were full of carbonaceous plant fragments, and some fairly 

 good leaves were found in the thin interbedded layers of sand or 

 sandy concretions. I do not know whether these beds should be 

 put in this series or in the Fort Pierre. I think it better to consider 

 them, until they are more thoroughly explored, as belonging to the 

 Fish Creek series. 



Fort Pierre. 



Above the beds just described are the Fort Pierre shales. This 

 represents a well-distinguished horizon, so well marked by litholcgi- 

 cal characters and by characteristic fossils that its position is 

 beyond doubt. The description of the Pierre in Colorado, Wyom- 

 ing, etc., would answer almost equally well for the formation here. 

 Dark, soft shales predominate. There are occasional thin bands of 

 sand and many brownish concretions which break into angular 

 fragments. These sometimes contain marine fossils and sometimes 

 a network of calcite seams. The best preserved invertebrate fossils 

 are in these concretions. The shells are those oi Ammonites^ Bacu- 

 Hies, Scaphifes, Nautili, and small Gasteropods and Cephalopods. 



