180 



GEOLOGY. 



Divisions. 



g 



M 

 O 

 ft 



P O 



5 fe 



« 



O 



Lead gray calcareous marl, weathering 

 to a yellowish or whitish chalky appear- 

 ance, containing many large scales of fish- 

 es and many ostrea congesta attached to 

 fragments of Inoceramus. Passing down 

 into yellowish and whitish limestone con- 

 taining many Inoceramus problematicus, 

 ostrea congesta, etc. 



Localities. 



Bluffs along the Mis- 

 souri below the Great 

 Bend, greatly devel- 

 oped below the mouth 

 of the Niobrara, and 

 on to Dakota County 

 along the Missouri. 

 West of this line ex- 

 tends an underlying 

 rock to Kansas. Most 

 extensive group of 

 cretaceous rocks in 

 Nebraska. Maximum 

 thickness 200 feet. 





K < 



Dark gray laminated clays, sometimes 

 alternating near the upper part with seams 

 and layers of soft gray and light colored 

 limestone. Many chambered shells and 

 other marine molluscan forms. 



Fort Benton on the 

 upper Missouri, and 

 along the latter from 

 ten miles above James 

 River to the Big 

 Sioux, Black Hills. 

 Found in Nebraska 

 beneath the Niobrara 

 Group, but rarely the 

 surface rock. Maxi- 

 mum thickness, 800 

 feet. 



p 

 26 



H g 

 O £ 



Yellowish, reddish, and occasionally 

 white sandstone with occasional alterna- 

 tions of various colored clays and beds and 

 seams of impure lignite. Also, silicifled 

 wood and casts of marine mollusks. Many 

 remains of the higher types of dicotyledo- 

 nous leaves from tree forms. 



Back of Dakota and 

 in surrounding coun- 

 try. Thence south- 

 westward into Kan- 

 sas and beyond. Max- 

 imum thickness, 400 

 feet. 



These groups are readily separated and distinguished on the up- 

 per Missouri and through Nebraska, except along the Republican 

 River. Had I first studied the Cretaceous on the Republican, 

 Meek and Hayden^s divisions would have appeared inapplicable as 

 these groups there shade into each other. The geologist, however, 

 who first studies the rocks of this period on the Missouri cannot 

 well deny the validity of this grouping. Since the rocks of this 

 period have been studied in the mountains by the United States 

 Surveys, two of the chiefs, Clarence King and Dr. Haydcn, have 

 agreed on a slightly different division. They retain No i, or the 

 Dakota Group as the basal member of the series. The next thrce ? 

 however, namely, the Fort Benton, Niobrara, and Fort Pierre 



