MEDIAEVAL OR MESOZOIC TIMES. 181 



Groups they now call the Colorado.* The Fox Hills Group, Dr. 

 Hayden's No. 5 becomes then, with this division, No. 3. 



The Dakota Group. 



This was so named by Hayden because of its great development 

 southwest from Dakota City in Dakota County. Beginning from 

 below, it consists in the main of a whitish clav frcm a few inches 

 to four feet in thickness, then various thicknesses of conglomerate 

 and concretionary sandstone averaging from one to ten feet; next 

 yellowish coarse sandstone from fifteen feet and upwards; and next 

 a red hard ferruginous sandstone containing impressions of plants, 

 leaves, wood, etc., from thirty to seventy feet in thickness. 



Extent of the Dakota Group Deposits. — The Dakota Group 

 towards the west extends under the Fort Benton and Niobrara 

 Groups and therefore its real breadth cannot be ascertained. I 

 have traced it, however, from east to west over a breadth of from 

 sixty to ninety miles. In the States of Iowa and Kansas Lesque- 

 reux estimates its breadth as slightly greater. Its eastern boundary 

 is that of the Cretaceous and can be seen in the accompanying geo- 

 logical map of the State. It is mainly found in the following coun- 

 ties: Dakota, Wayne, Winnebago and Omaha reservation, Burt, 

 Washington, Cuming, Stanton, Colfax, Dodge, Douglas, Sarpy, 

 Saunders, Butler, Seward, Lancaster, Cass, Gage, Jefferson, 

 Saline, and occasionally in the counties bordering on these. South- 

 westerly it has been traced to Texas. It crops out in numerous 

 places as the basal member of the cretaceous series in the mountains. 

 It covers a large part of northwestern Iowa, and extends towards 

 the northern limits of Minnesota. There are evidences of its 

 presence in British America. Prof. Heer has also described fossil 

 leaves from Greenland, some of whose genera and species are ident- 

 ical with those from the Dakota Group, and therefore it is probable 

 that it has been continuous, as Lesquereux remarks, from the Gulf 

 of Mexico to Greenland and other Arctic lands, or over thirty-five 

 degrees of latitude. 



Origin of the Dakota Group. — We have already seen that during, 

 at least the lower Cretaceous, Nebraska, with a large part of the 

 Rocky Mountain region was an extended land surface in process of 

 slow subsidence. By the time the middle Cretaceous began, this 

 subsidence had reached so low a level as to admit the Gulf of 



*llavden considers the Fort Pierre Group from its organic remains most closely allied to 

 the Fox Hills. 



