Gress: Fossil Plants of the Dakota. 279 



Some marine fossils are found in the uppermost parts of the forma- 

 tion. The four epochs of the Upper Cretaceous period of the Rocky 

 Mountain region seem to be sufficient proof of submergence followed 

 by emergence, the Dakota being chiefly a fresh-water formation, the 

 Colorado and Montana salt-water formations, and the Laramie, the 

 western coal epoch, a fresh-water formation. 



In discussing the " Dakota Formation," Chamberlain and Salisbury 

 ("College Text-book of Geology," 1909) say: "The formation was 

 formerly regarded as lacustrine, but it is perhaps to be regarded 

 rather as the joint product of subaerial and fluviatile deposition. The 

 presence of bird tracks in Kansas, and the widespread abundance of 

 fossil leaves of angiosperms, in a condition which precludes much 

 transportation, imply subaerial sedimentation to a notable extent at 

 least." For the deposition of the Dakota Group in Iowa, see Calvin, 

 Iowa Geological Survey, i. 1892, p. 132. 



Since the fossil leaves described in the following pages were col- 

 lected chiefly in Kansas (Ellsworth County and Fort Harker),- we 

 give here a more detailed account of the location of the Dakota Group 

 in that State. 



The central, northern, and western parts of the State of Kansas 

 are underlain by the Dakota sandstone. The depth varies from about 

 1,000 ft. in the central northwestern counties to probably 2,500 ft. in 

 the northwest counties. In Ellsworth County, near the central part 

 of the State, and the place from which most of our fossil leaves have 

 been collected, the sandstone lies at or near the surface, except in the 

 deeper parts of Smoky Hill Valley, in the southeastern part of the 

 county, where the underlying Permian rocks are exposed. 



It varies from a few feet to about 600 ft. in thickness, and outcrops 

 in a zone from about 12 to 30 miles in width, extending from Wash- 

 ington County in a southwesterly direction to the Arkansas River in 

 Rice and Benton counties. From here it extends in an easterly direc- 

 tion up the Arkansas River to Ford County, where it is covered by 

 Tertiary deposits. 



It again appears near the Colorado State line along the valleys of 

 the Cimarron River and some of its branches. In northwestern Kan- 

 sas the " Dakota Formation " is underneath the Benton, Niobrara, 

 Pierre, and Tertiary formations. It probably lies 2,000 ft. below the 

 surface in the extreme northwestern corner. In the north-central 



ANN. CAR, MUS., XIII. 19, FEB. 21, 1922. 



