146 



GEOLOGY. 



sition, for deposits of this class furnish their own adjustments. The 

 presence of bird tracks in the Dakota of Kansas l and the preserva- 

 tion of some 500 species of plant fossils, mostly the leaves of angio- 

 sperms, at various points and in conditions which forbid much trans- 

 portation, imply the prevalence of subaerial conditions to a notable 

 extent at least. 



Fig. 392. — A Dakota " hog back." The rock at the left is the Red beds; the ridge 

 near the center is occasioned by the outcrop of the resistant Dakota sandstone. 

 Near Boulder, Colo. (Lees.) 



The thickness of the formation is, on the whole, rather uniform, 

 averaging perhaps 200 or 300 feet, though greater thicknesses are known. 2 

 To the south (Texas), the Dakota formation rests on the Comanchean 

 system unconformably. Farther north it is often in apparent con- 

 formity with the Comanchean, though it often, as in the Wasatch 

 and Uinta Mountains, rests on older formations. 



1 Williston, Univ. of Kans. Geol. Surv., Vol. IV, p. 50. 



2 Darton, 19th Ann. Rept. U. S. Geol. Surv., Pt. IV, and Knight, Bull. 45, Wyo. 

 Experiment Station. 



