LIFE 575 



with the Eocene is close, 1 and its separation from the Miocene 

 distinct. This is in keeping with the phenomena of the Gulf 

 States. Limestone is the dominant rock in the Antillean region. 



The Oligocene stage is also recognized among the terrestrial 

 deposits of the western part of the continent. The White River 

 formation, now classed as Oligocene, occupies an extensive area in 

 northeastern Colorado, southwestern Wyoming, western Nebraska 

 (Untie and Chadron formations), and South Dakota, and perhaps 

 in Kansas. In the light of present knowledge, it seems probable 

 that all phases of land aggradation, lacustrine, fluvial, and eolian, 



Fig. 483. Chimney Rock, a detail in the Bad Lands of the White River coun- 

 try. The base of the column is Brule clay. (Darton, U. S. Geol. Surv.) 



are represented in this formation. 2 Even thin beds and lenses of 

 limestone and volcanic ash enter into it. The formation is said 

 originally to have covered most of the Black Hills region, and pos- 

 sibly all of it. 3 Remnants are found up to elevations of more than 

 6,000 feet, and the highest points of the hills are but little higher. 

 The Florissant beds in South Park, Colorado, consisting largely of 

 volcanic ash, and famous for their fossil insects, are classed as 

 Oligocene. So also are some of the beds of the John Day Basin of 

 Oregon, unconformable above the Eocene. Marine Oligocene beds 



1 Hill, Geology and Physical Geography of Jamaica, and Geological History 

 of the Isthmus of Panama and portions of Costa Rica. Bull., Mus. Comp. Zool., 

 Vols. XXVIII and XXXIV respectively. 



2 Fraas, Sconce, Vol. XIV, N. S., p. 212, and Matthew, Am. Nat., Vol. 

 XXXIII, p. 403, 1899. 



8 Darton, igth Ann. Kept., U. S. Geol. Surv., Pt. IV; 2ist Ann. Rept., U. S. 

 Geol. Surv., II. 



