240 GEOLOGY. 



wholly fluviatile, 1 and even as eolian. 2 The eolian origin has been 

 urged on the basis of the fossils, which are chiefly those of hind animals 

 (land tortoises and mammals),' but while much may be said for this 

 hypothesis as applied to parts of the formation, it does not seem appli- 

 cable to all of it, as the constitution of the beds shows. Gypsum, barite, 

 etc., in the series give some hint of the climatic conditions of the time. 

 In the light of present knowledge, it seems probable that all phases of 

 land aggradation, lacustrine, fluvial, and eolian, are represented in the 



Fig. 436. — Chimney Rock, a detail in the Bad Lands of the White River country. 

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



series. The formation is said to have originally covered most of the 

 Black Hills region, and possibly all of it, 3 for remnants are now found 

 up to elevations of more than 6000 feet, and the highest points of 

 the Hills are but little higher; but in so far as running water and wind 

 were concerned in its deposition, the present altitude and attitude 

 of the beds cannot be relied on as a measure of former extension or 

 later deformation. 



In these and other comparable formations, well-defined bedding has 

 often been relied on as conclusive evidence of lacustrine origin; but it 

 should be remembered that eolian sand is often as distinctly stratified 

 as that which is deposited by water (Fig. 437). The stratification 



1 Fraas, Science, Vol. 14, N. S., p. 212, holds that the earlier White River beds 

 were fluviatile, and that later ones were lacustrine. 



2 Matthew, Am. Nat., Vol. XXXIII, p. 403, 1899. • 



3 Darton, 19th Ann. Rept. U. S. Geol. Surv., Ft. IV; 21st Ann. Rept. U. S. Geol 

 Surv., II. 



