- 
about the head of the Missouri, &c. 231 
I, Granrre, Stratiriep Azoic, anp Eruprrve Rocks. 
showing the wide geographical area under which the elevating 
forces acted. 
_ Tallude in the first place to the Black Hills, the northern por- 
tion of which we examined on our route from Fort Pierre on the 
Missouri to the Yellow Stone river. These Hills form the most 
eastern outlier of the Rocky Mountains and would seem to be an 
independent elevation were it not for alow anticlinal which ex- 
tends across the plain country southward connecting it with the 
waramie Mountains. The central portion is composed of a coarse 
flesh colored feldspathic granite with a series of metamorphic 
slates and schists superimposed, and thence upon each side of 
the axis of clevation, the various fossiliferous formations of this 
Tegion follow in their order, to the summits of the Cretaceous, 
the whole being more or less inclined against the granitic rocks. 
The distance across the granitoid nucleus, is from fifteen to thirty 
miles and on each side of the crest or axis of elevation we find 
the corresponding portions of the fossiliferous beds from the Si- 
lurian to the summit of the Cretaceous. The evidence therefore, 
18 conclusive that all the unchanged sedimentary strata at a peri- 
od of comparatively recent date extended continuously over the 
Whole area occupied by the Black Hills. The eruptive rocks 
reveal themselves at various localities as at Bear Peak, Inyan- 
kara Peak, &. Bear Peak is a protrusion of very compact 
See rocks, almost isolated from the main ran ge of the Black 
rtant detached o stern | 
continent. This seems to trend nea 
mentary strata to the summit of the Cre- 
and including a portion of the Lignite Tertiary can be 
