IGNEOUS INTRUSIONS IN THE BLACK HILLS 39 
rhyolite core, while the northern one is a portion of a rampart of 
sedimentary beds which once entirely surrounded it. This hill, 
like the others in the series to which it belongs, owes its exist- 
ence, if the present writer’s views are correct, to the intrusion of 
a plug of igneous rock vertically upward through nearly hori- 
zontal sedimentary beds and the subsequent exposure of the 
intrusive rock by erosion. 
Terry and Custer Peaks. 
marks in the northern portion of the Black Hills. They have 
been described by Newton," as pustular outbreaks of igneous 
matter and belong to the series of intrusions to which attention 
is here directed, but so far as one can judge from the published 
descriptions, they offer no important features not already 
These are other prominent land- 
noticed in this paper. 
Warren Peak.—This is the largest and in fact the only truly 
mountainous mass of igneous rock on the outskirts of the Black 
Hills, and may differ materially in the mode of its formation 
from the plugs of crystalline rock we have already noticed. Its 
broad extent and the manner in which the surrounding stratified 
rocks dip away from it in all directions, seems to indicate that it 
is a true laccolite, very similar to those of the Henry Mountains. 
The description given below is copied from Newton.’ 
Warren peaks are the crowning points of the “ Bear Lodge 
Range,” an elevated, broken plateau between the Redwater 
valley and the Belle Fourche. The peaks are not remarkably 
prominent, but their total elevation above the sea, 6830 feet, is 
equal to that of some of the principal peaks in the central por- 
tion of the Black Hills, while their height above the Red Valley 
immediately south, is about 1800 feet. The trachytic area to 
which they belong is the largest of the whole group, and covers 
fifteen or twenty square miles. Around this the strata of the 
sedimentary series are uplifted and arranged in concentric 
circling outcrops, so as to make a sort of miniature copy of the 
hills. The trachytic nucleus has an extension from northeast to 
* Geology of the Black Hills, pp. 192-194. 
? Geology of the Black Hills, pp. 199-200. 
