IGNEOUS INTRUSIONS IN THE BLACK HILLS 41 
water, or westward to the Belle Fourche. On the south and 
east, facing Sun Dance hills, the Red Bed limestone forms the 
outer slopes, dipping under the red clay of the Redwater Valley, 
On the north and west, however, the Jura is well exposed and 
capping this the Dakota sandstone. 
Though on a grander scale and exposing a larger area of 
igneous rock than the other neighboring centers of intrusion, the 
Warren peaks show the same character of pustular eruption. 
In a recent paper on the geology of the Black Hills, W. O. 
Crosby,’ has devoted a few pages to the consideration of the 
igneous intrusion described in this paper, in which it is claimed 
that they are true laccolites. He also presents reasons for con- 
b 
cluding that the hypothesis of ‘‘ Pustular Eruptions,” advanced 
by Newton is not warranted by the facts. 
As I am unfamiliar from close inspection with the larger 
igneous masses of which Warren, Terry and Custer peaks are lead- 
ing examples, I am unable to offer an opinion as to whether they 
are laccolites or not. Their great size and the manner in which 
the surrounding sedimentary beds have been disturbed and 
altered in their vicinity, certainly seem to indicate that such 
was their mode of origin. In the case of the Sun Dance hills, 
Mato Teepee and the Little Missouri buttes, however, which I have 
examined and also in the case of Inyan Kara and Bear Butte, 
which I have seen from a distance, and of which detailed descrip- 
tions have been published, the evidence does not sustain the 
assumption that they are laccolites. 
The absence of volcanic débris except in the immediate vicin- 
ity of these hills, indicates that the intruded rock did not spread 
widely among the stratified beds or overflow the surface. This, 
as well as the fact that in a series of examples ranging through 
all degrees of erosion from the unbroken dome of Little Sun 
Dance hill to Mato Teepee with its majestic tower over 600 
feet high, and Bear butte, 1200 feet high, neither of which 
exhibit evidence of laccolitic expansion, seems sufficient to prove 
** Geology of the Black Hills of Dakota.” Boston Soc. Nat. Hist., Proc., Vol. 
XXIII. pp. 488-517. 
