IGNEOUS INTRUSIONS IN THE NEIGHBORHOOD OF 
tHe BUENCK HIS .;OE DAKOTA. 
On the northern border of the Black Hills of Dakota, and 
situated partly in Wyoming, there are about a dozen hills of 
igneous rock, which not only add variety and beauty to the pic- 
turesque region where they occur, but are unique topographic 
features and furnish examples of a type of igneous intrusions 
that does not seem to have been clearly recognized. 
The hills referred to, are known in the general order of their 
occurrence from east to west, as Bear Butte, Custer Peak, Terry 
Peak, Black Butte, Crow Peak in South Dakota, Inyan Kara, the 
Sun Dance hills, Warren Peaks, Mato Tepee or Bear Lodge, and 
the Little Missouri buttes in Wyoming. 
Only a few of these hills have been examined by me, but 
such observations as I was able to make together with the des- 
criptions given by N. H. Winchellt and Henry Newton* of those 
not visited, show that they all have a common history and may 
be classed in a single group. 
Each of these hills owes its existence to the injection from 
beneath of a column of molten rock into stratified beds, and the 
subsequent removal of the enclosing sedimentary strata so as to 
expose, with one exception, the inner core of plutonic rock. 
They differ from the laccolites described by G. K. Gilbert3, in 
the fact that the molten rock did not spread out horizontally 
among the stratified beds so as to form ‘‘stone cisterns,” 
*Geological Report. In report of a reconnoissance of the Black Hills of 
Dakota made in the summer of 1874, by William Ludlow, Washington (Engineer 
Department, U.S. A.), 1875, 4to, pp. 21-66. 
2Geology of the Black Hills (edited by G. K. Gilbert). In report on the geol- 
ogy and resources of the Black Hills of Dakota, by Henry Newton and Walter P. 
Jenney, Washington (Department of the Interior), 1890, 4to, 1-222. 
3 Report on the Geology of the Henry Mountains; Washington (Department of the 
Interior), 1877, 4to, pp. x.-160, Plates I.-V. 
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