IGINE OTS INTROSIONS JN THE BLACK HILLS Bi 
larger dimension has a bearing of about 30° West of North and 
upon which the rock is well exposed. It is a hard, highly feld- 
spathic trachyte, and on weathered surfaces large and well 
formed crystals of feldspar were seen in great abundance, giv- 
ing the weathered mass a porphyritic appearance. Its mass. 
is notably magnetic. The rock shows well marked cleavage 
or jointing planes, nearly vertical, in two series. The first runs. 
toward the northwest and the second towards the west, divid- 
ing the rock into prisms and producing a quasi-columnar 
structure. 
Though on such a large scale, the entire upheaval being prob- 
ably two or three miles in diameter, the peak has essentially the 
structure of the Sun Dance hills. Among the uplifted beds sur- 
rounding Inyan Kara no strata were recognized excepting Red 
Bed limestone and the underlying, impure, reddish sandstone, 
and beyond the immediate base of the outer slope no disturbance 
was indicated. Indeed, the red arenaceous clay is too nearly 
structureless to retain readily such evidence. 
The high angle at which the stratified rocks surrounding the 
base of Inyan Kara dip away in all directions from the central 
core, shows that there was more disturbance caused by this. 
intrusion than in any of the similar examples previously described 
in this paper. The igneous rock was also forced to a higher 
level than in the Sun Dance hills, and was greater in mass than 
in any of the similar hills in the neighboring part of Wyoming, 
with the exception of Mount Warren. 
Complete as is the exposure of the igneous rock in Inyan 
Kara, it is to be remarked that no expansion of the central plug 
so as to form a coulee or a laccolite, is mentioned by the geolo- 
gists who have examined and described it. 
Bear Butte—On the northeast side of the Black Hills and 
about six miles east of the town of Whitewood, there is another 
conspicuous butte, similar in many ways to Inyan Kara. This 
hill, known as Bear Butte, rises from Middle Cretaceous shales 
but is surrounded at its immediate base by older rocks which 
dip away in all directions. It has been described by F. V- 
