IGNEOUS INTRUSIONS IN THE BLACK AILLS 27, 
colored, compact and usually coarsely crystalline; and contains 
prominent crystals of sanadine, quartz, biotite and hornblende. 
In the hills examined by the writer, namely, the Sun Dance 
hills, Mato Teepee and the Little Missouri buttes, the rock 
appeared to be of the same character in each instance. In hand 
specimens it was impossible to distinguish any essential differ- 
ence. For a more exact description of the rocks in question 
the reader is referred to Mr. Caswell’s report which contains the 
results of the only systematic study of them that has been made. 
The stratified beds through which the plugs have been 
forced, vary in age from the Potsdam to the middle of the Cre- 
taceous. Ina conglomerate discovered by W. P. Jenney’ at the 
base of the Miocene in the neighborhood of the Black Hills, 
pebbles of igneous rock were found which must have been 
derived from the hills now claiming our attention. The date at 
which the plugs were formed is therefore somewhere between 
the Middle Cretaceous (Fort Pierre group), which was disturbed 
and altered by their intrusions; and the base of the Miocene, at 
which date they were exposed to erosion and contributed pebbles 
to neighboring streams. 
In discussing the date of the origin of these rocks, Newton? 
observes that the interval mentioned above, witnessed the depo- 
sition of the Niobrara, Fort Pierre, Fox Hill, Judith River and 
Fort Union terrains, which represent a total depth, in the upper 
Missouri region, of about 4000 feet of sedimentation. The date 
of the igneous activity is therefore very far from being definitely 
established, and its relation to the main Black Hills uplift is not 
determined. The igneous rocks may have been in place and 
even ancient when the elevation of the Black Hills began, or 
they may have been forced up while the greater movement was 
in progress. 
With this introduction, such facts of geographical and geo- 
logical interest as are available concerning these remarkable 
intrusions will be presented. 
* Geology of the Black Hills, p. 220. 
G20 220t 
