190 GEOLOGY OF THE BLACK HILLS. 



They are curiously limited within the region of survey by a parallel 

 of latitude. The line of 44° 10' divides the uplifted area — as marked out 

 by the outcrop of the Dakota sandstone — into equal parts. South of the 

 line there is no volcanic peak either in the Hills area or in the vicinity. 

 North of it they dot the country in every direction. They do not seem to 

 bear toward one another any relation of distribution or intensity of igne- 

 ous action; and though the maximum effect is seen in Warren Peaks, the 

 other peaks in that region do not appear to hold toward them any closer 

 relation, except in position, tlian they do toward the other igneous peaks. 



The main points of eruption are as follows : within the area of the 

 Hills proper, Custer, Terry, and Crow Peaks and Black Butte ; northeast 

 of the Hills on the edge of the plain, Bear Butte ; in the Red Valley on 

 the northwest side of the Hills, Inyan Kara, Sun Dance Hills, a nameless 

 peak northeast of Inyan Kara, and Warren Peaks of the so-called Bear 

 Lodge range ; on the Belle Fourche, Mato Teepee or Bear Lodge ; and 

 near the head of Little Missouri River, the Little Missouri Buttes. Besides 

 these prominent peaks, all of which are indicated on the geological map, 

 there are many others, less conspicuous, in the northern part of the main 

 area of the Hills. Terry is the crowning peak of a group the remaining 

 members of which lie to the north and to the south of that point. In the 

 schist area at its northern end, east of Terry Peak, there are probably 

 several small points of igneous rock. To the east of Black Butte, along 

 the northeastern margin of the wooded area of the Hills and not far from the 

 inner edge of the Red Bed limestone, several small peaks supposed to be 

 igneous were seen from a distance. Farther to the northeast also there 

 were observed several exposures of igneous rock that had barely penetrated 

 or disturbed the sedimentary rock, and were only exposed by the denuda- 

 tion of the overlying formations. 



The rocks Avhich form these numerous extrusions have in general a 

 close resemblance in sti'ucture and appearance, and are sometimes almost 

 identical from several peaks. Specimens from nearly all the prominent peaks 

 have been critically examined by Mr. Caswell, who finds that they have a 

 common fades, although some fall into the division of rhyolite and others 

 into that of sanidin-trachvte. The resemblances are on the whole more 



