TEEEY PEAK. . 193 



The margin of the plateau west of Custer Peak is exceedingly broken and 

 the composing rocks are not well exposed. A mile or so directly south, 

 however, outliers of the Carboniferous limestone were seen in their hori- 

 zontal position occupying the summits of conspicuous bluffs, and immedi- 

 ately west of the peak, within a short distance of its base, are small frag- 

 mentary outcrops of the Carboniferous, undisturbed and unchanged. East 

 of the peak and within a half mile of its base there are outcrops of the 

 Potsdam along the headwate;s of Box Elder Creek, showing the usual 

 character of the formation, lying nearly horizontal, and so far as observed 

 entirely unaffected by proximit} 7 to the igneous peak. 



Terry Peak is the crowning point of an igneous region of considerable 

 extent, having a maximum development in a northwest and southeast direc- 

 tion. The region is deserving of careful study, but because of the great 

 number of the points of exposure and the very rugged nature of the country 

 it was impossible to wholly examine it even superficially in the time at our 

 disposal. The peak stands upon the edge of the plateau ; the Potsdam is 

 extensively developed on its eastern side, and with it are occasional outliers 

 of the Carboniferous. Tts sharply pointed summit is the most conspicuous 

 landmark in the northern region of the Hills and is visible from the Plains 

 far to the north and east. Its altitude, by mercurial barometer, is 7,230 

 feet, and there are but two summits in the Hills that outrank it. It rises 

 fully 1,000 feet from its base. 



The igneous group or range of which Terry is the highest point begins 

 about three miles to the southeast in a cluster of peaks called the Deer 

 Mountains, and there is a nearly or perhaps quite continuous chain of 

 trachytic and rhyolitic outcrops all the way to Terry. The same line con- 

 tinues northward for a few miles beyond Terry. 



The disturbance of the sedimentary strata is exceedingly local and 

 confined to the immediate vicinity of the peaks themselves. Directly east 

 of Terry Peak, and almost at its base, the Carboniferous limestone lies 

 undisturbed, while against the ridge that culminates in the peak the Pots- 

 dam, much metamorphosed, was found at several places inclining at a 

 high angle. The sides of the ridge and of the peak are for the most part 

 covered by a talus of angular and slippery fragments of the rock, On the 

 13 B H 



