194 GEOLOGY OF TUE liLACK HILLS. 



summit of" the peak the rock is well exposed, and three or four ridges of 

 outcrop aj)pear to radiate a few hundred feet from the central crest, while 

 between them tlie slopes are composed of weathered frag-ments. The rock 

 of the sunmiit has a greenish gray color, and contains many conspicuous 

 crystals of sanidin. In liand specimens it appears like a feldspathic por- 

 phyry, but wliat seems to the unaided eye to be a groundmass is shown 

 by the microscope to be crystalline and to carry a large amount of quartz 

 The percentage of silica is so large that Mr. Caswell classes tlie rock with 

 the rhyolites. 



About a mile and a half to the eastward there is a dike of greenstone 

 running nortli and south for some distance, cutting through tlie sedimentary 

 rocks. It is probably connected wdtli a small peak northeast of Terry 

 which was reported to be composed of greenstone. 



Near the base of Deer Mountains a rhyolitic rock was observed over- 

 lying quartzite that was thought to belong to the Potsdam. This may be 

 either an overflow or an injection between strata, and it is the only case in 

 which the igneous and sedimentary rocks were seen in this relation. In 

 several places in the canons contiguous to the igneous masses the Potsdam 

 sandstone — here the best exposed of -the sedimentary series — was found 

 highly altered by the igneous rocks, which in many places penetrate it. It 

 is frequently altered into a hard brownish rock with crystals of feldspar 

 irregularly shot through the mass. Sometimes it is a hard granular rock 

 with a conchoidal fracture, and in places the conglomerate is so hardened 

 that it breaks as readily through the composing pebbles as in any other 

 direction. 



The igneous rocks of this region are varied in their appearance and 

 texture. 



Crow Peak is a pustular outbreak of volcanic rock through the Red 

 Bed limestone twelve miles northwest from Terry Peak. Though it does 

 not rank as one of the highest points of the Hills, it springs so abruptly 

 from its immediate surroundings as to make it locally a very conspicuous 

 point. Its approximate height above the Red Valley in its vicinity is 1,500 

 feet. As seen from the east or west it appears to be comi)osed of two peaks 

 closely unitf^d ; the southern one is the rliyolite core, while the northern 



