51 R 



GEOLOGY OF THE BLACK HILLS. 



rock is \'ery similar to [124] from Terry Peak, although this has larger 

 hornblendes and an abundance of magnetite, which is lacking in the rock in 

 question. The resemblance between the two hand specimens is not so 

 marked, [124] having large, porphyritic sanidin and hornblende crystals, 

 while [171] is more uniform and compact. There is a similarity in struc- 

 ture to [123] and [159], the small hornblende prisms in the two latter, 

 however, being much more numerous and the rocks in a fresher condition. 



The sanidin-trachyte [173] from southwest of Warren Peaks is light- 

 gra}' in color, and quite compact and uniform in structure. It has an 

 occasional large, glassy sanidin, while the entire rock is thickly besprinkled 

 with tiny, green hornblende crystals, easily seen with a loupe. Under the 

 microscope, the section appears to consist entirely of sanidin, partly in 

 well defined, clear crystals and partly in a cloudy mass, scattered through 

 which are seen the gi-een hornblendes. 



The clear sanidins present a new feature as yet not observed in any of 

 these rocks, namely, inclusions of small, rounded, light-brown glass masses. 

 These particles do not polarize at all and are abundant in almost every 

 crystal, having sometimes hexagonal and rectangular forms. They are also 

 partially transparent. It is interesting to note that the single macroscopical 

 sanidin cr3'stal in the hand specimen contains near its center a light yellow- 

 ish-brown, spherical inclusion about the size of a small mustard seed, 

 undoubtedly of the same character as the glass masses observed in the 

 slide, which unfortunately does not happen to have a large sanidin in it. 

 There are also a few transparent hornblende microlites in the sanidin. The 

 hornblende crystals are of a fine green color, slightly dichroitic and pris- 

 matic in shape, somewhat similar to those in the preceding rock, but broader 

 and more sharply terminated. 



The groundmass, if any can be distinguished between the sanidin 

 crystals, is cloudy and thick, but more granular than microlitic and having 

 but very slight traces of any fluid-like structure. The rock is similar to 

 [171], but not so much weathered, both the hornblende and sanidin being 

 sharper and more crystalline in form. 



The rock [174] from north of Warren Peaks is quite interesting and 

 different from any of the other specimens and rather difficult to determine. 



-II 



