BLACK HILLS GEOLOGY. 249 



forming here and there a noticeable dark speck or needle-like 

 crystal. The groundmass which constitutes almost the whole 

 body of the rock is a light grayish green, having a vitreous 

 lustre, the color being due to the aegerine needles, and the lustre 

 to the innumerable, fractured quartz grains. 



The feldspar forms both phenocrysts and the smaller crystals of 

 the groundmass and is invariably automorphic. Most of the 

 crystals possess a kaolinized rim and none are twinned. The 

 cores of many of these feldspars consist of quartz. yEgirine- 

 augite occurs in sparsely disseminated crystals throughout the 

 groundmass, and seems to have preceded both the quartz and 

 the feldspar in the order of crystallization. yEgirine is present 

 throughout the groundmass in fine acicular crystals exhibiting 

 the customary frayed extremities characteristic of the rocks of 

 the tinguaite series. The absorption is green to nearly colorless 

 and the extinction is invariably parallel. At times these fine 

 needles penetrate the quartz grains and the feldspar, but for the 

 most part they fill the interstices. 



Quartz is confined to the groundmass of which it is the chief 

 constituent, the remainder being composed of small square 

 crystals of feldspar and small needles of aegirine. The quartz 

 is very clear, httle if at all fractured, and apparently is the latest 

 formed ingredient of the rock. 



Microcline, albite, microperthite and hornblende are absent, 

 the last mineral being characteristically lacking in this and most 

 of the other post-Cretaceous intrusives from this region. The 

 rock differs from Broegger's type grorudite in the absence of all 

 of the last named minerals, and but for the predominant aegi- 

 rine might better be classed with the soda rhyolites than the 

 grorudites. 



A silica determination of the type gave SiO^ /i-SS- 



2. Elk Mountain Type. 



Megascopic Appearance. — This rock is much coarser-grained 

 than that from Lost Camp creek, and is markedly porphyritic. 

 The phenocrysts are numerous, and for the most part are sani- 

 dine. They are very much decomposed, and replaced by sec- 



Annals N. Y. Acad. Sci., XII, November 23, 1899 — 16. 



