BLACK HILLS GEOLOGY. 267 



The Third Variety of Pyroxene. — The third division of segirine 

 is exceedingly interesting, as it is present in almost every type 

 of the series from the rocks at the basic end to the exceedingly 

 siliceous quartz-segirite-porphyries. It comprises those fine hair- 

 like needles that may properly be termed microlites. These 

 needles penetrate the feldspar phenocrysts, the quartz in the 

 quartz-aegirite-porphyries, and the feldspar of the groundmass. 

 They are not, however, found as inclusions in the large aegirines, 

 in the nepheline or the nosean and titanites, nor do they ever 

 form parallel growths with the larger crystals. Hence it can be 

 inferred that their period of formation followed that of the larger 

 aegirines, and preceded the period of the feldspar phenocrysts. 



Nepheline. — This mineral occurs as idiomorphic crystals in 

 the majority of the tinguaites. It reaches the highest degree of 

 development in the phonolites proper, and in the trachytoid va- 

 rieties occurs only as interstitial masses. 



In the phonolite from Spearfish Peak (Black Butte) it has at- 

 tained a remarkable perfection. Sections show hexagonal 

 cross-sections, dark during the rotation of the stage and square 

 sections with parallel extinction and low single and double re- 

 fraction. An analysis of the rock will undoubtedly show a re- 

 markable amount of soda. The crystals show great numbers 

 of inclusions, most of them highly refractive, but too small for 

 determination. For this rock Caswell ^ gives : 



SiO^, 56.32 ^; soluble in HCl with very strong gelatiniza- 

 tion, 24.08 /6; which shows how large is the percentage of ne- 

 pheline. Again in the rock above Maurice the nephelines are 

 in great abundance. The rock is different from that from Black 

 Butte in its bluish color, its almost complete lack of greasy 

 lustre, and the presence of macroscopic sanidine phenocrysts. 

 Under the microscope also it shows the segirine in bundles and 

 shredded crystals instead of the irregular masses characteristic 

 of the Black Butte rock. Nosean is also abundant. The 

 nepheline, however, except that the crystals are smaller, bears 

 the same relations to the aegirines and feldspar of the rock. 



The rock from the bottom of Badger Shaft exactly resem- 



1 Rep. GeoL and Resources of the Black Hills of Dakota. LT. S. G. G. Survey. 

 1880. p 526. 



