402 



GEOLOGY OF THE BLACK HTLLS. 



optical position. Sometimes a little glass mass is seen between the niicro- 

 lites, but very sparing in quantity. 



Sanidin-oiigoclase-tracliyte, also characterized by its lack of quartz, 

 consists of crystallized sanidin and oligodase, with biotite and hornldende 

 as accessory minerals. The entirely feldspathic groundmass is composed 

 of a fine-grained or crystalline aggregation of these minerals, in which 

 cr}'stals of the same minerals are imbedded. The oligoclase can ])e dis- 

 tinguished from the sanidin b}' its twin lamellations, and is often present in 

 greater abundance than the sanidin. 



The Drachenfels trachyte is a rock of this class, and .some from the 

 Black Hills [144], [149], [151], and [157] (Plate II, Fig. 2), are of a simi- 

 lar character and may be regarded as the most typical of those thus far 

 examined. 



Phokolite. — Phonolite is a compact, dense rock, standing in appear- 

 ance between trachyte and basalt, from which latter it differs in its lighter 

 color, lower specific gravity, schistose structure, lack of olivine and the 

 constant presence of sanidin, all of which characteristics place it nearer 

 trachyte than basalt. Phonolite has a more compact groundmass than 

 trachyte, and is also free from quartz. 



The microscope has proved what the chemical analyses have always 

 indicated, namely, the presence of nephelite as an essential constituent of 

 all phonolites, and of nosite in nearly all. This occurrence of nephelite 

 satisfactorily explains the large percentage of soda in the portion of rock 

 soluble in hydrochloric acid, as well as the frequent presence of zeolites as 

 products of decomposition in the cavities and cracks. 



Phonolite consists, then, of sanidin, hornblende, and nephelite, some- 

 times nosite, in a compact or crystalline groundmass. The sanidin crystals 

 are generally sharp in outline, except when the rock is partially decom- 

 posed, in which case the formation of zeolitic minerals affects them also, so 

 that they lose their form and become filled with a fine fibrous mass. They 

 include minute, pellucid crystals of nephelite, generally in a row near the 

 edge of the crystal, and also exceedingly small, almost colorless born- 

 blende microlites. 



Nephelite is found in all true phonolites in great abundance in the 



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II 



