BLACK HILLS GEOLOGY. 259 



All of the rock types are here intrusive, and yet the tex- 

 ture in many cases dilTers in no essential degree from that 

 observed in Rosenbusch's typical phonolites. Instances are, 

 the rock from Ragged Top mountain, that from the sheet 

 above Maurice, and the rock described by Caswell from Spear- 

 fish peak. 



It is true that the texture known as tinguaitic occurs in almost 

 all of the smaller dikes and masses of phonolitic rocks, but 

 these are as frequently porphyritic as otherwise, the tinguaitic 

 texture being confined to the groundmass. Phenocrysts of very 

 large size often occur, some of them attaining the length of 

 one inch. Instances are the dike near the Rua mine and 

 the dike on the divide west of Twin Peaks and many others. 



Again tinguaitic texture is not restricted to these smaller 

 masses. Instances are rocks which make sheets of extremely 

 large size. Such is the large mass northwest of the town of 

 Englewood. 



The name tinguaite has, therefore, been arbitrarily used in this 

 paper to designate those rocks which possess a fine ground- 

 mass of interlocking needles of segirine, with sanidine and more 

 or less nepheline. Phenocrysts of sanidine and a;girine-augite 

 may or may not be present. 



Phonolites. 



The phonolites proper are, as here described, those in which 

 nepheline occurs in automorphic crystals, which may be read- 

 ily distinguished by the microscope. They contain aegirine- 

 augite, aegirine, orthoclase, nosean, and the accessories de- 

 scribed below. 



They differ from the tinguaite, into which they pass by in- 

 perceptible gradations, in the possession of much automorphic 

 nepheline, and in not having so marked an interlocking series 

 of smaller aegirines. On the other hand, they differ from the 

 trachytoid varieties in the absence of the trachytic arrangement 

 of feldspar in the groundmass, and in the possession of much 

 nepheline. The transition in this direction is also gradual. 



