BLACK HILLS GEOLOGY. 263 



As we pass from the phonolites to the trachytoid phonolites 

 the groundmass becomes coarser and of a Hghter hue. 



The feldspar phenocrysts of the phonolites as a rule contain 

 many inclusions; aegirine-augites, in short stumpy crystals, fine 

 needles of aegirine that seem to be among the earliest formed 

 ingredients of the rock, titanite and nosean. The aegirine 

 needles are scattered through the feldspar sometimes in great 

 abundance, so that, when viewed between crossed nicols, the 

 darkened mineral is penetrated by a network of lighter lines 

 made by the differently oriented needles of aegirine. This is 

 especially noticeable in the rock from Raum's Drill. These 

 needles are more thickly developed near the border of the 

 crystal and frequently leave the cores quite clear. The in- 

 cluded noseans are often large and almost as frequent in the 

 feldspar as elsewhere in the groundmass. 



The high percentage of soda in relation to potash in almost 

 all of these rocks together with the investigations of L. V. 

 Pirrson on rock from the Devil's Tower, in which the pheno- 

 crysts were found to be soda-orthoclase, have led the writer to 

 believe that much of the feldspar in these rocks is of the same 

 character. Many of the phenocrysts show a core of microcline 

 which gradually shades into a clear, unstriated border of san- 

 idine. Even the freshest sanidines show an exceedingly fine, 

 longitudinal striation, which may perhaps be due to a very fine, 

 almost sub-microscopic twinning arising from the triclinic char- 

 acter of the mineral. 



The feldspars of the groundmass fall into two divisions : 

 those which are long, lath-shaped and almost automorphic, 

 and those which present an irregular jagged outline, and make 

 up a groundmass almost granitoid in appearance. The former 

 are characteristic chiefly of the trachytoid variety of phonolite. 

 The rocks from Squaw Creek and Sugar Loaf Hill are good 

 instances of this. The feldspars are sanidines, generally 

 twinned, and with typical structure ; sometimes entirely without 

 definite orientation, impinging one upon the other in slightly 

 irregular lines so as to form a hypidiomorphic texture. Quite 

 frequently one may observe, scattered among them, laths of an 



