BLACK HILLS GEOLOGY. 261 



pass to the more trachytoid varieties. This peculiar lustre has 

 been supposed to arise from the presence of nepheline, but, while 

 the Black Butte phonolite in which it is most strongly marked 

 contains a remarkable amount of this mineral, many of the more 

 bluish varieties in which the microscope has shown great quan- 

 tities do not exhibit it in the smallest degree. The dense tingu- 

 aitic varieties, however, show it frequently, and from this it seems 

 probable that it arises from the presence of nepheline in the 

 groundmass rather than from the crystals which may be identi- 

 fied by the microscope. If this be so it will be of great service 

 in the determination of this mineral, for, unless observed in 

 automorphic crystals it[is practically impossible to establish its 

 presence without resort to the test of gelatinization. 



As we pass from the tinguaites and phonolites, into the 

 phonolitic trachytes, we encounter a much lighter colored 

 series of rocks, most of them being of a coarse porphyritic 

 texture, and showing large crystals of sanidine of i^ inch and 

 more in diameter. The groundmass frequently becomes incon- 

 spicuous, as in the rock from Raum's Drill and the " Spook " 

 shaft near Balmoral (the peripheral phase of the Ragged Top 

 mass) where the large phenocrysts are crowded so closely to- 

 gether as to comprise almost the whole body of the rock. The 

 groundmass is, however, present, and in it are embedded large 

 crystals of segirine-augite, whose octagonal cross section is fre- 

 quently to be marked without the aid of a glass. 



These pyroxenes attain a size of ^ inch in length, and vary 

 from that down to those just barely observable with the naked 

 eye. The very fine microscopic aegirines that give the greenish 

 color to the groundmass in the tinguaites are only sparingly 

 present in the trachytoid varieties so that the groundmass of 

 these rocks is prevailingly of a grayish to almost whitish tinge. 



Microscopic Characters. — The microscope shows the following 

 series of minerals : orthoclase (anorthoclase) microcline, pyrox- 

 ene, nepheline, nosean, haiiyne, biotite, magnetite, titanite, garnet 

 (melanite variety) and a mineral formerly supposed to be leucite. 



All of these minerals do not of course occur in the same 

 specimen, but are found in the different varieties that comprise 



