BLACK HILLS GEOLOGY. 255 



Zonary banding and twinning after the Carlsbad law are ex- 

 ceedingly common. Inclusions of albite occasionally occur in 

 square, almost automorphic crystals. How much of the sani- 

 dine may prove on closer study to be anorthoclase cannot be 

 said, although it is exceedingly probable that in a rock contain- 

 ing soda pyroxene in such amount, there may be a considerable 

 quantity. Triclinic characters could not be determined in any 

 of the phenocrysts observed in the rock, and it remains for care- 

 ful chemical investigation to prove the development of this va- 

 riety of feldspar. 



Phenocrysts of an exceedingly acid plagioclase are also pres- 

 ent in considerable quantity, but are very small as compared 

 with the larger sanidines, being only observable with the micro- 

 scope. The maximum angle, measured in sections perpendicu- 

 lar to albite lamellae, varies from 4 degrees to 5 degrees. The 

 crystals are scattered here and there through the groundmass 

 and, from their inclusion in the later phenocrysts, they seem to 

 have been formed very early in the consolidation of the magma. 

 The groundmass of the rock is exceedingly fine grained. It 

 consists of a doubly refracting granular aggregate which is 

 penetrated in all directions by needles of aegirine, the latter far ex- 

 ceeding the other individuals in size. How much of this ground- 

 mass is quartz and how much feldspar, it is impossible to say, but, 

 judging from the general analogy of the rock with that of 

 Terry peak, to which it seems quite closely related, it is probable 

 that much quatrz is present. The quartz-phenocrysts, which 

 constitute the most noticeable as well as the most unusual fea- 

 ture of the rock, are extremely large, varying from yL to — in 

 rare instances — ^ inch in diameter. They are strongly con- 

 trasted with those of the Annie creek mass, in that they are 

 very much resorbed, for the groundmass frequently extends far 

 into them in bottle -shaped embayments. Fractures have often 

 allowed the groundmass to fill the interstices between the 

 broken portions. 



The bisilicates are present in two generations ; the larger 

 automorphic crystals of aegirine-augite, and the finer shredded 

 needles of aegirine, which in the specimen from the shaft are 



