BLACK HILLS GEOLOGY. 265 



Pyroxenes of the First Variety. — The crystals of this group 

 are prevaiHngly idiomorphic, the only modification being made 

 by the addition of later deposited aegirines — crystals which are 

 attached to the large cores, sometimes in parallel orientation 

 and sometimes without definite arrangement. 



The faces most commonly developed areco/^(iio) oo/^oo 

 (lOO) and 00/^00(010). The tabular habit, ascribed to this 

 mineral by Rosenbusch ^ through the development of the face 

 00 Po) was often observed, but as frequently absent, for in sec- 

 tions perpendicular to the vertical axis the crystals are often 

 square, or when flattened only slightly so. The other faces ob- 

 served were oP (001), and /^ (i 1 1). The face /'(in) is rare, 

 and has been identified but once with certainty. Determination 

 of the axes of elasticity shows no variation from the usual type 

 as described under the quartz-segrite-porphyries. 



An irregular zonal structure is almost always present. The 

 cores of the crystals are slightly pleochroic or completely color- 

 less augite. The aegirine' molecule increases as we pass out- 

 ward, and the whole is often surrounded by a deep green, 

 highly pleochroic mantle of cegirine. 



The different zones have decreasing extinction angles as 

 we proceed from the center outwards until, in the aegirine 

 mantle, they are practically zero. The aegirine and augite mole- 

 cule vary greatly in their relations to one another. In the rock 

 from the dike east of the Rua Mine the augite molecule is only 

 sparingly developed, the large automorphic crystals being deeply 

 colored and strongly pleochroic throughout, and possessing a 

 low angle of extinction. In the tinguaite from Englewood, we 

 have a green, quite strongly pleochroic aegirine-augite, with a very 

 large extinction angle of 30 degrees, but only slight zonal de- 

 velopment. From this we pass to the most common types in 

 which zonal banding is more pronounced. Finally at the augite 

 extreme we have in a phonolite from False Bottom Creek 

 pyroxene crystals, which lack the aegirine molecule altogether. 

 Others alongside of them are quite pleochroic, and still others, of 

 almost colorless augite, have not only been surrounded by a 



' Mikroskopische physiographie, Dritte Auflage, p. 538. 

 Annals N. Y. Acad. Sci., XII, December 5, 1899 — 17. 



