PETROGRAPHY— ORTHOCLASE. 



525 



SO than in [213], and is also in part altered to magnetite. The latter 

 occurs in large masses, mostly in former biotite cr3\stals, but is very spar- 

 ing in the groundmass and not at all so uniformly distributed as noticed 

 before. Large, opaque, brown masses of hydrated oxide of iron are also 

 present. 



The groundmass is very fine indeed, it being very difficult to trace any 

 outline of the small sanidin sections, which appear as mere whitish spots. 

 A few colorless, but sharp and clear apatite needles were recognized. One 

 of them of columnar form has a very distinct black center, Avith a light 

 exterior border. The whole appearance of the rock, both in hand specimen 

 and section, shows it to be considerably weathered. 



The section of feldspar from the granites of the Black Hills is ortho- 

 clase very beautifully striated, so that it much resembles plagioclase. It 

 is similar to the stiiation of the amazon stone from Siberia, and differs from 

 the banding of plagioclase because the lines are not exactly parallel, but 

 rather pointed and sometimes at right angles to each other. It is a beauti- 

 ful example of this occurrence of banded orthoclase. Zirkel, in the N. Jb. 

 f Min. 1872, p. 12, explains this banding of orthoclase in polarized light as 

 due to particles of some foreign substance, arranged in layers and strata, 

 so that between them is seen the pure and clear feldspar, which causes the 

 phenomenon by its bright colors. The dark bands are made up of long- 

 shaped cavities, varying somewhat in size, and yellowish needles and leaves. 

 When these bands are interrupted by a layer of clear orthoclase, the rows 

 of cavities become very thin and finally disappear in the feldspar with 

 delicate comb-like borders, very beautiful, but not to be confounded with 

 the striations arising from the twin lamellation of triclinic feldspar, the latter 

 being sharper, more exactly parallel to each other and more distinctly 

 defined. This feldspar has inclusions of long needles of apatite, horn- 

 blende and probably mica. Masses of uncrystallized orthoclase run through 

 it in irregular shapes, and some quartz and magnetite are also to be seen. 



