r>20 



GEOLOdY OF THE BLACK HILLS. 



ill their tiiin sliai[), square crystals of magnetite, forming a double inclusion 

 )iot often met with (Plate II, Fig. 4). The partially altered biotites are very 

 (lark, full of magnetite and nearly opaque, while the light-brown, transparent 

 j)rismatic sections often have apatite inclusions. The magnetite is very 

 abundant in large grains and masses. Apatite is present and is recognized 

 in the hexagonal white sections, which change only from light to dark with 

 the revolution of the nicols. The long, slender sections parallel to the 

 prism are seen to be doubly terminated by a pyramid, while they are of a 

 faint violet color with crossed nicols. The basal sections are quite sharp 

 and clear in outline, and of large and small size, the latter being most 

 brilliant That this mineral is apatite and not nephelite is confirmed by 

 the chemical reaction obtained for phosphoric acid as well as the form of 

 the long, acicular prismatic sections, which are more usual for apatite than 

 nephelite. The pyramidal terminations belong also to apatite, the crystals 

 of which in this rock are unusually large and fine. The calcite is in irreg- 

 ular, colorless masses, which appear to fill up cavities and also to be inclu- 

 sions in hornblende and biotite. It changes only from light to dark, and 

 does not have in the section any of the usual lines of twinning and cleavage. 

 A fragment broken from the large mass previously mentioned showed the 

 characteristic cleavage very distinctly under the microscope, while the 

 effervescence with acid also proves its character. It may be the result of 

 decomposition, or more probably it has been formed in the cavities of the 

 rock from extraneous sources as an infiltration. 



The grouudmass is made up of sanidin crystals, confusedly inter- 

 grown and interlaced, a few of which are still fresh enough to show their 

 forms as bright sections between crossed nicols, while the whole mass is also 

 sufficiently clear to make its structure apparent. 



The sanidin-trachyte [175] from southwest of Warren Peaks has a 

 dark-green color and is very compact and solid, resembling [123] and 

 [167]. Scattered through it are whitish crystals of feldspar, quite conspicu- 

 ous in the green mass. Tiie thin section is seen under the microscope to 

 consist of a remarkable mass of delicate, acicular microliters of hornblende, 

 in which are a few sanidin and large hornblende crystals, whh magnetite. 

 The sanidin crystals having a distinct outline are rather scarce. They are 



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