41>4 (JHOLOGY OF THE BLACK HILLS. 



tlie beginning- «»t" decuuipositiou, which, when further iidvaiiced, cau.se.s a 

 slijj-lit j)(>lai"ization of a newly formed mineral. 



Magnetite also occurs in j)honolite quite frequently. 



Tlio groun<lmass is usually crystalline and, for the trachytic phonolites 

 or those containing considerable feldspar, a fluid-like structure is ver\' char- 

 acteristic. Of the Black Hills rocks, the specimen [130] from Black Butte 

 is a phonolite, being- made u[) of sanidin and hornblende, with nephelite in 

 great abundance. 



Examination of the thin SKCTioNS.-^The rock [101] from the top of 

 'i'erry Peak has a light gray to jjinkish color, large crystals of feldspar, 

 with small ones, being thickly crowded together in a feldspathic ground- 

 mass, in which quite small, black needles and crystals are occasionally visi- 

 ble. It has a somewhat i)or})hyritic structure, except that there is very 

 little groundmass to be distinguished macroscopically from the crystals. 

 In the section under the microscope, the rock appears to consist of large 

 and weathered or dusty cr3'stals of orthoclase or sanidin, with numerous 

 large and small, columnar hornblendes imbedded in a partially crystalline, 

 (juartzose groundmass, made up o( feldspar and quartz granules and some 

 large quartz masses. No mica was observed, except a few opaque, white, 

 and decomposed masses, which may have been muscovite when fresh. The 

 sanidin is quite dusty, but the interior of some of the crystals is still clear 

 enough to give color with polarized light, a few of the small ones being 

 (piite transparent. Their outlines and crystalline form are very distinctly 

 defined against the brightly colored groundmass, very interesting being the 

 appearance of a bright line around the cloudy central portion, having the 

 usual termination of base and hemi-orthodome, while outside of this line 

 the crystal is finished flat and quite opaque and the terminal faces replaced 

 b}' one plane, making the exterior form rectangular. This is the very char- 

 acteristic, zonal formation of sanidin, which is not so often observed in the 

 orthoclases of granite. There are some twin crystals; and the interior of 

 some of the large crystals is filled up with the groundmass, leaving only a 

 dark rim of the feldspar. A very little })lagioclase was noted, distinguished 

 by its twin lamollations. 



(Quartz iij in small grains, evenly distributed through the groundmass, 



4 



