494 GEOLOGY OF THE BLACK HILLS. 



the beginning of decomposition, which, when further advanced, causes a 

 slight polarization of a newly formed mineral. 



Magnetite also occurs in phonolite quite frequently. 



The groundmass is usually crystalline and, for the trachytie phonolites 

 or those containing considerable feldspar, a fluid-like structure is very char- 

 acteristic. Of the Black Hills rocks, the specimen [139] from Black Butte 

 is a phonolite, being made up of sanidin and hornblende, with nephelite in 

 great abundance. 



Examination of the thin sections. — The rock [101] from the top of 

 Terry Peak has a light -gray to pinkish 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 porphyritic 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 crystals of orthoclase or sanidin, with numerous 

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

 quartzose groundmass, made up of 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 

 quite transparent. Their outlines and crystalline form are very distinctly 

 denned 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 

 by 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 plagioclase was noted, distinguished 

 by its twin lamellations. 



Quartz is in small grains, evenly distributed through the g-roundmass, 



