W. P. Blake—NMinerals from Dakota. 235 
the hornblende playing the same réle as the augite of many 
labases, its irregular form filling the spaces between well- 
developed plagioclase crystals, which in part appear to be 
anorthite and have crystallized before the hornblende. There 
1s a little pyroxene present and considerable tridymite. 
Art. XXVIL.—Cassiterite, Spodumene and Beryl in the Black 
Hills, Dakota ; by Wiiu1aM P. BuaKE. 
CASSITERITE is found in place and in stream deposits in the 
central region of the Black Hills, Dakota, about two miles 
from Harney in Pennington County. This tin ore occurs in a 
vein, or mass, of coarsely crystalline granite rising through the 
fine-grained micaceous and siliceous slates of the Huronian — 
period flanking the granitic axis of the Harney range of 
mountains. 
mica, but a few crystals of a black mineral, believed to be wol- 
framite, have been seen in the mixture of spodumene and feld- 
Spar. No topaz has yet been identified. 
bedded in a matrix of quartz, resembling the beryls found in — 
a mica mines of Acworth, New Hampshire, though not so 
arge, 
* 
