36 
CORUNDUM, ITS OCCURRENCE AND DISTRIBUTION. 
The gTOundmass of this amphibolite is a grass-green ainphibole, 
containing IT per cent of akuiiina, nearly 12 per cent of lime, and 
one-half of 1 per cent of magnesia, which is best classified under the 
edenite variety of aluminous amphiboles. The rich green color of the 
edenite is undoubtedly due to the presence of a small amount of chro- 
mic oxide, the analysis showing the presence of 0.38 per cent of this 
oxide. Many microscopic grains of picotite or chromite are scattered 
through the groundmass of edenite. There is also present, in widely 
varying proportions, the plagioclase feldspar anorthite. The feld- 
CONTOUrf INTCRVAL 50 FEET 
Fig. 9. — Map of the Buck Creek peridotite area, showing the relation of the amphibolitt 
dikes. 
spar is not constant in all of the amphibolites, but where it does 
occur it varies in size from minute particles to masses as large as a pea. 
The rocl^ has often a strikingly laminated structure, and grades 
from this extreme to a structure showing no lamination at all. It is 
exceedingly tough and very fine grained. The corundum, which i 
occurs in the amphibolite as an accessory mineral, varies in size from 
minute particles to masses several inches in diameter, in which there 
are usually developed parting planes parallel to the unit rhombohe- 
dron. In color it varies from almost white to a deep ruby-red, but 
the prevailing color is a deep pink. Very rarely deep blue corundum 
has been found in these amphibolites. 
On account of the exceeding toughness of the rock, and more par- 
ticularly on account of the low percentage of corundum, these am- 
phibolites are not of commercial value as a source of corundum ; they 
