84 
CORUNDUM^ ITS OCCURRENCE AND DISTRIBUTION. 
there occur masses of block corundum, large rough crystals, small, 
well-developed crystals, and particles or grains of corundum. 
The differentiation of the basic peridotite magma, upon cooling, is 
similar to what Vogt " and Adams ^ have described in the case of the 
separation of sulphide deposits from a molten gabbro magma. The 
Ounite 
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Fig. 13. — Ideal vertical cross section showing a somewhat gradual transition from coruu 
dum to peridotite. 
segregation or concentration of these ores is usually near the contact 
of the gabbro with the gneiss and is ahvays sharply separated from 
the gneiss. AVhile there is sometimes a distinct line of sej^aration 
between the ore and the gabbro, at others there is a gradual transition 
from the ore to the pure gabbro. It might be expected tliat the 
" Zeitschr. fiir prak. Geol., Nos. 1, 4, and 7, 1893. 
"On the Igneous Origin of Certain Ore Deposits; read before the Gen. Min. Asso. of the 
I'rov. of Quebec, Montreal, Jan. 12, 1894. 
