16 
CORUNDUM IN THE UNITED STATES. 
[bull. 180. 
corundum found in the peridotite rocks there is a scarcity of chro- 
mite, and where there is a large quantity of chromite there is a 
scarcity of corundum. 
In a recent paper 1 I have discussed the origin of the corundum in 
the peridotite rocks, and have accepted the theory that the corundum 
was held in solution in the molten mass of the peridotite when it was 
intruded into the country rock, and that as the mass began to cool 
it was among the first minerals to separate. In these molten magmas 
the more basic minerals, corundum and spinel, would be the first to 
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Fig. 3. —Ideal vertical cross section of a mass of peridotite soon after its intrusion in gneiss. 
The corundum zone is greatly exaggerated. 
separate, and this separation would take place along the outer border 
of the mass, for there it would first cool. Convection currents would 
then tend to bring into this outer zone a new supply of material car- 
rying alumina, and when this zone was reached crystallization would 
take place and the alumina would be deposited as corundum. 
Fig. 3 represents an ideal appearance of a vertical cross section 
of a mass of dunite holding corundum in solution, soon after its 
intrusion into a gneiss. In this figure the. corundum zone has been 
greatly exaggerated, in order to illustrate better the cross section. 
The corundum would be concentrated toward the borders of the 
Am. Jour. Sci., 4th series, Vol. VI, 1898, p. 49. 
