MINERALOGY OF BLACK LAKE AREA. 
17 
The principal deposits of chromite have been found in a rock inter- 
mediate in composition between peridotite and pyroxenite, which 
represent the two most basic, and earliest consolidated, phases of the 
differentiation of the magma which has given rise to the igneous complex 
known as the serpentine belt. In shape, the ore-bodies are roughly 
lenticular, and they vary in size from small pockets up to masses contain- 
ing thousands of tons. As a general rule there is no sharply defined 
wall bounding these lenses, but the ore passes by rather gradual trans- 
itions into the country rock, through which chromite is then dispersed 
as scattered nodules and grains the size of a pea or smaller. When 
examined in thin section under the microscope, the chromite in these 
rocks is seen to occur in isolated grains, and it displays the relations of 
a primary mineral. 
A very interesting occurrence of chromite is that at the Martin 
Bennett property (lot 28, range I, Ireland township). Besides occurring 
in massive form the mineral is also found as disseminated nodules enclosed 
in the serpentine adjacent to the massive chromite ore-bodies. These 
nodules are of all sizes, from mere grains up to masses several centi- 
metres in diameter; one of the largest collected had dimensions 3| x 2J x 
1| cm. At first sight they appear to be made up entirely of massive 
chromite, but, when broken open, it is found that there is, in addition, 
a fair amount of interstitial serpentine, in part altered to brucite. 
At the Hall chrome pit (Plate V) a granular chromite was observed, 
which is so friable that it can be readily crumbled In the hand. 
Analyses show that all the rocks of the serpentine belt contain 
traces or small percentages of chromium ; this is probably present mainly 
in the form of chromite, disseminated through their mass in an extremely 
fine state of division, though it is possible that a little chromium may 
also enter into the composition of some of the ferromagnesian minerals 
which these rocks contain. 
Origin . 
Considerations such as those just enumerated, regarding the manner 
and attitude in which the ore occurs, have led to the view that the 
chromite was an original constituent of the magma, from which it 
separated by differentiation. This hypothesis was first advanced by 
F. D. Adams 1 in 1894, and investigators who have since examined the 
deposits in the field have been practically unanimous in confirming this 
view. The evidence for this mode of origin is thus summarized by 
Dresser:* “The microscopic evidence that chromite occurs in Isolated 
grains as a primary mineral, its general occurrence, in traces at least, 
i Trans. Province of Quebec Mining Association, 1894. 
* 0p> cit., p. 90. 
