20 
MUSEUM BULLETIN NO. 27. 
Montreal chrome pit. The vesuvianite is found either deposited directly 
upon the compact chromite along narrow seams and partings, or else 
lining druses within the massive white diopside which cements the 
brecciated ore. It would thus seem that the magmatic waters, or 
“extract,” which in an earlier section have been credited with exerting 
a powerful solvent action on the peridotite and other rocks they bathed, 
have also been capable of dissolving the chromite with which they came 
into contact. It is also possible that some of the chromite thus taken 
into solution was later redeposited as a secondary mineral, since small 
veinlets of chromite are found, not only within the country rock, but 
also penetrating the “dykes” of diopside and vesuvianite. 
Microscopic Character. 
Microscopic examination of thin sections of the ore by Dresser 
revealed the interesting fact that it is not homogeneous but is made up of 
two distinct varieties of chromite, viz., a reddish-brown translucent 
substance and a black opaque material. The relative amounts of these 
present in the ore are variable, and Dresser was able to establish the 
fact that the high-grade ores contain as much as 90 per cent of the 
translucent variety, while in poor ores the black variety is greatly in 
excess. With regard to the microscopic character of the ore, he says: 
“Specimens of medium grades of ore show the two portions in the 
thin section to be definitely distinct from each other, though often 
intricately intergrown. In a few cases they had the appearance of inter- 
locking octahedral crystals, but in general crystal outlines cannot be well 
distinguished in either. In reflected light the two kinds of material are 
absolutely indistinguishable.” 1 
The microphotograph of a thin section reproduced in his report 
shows the translucent variety with fairly sharply defined crystal outline, 
while the black material fills irregular spaces and sends off arms into the 
other. Judging from the photograph it would seem that these arms 
follow planes of cleavage or partings, since they show an obvious tendency 
to parallelism, both among themselves and also to the crystal outlines. 
In this connexion Dresser remarks: “In some sections the relative 
positions of the translucent and opaque portions are such as to suggest 
that the opaque might be an altered form of the other, but in others both 
appear to be primary.” 2 
Chemical Composition. 
Dresser treated a quantity of the crushed ore on a Wetherell magnetic 
separator, in an attempt to isolate the two varieties, the translucent 
1 Op. dt., p. 77, 
«Op. dt. ( p. 79. 
