18 
MUSEUM BULLETIN NO. 27. 
in all the rocks of the series, the shape of the deposits, their relation to 
the wall rock, and the fact that they occur principally in a particular 
phase of the intrusive complex, viz., in transition rock between peridotite 
and pyroxenite, all go to support the opinion that the deposits were 
segregated from the magma of the original rock before it was completely 
solidified.” 
Further evidence supporting the view that the chromite is primary 
is afforded by the discovery, made by R. A. A. Johnston, of microscopic 
crystals of diamond enclosed within the mineral. The theory that the 
ore was an original constituent and an early differentiate or segregation 
product of the magma receives additional support from the fact that the 
granitic dykes, which represent the last phase of differentiation of the 
same magma, and which may be seen accompanying the chromite in 
most of the pits, have certainly been injected at some time subsequent 
to the formation of the ore-bodies. 
The frequent occurrence of granitic dykes within, or close to, the 
ore-bodies has been noted by many observers. F. Cirkel, although 
expressing the belief elsewhere in his report that the chromite was 
formed during the cooling of the magma, goes so far as to state that these 
intrusive dykes exert "quite a favourable influence upon the deposition 
of the mineral .” 1 The field evidence, however, all points to the ore as 
having been solid before the dykes were injected, and the present writers 
cannot subscribe to the opinion that the latter could have had any 
influence, favourable or otherwise, on its deposition. 
If the dykes are actually more numerous in the vicinity of the 
rich ore lenses than elsewhere within the peridotite and other associated 
rocks, a point which has not hitherto been clearly demonstrated, it might 
be explained on the assumption that, as compared with the normal 
igneous rocks, the ore would constitute a brittle mass much more suscept- 
ible to fracturing than the relatively plastic peridotite. Further, the 
contact between the ore and the country rock, although nowhere sharply 
defined, should form a zone of weakness peculiarly prone to the develop- 
ment of fractures to be later filled by intrusives. This might explain 
the fact which has been repeatedly noted, that the ore-bodies are at 
times completely cut off by dykes. 
Where such magmatic injections have passed along fissures within 
the ore, they have frequently ripped off and trapped fragments from the 
walls, and these are now seen embedded within the dykes. The general 
shattering of the rocks which preceded or accompanied this last phase 
of igneous activity, and whose effects are seen especially in the peridotite, 
extended into the ore-bodies; as a consequence, the ore in many places 
1 Mine* Branch, Can.. “Chrome iron ore depoeita in the Eastern Townships, Quebec,” Report No. 29 
1909 , p, 25. 
