53 
The alteration of limestone by the intrusion of the 
nepheline syenite is well seen at this quarry. The zone of 
alteration is not wide, but the alteration along the immedi- 
ate contact is intense, the limestone in places being changed 
to a coarse grained white marble. Certain exomorphic 
contact minerals have also been developed in parts of the 
altered limestone. In 1892 a vein of nearly pure native 
arsenic was found cutting the nepheline syenite near the 
contact, and about 4o lbs. of this mineral was obtained 
from the vein. The same mineral has been found, although 
in smaller amount, in driving the Canadian Northern 
tunnel, which passes through the contact below the surface 
at no great distance from this quarry. 
Many of the dykes and small stock-like intrusions 
about Mount Royal contain angular fragments of the sedi- 
mentary strata through which the magma passed in its 
upward course. In certain places, these fragments are so 
abundant within the igneous rocks that the latter become 
igneous breccias. 
An area within which these breccias are developed 
extensively is situated in Outremont, to the South of St. 
Catherine road and extending from Rockland avenue 
through the Golf Links to Mount Royal Heights. Here 
_the prevalent type of breccia possesses a camptonite base 
and includes a great number of fragments of Trenton 
limestone, and, in places, a few of Potsdam sandstone. 
The limestone fragments have been, more or less changed, 
from their normal blue colour to white, and in some cases 
have been recrystallized, though in many of them the 
Trenton fossils can still be distinguished. 
Upon Mount Royal Heights, many of the fragments 
of the Trenton limestone within the camptonite are very 
large, in places the stratification of this formation has been 
little disturbed, but it is traversed by irregular tongues of 
camptonite extending in every direction. Intrusive into 
the breccia is a small body of nepheline syenite, containing 
a large number of fragments of Potsdam sandstone. A few 
dykes of aplite (see analysis II, page 46) which are pre- 
sumably genetically related to the nepheline syenite also 
break through the camptonite breccia at this point. At 
a depth of 160 feet below the surface the tunnel of the 
Canadian Northern Railway penetrates both of these 
igneous rock types. At this depth, the fragments of lime- 
stone within the camptonite are not so abundant as at the 
