440 INTRUSIVE ROCKS OF THE DISTRICT OF MONTREAL. 
an ageregation of large crystalline grains of what appears to be a 
reddish orthociase, often without any cementing medium ; at other 
times the feldspar crystals are imbedded in a fine grained grayish 
base, and the rock closely resembles the trachytic porphyry of Cham- 
bly. Quartz and hornblende are both however sometimes present, 
the rock passing into a granite or syenite. These rocks are cut by 
thin veins or dykes of a hard reddish-brown jasper-like feldspathic rock. 
A portion of Rigaud Mountain however consists of a rather coarse 
grained diorite, which is made up of a crystalline feldspar, white or 
greenish in colour, with small prisms of brilliant black hornblende 
and crystals of black mica, im some specimens the feldspar and in 
others the hornblende predominating. These diorites resemble 
closely those of Belceil and Rougemont. 
The rocks of all these mountains, and especially of Montreal and 
Rigaud, still demand a great deal of study, and these observations 
and analyses are to be looked upon only as preliminary to a more 
extended examination, which shall determine the mutual relations of 
the trachytes, diorites, dolerites and olivinitie rocks above described, 
as well as their probable relations to the stratified deposits of more 
ancient periods. 
The eruption of these augitic and olivinitie rocks was evidently 
antecedent to the deposition of the Lower Helderberg rocks, since 
in the dolomitic conglomerate of that age we meet with fragments 
of augite, olivine and mica identical with those found in the dolerites 
just described (Report 1857, p. 202.) 
The metamorphic action exerted by these intrusive masses upon 
the Silurian strata in their immediate vicinity appears to have been 
very local, but it is not less worthy of study, inasmuch as its results 
on a small scale resemble those produced by the wide-spread action 
which has altered such vast areas of similar rocks in the Green 
Mountain chain, far removed from the influence of intrusive rocks. 
Among the sandstones and shales of the Hudson River group 
which surround Rougemont, there occur beds of those highly ferru- 
ginous dolomites so often met with in this formation, and similar to 
those which I have described in previous Reports. 
In one of these, which is conglomerate or concretionary in its 
structure, the paste has been converted into a dark greenish erystal- 
line hornblende, which retains its colour on the weathered surfaces, — 
while the nodules of buff coloured dolomite have become reddish- 
brown and pulverulent. 
