33 
t must be noted that while six of these mountains 
_.rom the horizontal strata of the plain, the two most 
-terly members of the group, namely Shefford end Brome, 
2 still to the west of the axis of that range, lie well 
in the folded belt of the Appalachians, although, 
‘x to the extensive denudation from which the region 
ffered, this folding has had but little influence on 
cal topography. About La Trappe, at the extreme 
_esterly extension of the Monteregian area, the dykes ot 
the series cut rocks of Laurentian age, which here form an 
outlier of the great Laurentian protaxis on the north. 
The Monteregian Hills form an exceptionally distinct 
and well marked petrographical province, being composed 
of consanguineous rocks of very interesting and rather 
unusual type. These are characterized by a high content 
of alkali and in the main intrusion of almost every moun- 
tain two distinct types are found associated with one 
another, representing the products of the differentiation of 
the original magma. 
These are— 
(a) Nepheline syenite, in some cases replaced by or 
associated with pulaskite, tawite, akerite or nord- 
markite. 
(b) Essexite, in some cases represented by theralite, 
yamaskite, rougemontite, or rouvillite. 
It may be mentioned that yamaskite is a very basic 
rock type characterized by a great predominance of pyro- 
xene, basaltic hornblende and ilmenite, with about two 
per cent of anorthite. Rougemonuite consists largely of 
anorthite with pyroxene as the only important ferro-mag- 
nesian constituent. Rouvillite is a highly feldspathic 
variety of theralite. 
GEOLOGY Cr MOUNT ROYAL. 
Mount Royal consists of a body of intrusive plutonic 
rock penetrating the nearly horizontal limestone of the 
Trenton formation (Ordowv’cian). It consists of two main 
intrusions composed o’  sexite and nepheline syenite 
respectively, of which nepheline syenite is the later. 
followed by a swarm of kes and sheets of consanguin- 
eous rocks which cut r- only the main intrusions, but 
also penetrate the surre ding limestones in all directions. 
The intrusive rock in. e places tilts up the limestones 
while elsewhere about ti 2 mountain these maintain their 
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