THE MONTEREGIAN HILLS 247 



phonolite, diorite, and dolerite, respectively. In this classifica- 

 tion no distinction was made between rocks occurring as dykes 

 and the great igneous intrusions which form the body of the 

 hills; differences in structure resulting from mode of occurrence 

 were not considered, the classification being based upon min- 

 eralogical composition alone. 



Recent investigations have shown that Hunt's names do not 

 convey an accurate idea of the petrography of these hills, nor 

 do they set forth the interesting relationships of the various 

 rocks composing them. It is necessary for this purpose to adopt 

 a more modern nomenclature, for all the mountains of the group 

 are composed of a family of consanguineous rocks, and taken 

 together they present one of the finest examples of a petrographi- 

 cal province hitherto discovered. They consist, furthermore, of 

 a rather rare class of rocks characterized by a high content of 

 alumina and alkalies, especially soda. 



The rocks forming the great intrusions which make up the 

 mass of these mountains belong to two well-characterized types 

 — one light in color, poor in iron-magnesia constituents, and 

 comparatively high in silica ; the other dark in color, rich in 

 iron-magnesia constituents, and with a lower content of silica. 

 They may be classed as follows, if Rosenbusch's nomenclature 

 be followed : 



1. Alkali-syenite, nepheline-syenite, or sodalite-syenite. 



2. Essexite. 



The first is an alkali-syenite, always containing a little neph- 

 eline, but this mineral in some cases becoming so abundant that 

 the rock passes into a true nepheline-syenite, or, by the replace- 

 ment of the nepheline by sodalite, into a sodalite-syenite. This 

 in the case of Mount Johnson and Shefford mountain is repre- 

 sented by the variety known as pulaskite ; in Brome mountain 

 it is stated by Dresser to resemble Brogger's laurvikite,' while in 

 Mount Royal and Beloeil it is a nepheline-syenite. At the latter 

 mountain a sodalite-syenite also occurs in association with the 

 nepheline-syenite. Nepheline-syenite is also known to form 

 part of Yamaska mountain. In addition to the syenite of the 



''Summary Report of the Geological Survey of Canada, 1901, p. 187. 



