The Monteregian Hills. 2 07 



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



2. Essexite. 



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

 nepheline, but this mineral in some cases becoming so 

 abundant that the rock passes into a true nepheline- 

 syenite, or, by the replacement of the nepheline by soda- 

 lite, into a sodalite-syenite. This in the case of Mount 

 Johnson and Shefford mountain is represented by the 

 variety known as pulaskite ; in Brome mountain it is 

 stated by Dresser to resemble Brogger's laurvikite, 1 while 

 in Mount Eoyal and Beloeil it is a nepheline syenite. At 

 the latter mountain a sodalite-syenite also occurs in asso- 

 ciation with the nepheline-syenite. Nepheline-syenite is 

 also known to form part of Yamaska mountain. In addi- 

 tion to the syenite of the pulaskite variety, Dresser found 

 in Shefford mountain a large development of a distinctly 

 more acid type of the syenite magma, the rock showing 

 occasionally a few grains of quartz. This rock he has 

 classed as nordmarkite. These light colored syenites, 

 together with certain dykes of bostonite having a general 

 similarity in composition, were the rocks classed by Dr. 

 Hunt as trachytes. 



To the essexites belong the dolerites and diorites of 

 Hunt, when he applied these terms to the great igneous 

 intrusions of the mountains and not to mere dykes. They 

 usually contain both hornblende and pyroxene, but the 

 relative proportion of these two minerals varies consider- 

 ably in the different occurrences. Olivine is sometimes 

 present. Hunt did not recognize the presence of nephe- 

 line in these rocks, nor the highly alkaline character of the 

 magma which they represent, and classified them as dol- 

 erite or diorite according to the preponderance of pyroxene 

 or horneblende, noticing certain occurrences in which the 

 former rock passed into a pyroxenite or peridotite. 

 . The greater part of Mount Royal is composed of an 

 essexite, usually very basic, the dark-colored constituents 



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



