248 FRANK D. ADAMS 



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 simi- 

 larity 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 considerably in the different occur- 

 rences. Olivine is sometimes present. Hunt did not recognize 

 the presence of nepheline in these rocks, nor the highly alkaline 

 character of the magma which they represent, and classified 

 them as dolerite or diorite according to the preponderance of 

 pyroxene or hornblende, 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 forming a very 

 large proportion of the whole rock. This was classed by Hunt 

 as a dolerite, but is almost identical with the essexite of Mount 

 Johnson, which Hunt classes as a diorite. This same rock is 

 stated by Hunt to make up the greater part of Montarville and 

 Rougemont and to form a portion of Yamaska mountain. An 

 examination of thin sections of specimens of the Rougemont 

 rock in the petrographical collection at McGill University shows 

 it to be an essexite, rich in olivine. Dresser has found it to con- 

 stitute approximately one-half of Shefford mountain and also to 

 form large areas in Brome mountain. It makes up the greater 

 part of Mount Johnson and forms the mass of Beloeil. 



It is thus seen that the essexite magma is represented in 

 every one of the eight mountains, and that in six of them at least 

 it is associated with the syenite magma. The remaining two, 

 Montarville and Rougemont, which have not been thoroughly 

 examined as yet, while certainly composed chiefly of essexite, 

 will probably be found, on further study, to present a develop- 

 ment of the syenite in some portions of their mass also. 



