208 Canadian Record of Science. 



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 Eougemont and to 

 form a portion of Yamaska mountain. An examination 

 of thin sections of specimens of the Eougemont 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 Eougemont, which 

 have not been thoroughly examined as yet, while certainly 

 composed chiefly of essexite, will probably be found, on 

 further study, to present a development of the syenite 

 in some portions of their mass also. 



In addition to these bodies of intrusive rock which 

 form the mass of the mountains, great numbers of dykes 

 occur cutting both the surrounding sedimentary strata and • 

 the intrusions. These are, of course, especially numer- 

 ous in and around the mountains themselves, but are also 

 occasionally found far removed from the centers of activ- 

 ity. The relative abundance of these dykes in the 

 vicinity of the several mountains varies greatly. They 

 swarm through the Paleozoic strata about Mount Eoyal, 

 cutting the limestones in all directions and also traversing, 

 although less frequently, the igneous rock of the main in- 

 trusion as well. No less than twenty-nine dykes and 

 flows, belonging to at least four and possibly five separate 

 series, each cutting the preceding set, were mapped by Dr. 

 Harrington some years ago in an excavation measuring 

 220 yards by 100 yards which was opened up in the 





