The Monteregian Hills. 211 



essexite which constitutes the greater part of the moun- 

 tain was the earliest intrusion. When this had become 

 solid the nepheline-syenite broke through it, sending arms 

 into it and catching up detached fragments of the shat- 

 tered essexite. The same sequence in time is, according 

 to Dresser, to be seen in Shefford Mountain. The basic 

 essexite here forms the earliest intrusion, and was suc- 

 ceeded by the pulaskite and more acid nordmarkite. 

 Mount Johnson, however, presents the two rocks in an en- 

 tirely different relation. Here, as will be shown later, 

 there was but a single period of intrusion. For although 

 both rocks are present in the mountain, the essexite forms 

 the central portion of the mass and passes over into 

 pulaskite about the periphery of the neck. The mountain 

 thus consists of essexite in its center, surrounded by a 

 zone of pulaskite, the two rocks passing imperceptibly 

 into one another. Mr. Leroy considers it probable that a 

 similar passage takes place in the case of Beloeil moun- 

 tain, but it is there difficult accurately to determine the 

 relations of the magmas to one another on account of 

 the covering of drift which obscures the contact. 



It is thus evident that the two rock types constituting 

 the Monteregian hills are differentiation products of a 

 single magma, the separated magmas, however, in the case 

 of Mount Eoyal and Shefford having been erupted in suc- 

 cession instead of simultaneously. In connection with 

 the question of differentiation, another noteworthy fact is 

 that the more easterly mountains contain proportionately 

 more syenite and the western hills a greater proportion 

 of the essexite. The bearing of this fact on the character 

 of the differentiation which took place in the subterranean 

 magma basin can be more profitably discussed at a later 

 date when the precise character and relative extent of the 

 intrusions in Yamaska, Rougemont, and Montarville have 

 been determined. 



With regard to the structure of these mountains, it may 

 be noted that Logan, who first examined them, refers to 



