THE MONTEREGIAN HILLS 25 I 



boro, N. H., is, however, closely related to the Monteregian 

 pulaskite in character and composition, and may prove to be 

 such a center. 



STRUCTURE AND ORIGIN OF THE MONTEREGIAN HILLS. 



The question of the mutual relations and relative age of the 

 several rock types constituting these hills presents many points 

 of interest. In the case of Mount Royal the essexite which con- 

 stitutes the greater part of the mountain 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 shattered 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 succeeded by the 

 pulaskite and more acid nordmarkite. Mount Johnson, however, 

 presents the two rocks in an entirely 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 pulas- 

 kite, the two rocks passing imperceptibly into one another. Mr. 

 Leroy considers it probable that a similar passage takes place in 

 the case of Beloeil mountain, 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 Royal 

 and Shefford having been erupted in succession instead of 

 simultaneously. In connection with the question of differentia- 

 tion, 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 subter- 

 ranean magma basin can be more profitably discussed at a later 

 date when the precise character and relative extent of the intru- 



