254 ROYAL SOCIETY OF CANADA 



The most easterly of the mountains, Shelîord. and Brome, lie Just 

 within the border of tiie belt affected by the Appalachian uplift, and 

 show the result of the movement. As has been pointed out by Dresser,^ 

 tins offers a basis for determining the time of the intrusions. '' The 

 latest sediments amongst which they have been intruded are of the Lower 

 Trenton formation, while the earliest members of the Palaeozoic system 

 in eastern North America that have not been disturbed by the Appa- 

 lachian uplift are the Permo-carboniferous of Prince Edward Island and 

 the adjoining mainland. Accordingly were it established that the final 

 folding throughout all parts of the northern Appalachians took place 

 simultaneously, the intrusion of the ShefEord mass would necessarily 

 have occurred between early Trenton and later Carboniferous time. But 

 the simultaneous folding of so great a belt as the Appalachian system 

 here comprises, cannot be safely assimied without a better correlation of 

 its complex structural details than is at present possible, and in conse- 

 quence the latest date at which the intrusions of Shefford mountain 

 could have taken place must meanwhile remain somewhat less precisely 

 defined." 



The Monteregian intrusives consist of a rather rare class of rocks 

 characterized by a high content of alumina and alkalies, especially soda. 

 In the case of Mount Royal the rocks belong to two quite definite types, 

 one light in colour, poor in ferro-magnesian constituents and compara- 

 tively high in silica, — nepheline syenite; the other dark in colour, rich 

 in ferro-magnesian constituents, and with a lower content of silica, — 

 essexite. Dykes are abundant, all the types being found which are any- 

 where known to be associated with magmas of the theralite or nepheline 

 syenite groups. The list comprises, — bostonites, solvsbergites, tinguaites, 

 alnoites, fourchites, camptonites, monchiquites. These dykes are not 

 confined to the immediate vicinity of the moimtains, but are found cut- 

 ting the rocks of the plain at considerable distances from tlie parent mass. 

 The farthest yet recorded is at St. Lin, twenty-four miles north of Mount 

 Eoyal, the nearest of the hills. There will be occasion to refer to this 

 dyke again. 



The essexite which constitutes the greater part of Mount Eoyal, was 

 the earliest intrusion. When this had become solid, the nepheline syenite 

 broke through it sending arms into it and catching up detached frag- 

 ments of the shattered esse site. Tt has been shown that in general these 

 two rock types constituting the Monteregian Hills are differentiation 

 products of a single magma,^ the separated constituents in this parti- 

 cular case having been erupted in succession instead of simultaneously. 



1 Geological Survey of Canada, Vol. XIII, pt. L., p. 35, 1902. 

 1 Monteregian Hills, p. 211. 



