162 THE CANADIAN NATURALIST. [June,, 



occupies a large area to the south of Shefford, approaches within 

 two miles of it. In like manner, a few miles to the south of BeloeiL 

 is another intrusive mass known as Mount Johnson or Monnoir ; 

 making in all nine hills of eruptive rock belonging to the Mont- 

 real group. Besides these, numerous smaller intrusive masses in the 

 form of dykes are met with around and between the hills. From 

 Mount Royal to Rigaud Mountain, a distance of about thirty miles, 

 a gentle undulation of the strata is observed, which increases to the 

 westward of Rigaud, and finally gives place to a considerable fault.. 

 This disturbance has been traced to the Laurentide hiUs on the Lac 

 des Chats, 140 miles west of Montreal ; but to the eastward the strata 

 exhibit no evidence of this transverse undulation, unless the ap- 

 pearance of the intrusive rocks already mentioned be supposed to 

 indicate the prolongation of a fracture without sensible dislocation.. 



The whole of these eruptive rocks rise through unaltered paleo- 

 zoic strata, which however, in the immediate vicinity of the intru- 

 sive rocks, exhibit a local metamorphism. The hills of Shefford, 

 Brome, and Yamaska break through the strata of the Quebec 

 group, and lie a little to the east of the great line of dislocation 

 which, in this region, brings up the lower members of the paleozoic 

 series against the superior portion of the Lower Silurian, and di- 

 vides into two districts the great paleozoic basin. (Geology of Can- 

 ada, pp. 234, 597.) The other hills all belong to the western di- 

 vision of this basin, and break through various members of the 

 Lower Silurian series from the Potsdam to the Hudson River 

 formation. Among the numerous dykes which traverse not only the 

 sedimentary strata but the intrusive masses, there are some which 

 intersect the conglomerates of St. Helen's Island. These are of un- 

 certain age, but repose unconformably on the Lower Silurian series, 

 and enclose pebbles and masses of Upper Silurian limestone charac- 

 terized by fossils of the Lower Helderberg period. (Ibid., p. 356.) 



This group of intrusive rocks offers very great varieties in com- 

 position ; thus Shefford and Brome consist of what we shall de- 

 scribe as a granitoid trachyte, while the succeeding mountain, 

 Yamaska, and the most western, Rigaud, both consist in part of a 

 kind of trachyte, and in part of diorite. Monnoir and Beloeil also 

 consist of diorites, which however differ from the last two, and from 

 each other ; while Rougemont, Montarville, and Mount Royal con- 

 sist in great part of dolerites, presenting however many varieties 

 in composition, and sometimes passing into pyroxenite. The dole- 



