244 FRANK D. ADAMS 



nowhere visible, so that it is impossible to determine whether 

 the mass of Rigaud mountain cuts through the strata in ques- 

 tion, as in the case of the Monteregian hills, or whether it is 

 pre- Paleozoic in age. The same is true of the mass in Chatham 

 and Grenville, the actual contact here also being found by Mr. 

 Leroy to be banked up with drift. The narrow margin of gneiss 

 shown on Logan's map^ between the Chatham syenite and the 

 Paleozoic is also conjectural, the area being likewise drift- 

 covered. Rigaud mountain is furthermore of a different shape 

 from the mountains east of Montreal, being six miles in length 

 and only two and one-half miles wide ; at the eastern end of it, 

 moreover, there is found an occurrence of ordinary Laurentian 

 gneiss. The abrupt and straight southern boundary of the Lau- 

 rentian plateau along this part of its course probably marks a 

 fault. Ells has noted the existence of other faults in this dis- 

 trict, one of which he believes to follow the north side of Rigaud 

 mountain. It is thus highly probable that the ridge known as 

 Rigaud mountain does not belong to the Monteregian hills, but 

 that it is a portion of the Laurentian plateau separated from the 

 main area by faulting and stripped of its original covers of Pale- 

 ozoic strata by denudation. It is probable that Mont Calvaire, 

 as regarded by Logan, is also an outlying portion of the Lauren- 

 tian plateau. 



The hills on the west side of Lake Memphremagog and to 

 the northeast toward the Chaudiere river, referred to by Dr. 

 Ells, so far as is known, are quite different in petrographical 

 character from Mount Royal and the other members of its group. 

 They constitute a chain of hills occupying a tract of country 

 some four miles wide and thirty-five miles in length, in the 

 heart of the Appalachian uplift and following the strike of the 

 Appalachian folding. Many of them, as Owl's Head and Orford 

 mountain, rise to a very considerable height, these peaks having 

 a height of about 2,400 and 2,800 feet respectively; forming, in 

 fact, the highest elevations in this part of Canada. So far as has 

 been ascertained, these mountains are in all cases composed of 



' Atlas to accompany the Geology of Canada, 1863, Map No. 2. 



