The Monteregian Hills. 203 



plateau, a few miles to the north of Rigaud mountain. 

 Owing to the drift which mantles this district, the actual 

 contact of the igneous rock of Rigaud mountain and the 

 Paleozoic strata of the plain is nowhere visible, so that it 

 is impossible to determine whether the mass of Rigaud 

 mountain cuts through the strata in question, as in 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 1 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 occur- 

 rence of ordinary Laurentian gneiss. The abrupt and 

 straight southern boundary of the Laurentian plateau along 

 this part of its course probably marks a fault. Ells has 

 noted the existence of other faults in this district, 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 Paleozoic strata by 

 denudation. It is probable that Mount Calvaire, as re- 

 garded by Logan, is also an outlying portion of the Laur- 

 entian 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 petro- 

 graphical character from Mount Royal and the other 

 members of its grDup They constitute a chain of hills 

 occupying a tract of country some four miles wide and 



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



