[HOWARD] SOME OUTLIERS OF THE MONTEREGIAN HILLS 



69 



Breccias and Other Inirusives, La Trappe 



The village of Oka lies on the north shore of the Lake of Two 

 Mountains directly across the lake from Como; at this place there is 

 a narrow fringe of Potsdam sandstone which is covered by a heavy 

 overburden of drift, and which circles the Pre-Cambrian rocks of the 

 Oka Mountains immediately to the north. Four miles north-east 

 of Oka is the Monastery of La Trappe which lies on the south-eastern 

 border of these mountains. 



Several occurrences of breccia and basic intrusives of undoubted 

 Monteregian character cut the Gren ville limestones and quartzites 

 in the vicinity of the monastery. These have been briefly described 

 by Harvie, and his paper is frequently quoted in the following descrip- 

 tion of the occurrence and the relationships of the rocks of which it is 

 composed. 



The mineralogical composition of the basic intrusive at La Trappe 

 and the associated dykes is as follows : 



Biotite- Peridotite. —The main intrusive in this vicinity forms a 

 small hill, about 100 feet high and a quarter of a mile across, and is 

 situated to the west of the ravine immediately west of the Monastery. 

 Harvie^ describes this hill as follows: "It is formed by an intrusive 

 plug which has pierced the Grenville Limestone and Laurentian 

 gneiss near the contact of the two. The main mass is almost pure 

 intrusive material but the border zone is a breccia formed from the 

 gneiss, limestone, and Potsdam sandstone. The Grenville is shot 

 through with stringers of the igneous material and there are several 



^Op. cit. pp. 256-258. 



