74 THE ROYAL SOCIETY OF CANADA 



more especially the earlier minerals and their alteration products, 

 this speculation is correct, the more acid part of the magma was in all 

 while the residual magma has to a great extent been removed. If 

 likelihood forced upwards by the slow settling of these more basic 

 constituents and concentrated in the breccia dykes. 



As inclusions of crystalline limestone are noted in other parts of 

 the neighbourhood, some of the calcite present may have been derived 

 from inclusions of the country rock. 



The rock as it is at present constituted can best be described as 

 a biotite-peridotite, although it resembles an alnoite very closely 

 except for the absence of melilite. 



Western Dykes and Breccia: — Besides the ma^in intrusion de- 

 scribed above, there are several outcrops of breccias and associated 

 dyke rocks, of which only two groups will be taken up here. 



The first group consists of the breccia described by Harvie to 

 the west of the main intrusion, a dyke which appears to be very closely 

 associated with it, and a small intrusive mass on the road just west 

 of the Mona'stery gates. 



The breccia is situated a quarter of a mile to the west of the main 

 intrusive and is "a dyke about twenty-five feet wide, which may be 

 followed by occasional outcrops for a distance of half a mile." Al- 

 though it cuts the Pre-Cambrian, it contains "fragments of Potsdam 

 sandstone and one of a rock probably the Calciferous, both of which 

 are stratigraphically higher than the gneiss." The groundmass is 

 very similar to that of the dyke described immediately below, that is, 

 it consists of an altered feldspathic rock with calcite and iron ore. 



In the open field 250 yards north of the Oka-La Trappe road, 

 near the bridge just west of the Monastery, a small dyke was found 

 cutting the Pre-Cambrian quartzite. This dyke, in the hajid speci- 

 men, is dull grey in colour and of a very fine grain. The only dis- 

 cernible minerals are minute flakes of augite, leg's than a millimetre 

 in diameter, and occasional white grains of calcite. 



This dyke is abc^t one foot in width and has a strike of 240° 

 and a dip of 60° to the northwest. It is exposed along twenty-five 

 feet of its length, and breaks up towards the west into tiny stringers, 

 finally disappearing within a few feet. Towards the east it is covered 

 by drift. 



A microscopic examination showed that the rock is composed 

 of a network of laths of biotite and augite in a fine-grained ground- 

 mass and is very highly altered. 



Augite is present as laths about a millimetre in length and 0.1 mm. 

 in width in the largest individuals, the average being 0.2 by 0.04 mm. 



