[HOWARD] SOME OUTLIERS OF THE MONTEREGIAN HILLS 75 



In the larger laths twinning along 100, and the typical hour-glass 

 structure of auglte were noted. The mineral is comparatively fresh. 



Biotite, which has been highly altered to chlorite and iron ore, 

 also occurs in minute laths rarely more than 0.1 mm. in length, and 

 0.01 mm. in width. In places plates of mica are noted with a regular 

 hexagonal outline, which have been altered to a brownish isotropic 

 mineral, while the iron content has been leacjied out and forms 

 clusters of irregular grains close to the altered mica. 



These biotite and augite laths form a complicated network, and, 

 with the iron ore, constitute the greater part of the rock. The ground- 

 mass is indeterminate in character, and has been greatly altered to 

 calcite. It appears to have been originally a calcic feldspar. 



This dyke is undoubtedly associated with the breccias to the 

 west similar to the association between the breccias described in 

 Harvie's paper and the accompanying lamprophyric dykes. 



The next locality in this series is an irregular outcrop of b3.sic 

 rock, very similar in appearance to the main intrusive which lies on 

 the north side of the road from La Trappe to Oka just west of the 

 bridge near the Monastery gates. The contact between this irregular 

 mass and the Grenville limestone is visible in places, and the limestone 

 h^as been very much altered by the intrusive rock. Inclusions of lime- 

 stone in the rock itself frequently occur. 



In the hand specimen, the chief differences between this rock 

 and the main type are that the phenocrysts of augite and biotite are 

 neither so large nor so highly altered. Pyrite and pyrrhotite are 

 present, and there are some patches of the latter mineral several 

 inches in diameter. Although alteration has not proceeded to such 

 an extent as in the main mass, there are a large number of round 

 white grains a millimetre or so in extent, which have been derived 

 from small augite and olivine grains. 



In this section the freshness of the rock as compared with that 

 of the larger intrusive is at once apparent, and the succession of 

 minerals can be more readily determined owing to the relatively 

 small amoujit of resorption and alteration. 



The first mineral to crystallize was apatite, which occurs as 

 small hexagonal crystals sometimes 0.3 mm. in diameter, as well as 

 in lopg narrow laths up to 2 mm. in length and rarely more than 

 0.2 mm. in width. 



Olivine entirely of the iron-rich variety is not so abundant as in 

 the main intrusion, and is almost completely altered to iron ore and 

 a semi-opaque mineral similar to the alteration product of the olivine 

 of the principal exposure. Grains of calcite are also present within 

 the original olivine grains, the limits of which are plainly visible. 



