[HOWARD] SOME OUTLIERS OF THE MONTEREGIAN HILLS 79 



Principal Type. — The larger part of the rock is composed of 

 phenocrysts of pale biotite, augite, diopside, and olivine with a large 

 amount of black iron ore in a granular groundmass which is very 

 largely altered to calcite but contains some andesine, nepheline, and 

 glass. 



Biotite occurs in irregular grains and hypidiomorphic crystals 

 which may be as much as a centimetre in length. The mineral is 

 light greenish brown in colour, and the pleochroism is weak. The 

 crystals are deformed in places indicating movement after partial 

 cooling, and a small amount of resorption has taken place Some of 

 the iron content has leached out giving rise to small grains of magne- 

 tite along the cleavage cracks and also around the edges of the grains. 



Augite is present in equi-dimensional rounded grains from 1.5 

 to 2 mm. in diameter. It is slightly tinted in yellow and puiple 

 shades and contains inclusions of iron ore arranged along definite 

 lines through the grains; clusters of iron ore around the augite are 

 common. The purple tint indicates that the mineral is rich in 

 titanium. Augite is the most abundant ferro-magnesian constituent 

 present, the two next in order of abundance being phlogopite and 

 diopside. 



Diopside is found in subordinate amounts in irregular colourless 

 grains about a millimetre in diameter. Some of these diopside grains 

 are surrounded by a rim of iron-rich augite. In such cases, the interior 

 of the grain extinguishes at —44° and the bordering augite at 

 — 56°. This augite is colourless and tends toward the more sodic 

 aegirite-augite. Frequently the border is equal in width to the 

 unaltered interior. Alteration also proceeded along cracks in the 

 diopside by means of iron-rich solutions, and the superfluous iron 

 was deposited as black iron ore along the cracks and around the 

 border. 



Olivine is present in very subordinate amount and occurs as 

 idiomorphic crystals up to 2 mm. in length and also in rounded grains 

 surrounded by a rim of secondary minerals such as calcite and anti- 

 gorite. 



The groundmass is largely altered to calcite and other secondary 

 minerals in minute grains, but andesine or acid labradorite and 

 nepheline can be distinguished. The minerals forming the ground- 

 mass usually form rounded grains less than 0.5 mm. in diameter, 

 and some isotropic grains which appear to be glass were observed. 



Magnetite and apatite are present as accessories, the former in 

 irregular grains ranging from less than 0.01 to 1.5 mm. in diameter. 



