[HOWARD] SOME OUTLIERS OF THE MONTEREGIAN HILLS 71 



be understood that other investigators visiting this area may find 

 still other minerals that were not found by their predecessors. 



The rocks collected by the writer from this place were ex- 

 amined in thin section and are composed of olivine, augite, and 

 biotite, all very much altered, with rounded grains of labradorite 

 and apatite in a fine-grained groundmass. The accessory minerals 

 consist of magnetite, ilmenite, pyrite, pyrrhotite, and perovskite, 

 most of which appear to be of secondary origin as they do not occur 

 to such an extent in the fresher rocks described below. Serpentine, 

 calcite, and a semi-opaque mineral very similar to leucoxene are 

 undoubtedly secondary. Amygdules filled with natrolite also are 

 present. 



Magnetite. — A most striking feature of the rock is the large 

 grains of black iron ore present. These grains are often over three 

 millimetres in diameter, and are frequently surrounded by a number 

 of smaller grains. Owing to the absence of leucoxene around these 

 grains and the fact that Ti02 is not abnormally large in the rock, this 

 mineral is supposed to be largely magnetite, although some ilmenite 

 is undoubtedly present. Besides the clusters about the larger grains, 

 small grains of magnetite are scattered throughout the rock and there 

 is a rim of magnetite about 1 mm. in width around the flakes of biotite. 

 This association is described more fully below. 



With the magnetite is associated pyrite and pyrrhotite, either 

 in small separate individuals or speckled through the larger masses 

 of iron ore. 



There are two varieties of olivine present, one apparently richer 

 in iron than the other, and further variation is noted in the decom- 

 position products of the two varieties. 



An iron-rich olivine occurs as large grains and is colourless 

 but feebly pleochroic, in pale yellowish tones, C being colourless. 

 The birefringence 7 — a = .050, and 7-/3 = .010; the mineral is biaxial 

 and distinctly negative in character. Olivine which is apparently 

 slightly different in composition from this pleochroic variety is also 

 present. It has a slightly lower birefringence and is non-pleochroic. 

 This variety is invariably surrounded by a semi-opaque rim of 

 alteration products, and is rarely distinguishable in the hand speci- 

 men, while the pleochroic variety is fresher and is transparent in 

 the hand specimen. 



The iron-rich variety which occurs in large grains, several 

 millimetres in diameter, is surrounded by a uniform border, about 

 0.1 mm. in width, of a mineral which is very similar to that forming 

 the bulk of the groundmass. Iron ore is absent from this border, with 



