56 THE ROYAL SOCIETY OF CANADA 



Isle Cadieux. Outlines of augite crystals which attain a diameter 

 of over a centimetre are present. These phenocrysts are almost 

 completely resorbed, leaving only the outlines, while the interior 

 consists of mica and the indeterminate groundmass. In some 

 individuals traces of the original cleavage are quite plainly 

 visible. Altered olivine phenocrysts are also not uncommon. 

 The weathered surface is light brown in colour, and flakes of mica are 

 plainly visible. This weathering is quite superficial and the rock 

 appears to be quite fresh within half an inch of the surface. 



Under the microscope the principal type was found to consist 

 of phenocrysts of chrysolite, augite, and biotite, all of which have been 

 greatly resorbed, followed by the formation of monticellite, biotite, 

 melilite, and carbonates. Opaque iron ore, perovskite, and apatite 

 are present as accessories. 



Biotite is the most abundant constituent in the rock. It is light 

 in colour, the deepest shade being a dark buff, indicating that the 

 mineral is low in iron. 



Some of the biotites present have idiomorphic outlines and are 

 quite fresh, while others are embayed and are considerably altered 

 and the rims of the laths and flakes in which the mineral appears have 

 been bleached and are practically colourless. This, together with the 

 fact that the birefringence of basal sections is below .008, would indi- 

 cate that the mineral is rich in alkalies. Iron ore and perovskite occur 

 scattered throughout the mica, and the colourless portions are not al- 

 ways confined to the rims but are occasionally found within the grains, 

 in which case they are usually bounded by cleavage cracks in the 

 parent mineral. The idiomorphic mica, which is very slightly or 

 not at all altered, forms probably a second generation of mica, while 

 the earlier poikilitic flakes have been considerably resorbed. One 

 individual possessing a regular hexagonal outline and which is 0.5 mm. 

 in diameter has a regular border of colourless mica, only 0.01 mm. in 

 width. The mica is nearly uniaxial in character. 



Chrysolite is present in two forms, probably representing different 

 generations. Those of the earlier generation are irregular in outline 

 and are frequently altered along cracks. Hydrated iron oxide is one 

 alteration product, and usually the mineral is surrounded by a rim 

 of monticellite. The difference in birefringence and the fact that the 

 monticellite is distinctly negative in character render the identifica- 

 tion of the two olivines comparatively simple. The chrysolite appar- 

 ently contains about 10 per cent FeO as the a^igle 2V is practically 

 90°. 



