72 THE ROYAL SOCIETY OF CANADA 



the exception of a few minute grains along the interior. The border 

 is composed of minute grains of a colourless non-pleochroic mineral 

 which has a refractive index slightly lower than that of the olivine. 

 These grains are rarely over 0.03 mm. in diameter, and are closely 

 packed together, although a tendency to alignment normal to the 

 edges of the olivine is noticeable. The highest interference colour 

 observed is a deep yellow of the first order, making the maximum 

 birefringence about .011, or slightly below that of monticellite. The 

 mineral is biaxial and negative, and there is a very poor cleavage, 

 which may be simply tiny irregular fractures. If this is the case the 

 mineral can be assumed to be fibrous serpentine and the olivine from 

 which it has been derived therefore contains some magnesia. The 

 interior of the olivine is quite fresh, and there is practically no 

 alteration of any kind even along the cracks. 



Throughout the groundmass there are small grains of olivine 

 less than 0.1 mm. in length surrounded by fibrous serpentine and 

 the latter mineral is profusely scattered through the rock in 

 small grains; frequently small areas are found in which the ojivine 

 has been completely altered to this mineral. The presence of these 

 smaller grains of olivine and serpentine throughout the whole of the 

 rock is very probably due to a grinding motion of some sort which 

 ripped off fragments from the phenocrysts and scattered them through 

 the magma. This action is also noted below in the case of thebiotite. 



The otWr form of olivine present is similar to the variety thus 

 described, except that it is non-pleochroic and is surro^uhded by a 

 semi-opaque border of a mineral resembling leucoxene. These rims 

 vary greatly in character, and to describe them fully woiild require 

 a detailed description of every individual of olivine present in the 

 rock. Suffice it to say that these olivine grains, sometimes a milli- 

 metre in diameter, are surrounded by a narrow rim of serpentine, 

 which is in turn enclosed by a rim about 0.35 mm. in width of a 

 semi-opaque mineral which resembles leucoxene very closely, although 

 it is not derived from ilmenite, and must, therefore, differ from leu- 

 coxene in composition. 



It is quite within reasdn to suppose that the larger grains which 

 are only surrounded at present by serpentine were formerly enclosed 

 in a semi-opaque rim as well, but that this rim has been removed by 

 subsequent motion within the magma. 



Augite is present in variable amounts, and is of the ordinary 

 variety. It is also surrounded by alteration rims very similar to 

 those surrounding the non-pleochtoic olivine. 



