64 THE ROYAL SOCIETY OF CANADA 



very small, and the grain is allotriomorphic. The mineral is distinctly 

 positive and is, therefore, poorer in iron than the Como chrysolite 

 whose optical character was difficult to determine. 



Monticellite is present as resorption rims about the chrysolite, 

 as has been described above, and also occurs as prismatic laths at- 

 taining dimensions of 0.1 by 0.3 mm. It is usually hypidiomorphic in 

 outline, but occasionally occurs as regular crystals terminated by 

 domes. Besides occasional magnetite grains, it contains minute 

 grains of calcite. In places it is found as chadacrysts in the biotite, 

 in regular crystals, although usually the phenocrysts occur through- 

 out the rock with no definite mineralogical associations. 



Occasionally this mineral appears in the groundmass and where 

 it is in contact with the biotite this contact is usually a straight line, 

 although otherwise the grains are decidedly allotriomorphic. It, 

 therefore, appears as if the monticellite had begun to crystallize before 

 the biotite, but had continued until the formation of this mineral was 

 completed. 



Biotite, the most abundant mineral present in the rock, occurs 

 in two generations. The earlier generation consists of small hypidio- 

 morphic grains, 0.2 by 0.4 mm. These are often included in the 

 poikilitic biotite which constitutes the second generation, and often 

 partially enclose phenocrysts of olivine. The only inclusions noted 

 are iron ore and perovskite. 



Most of the mica present in the rock is present as oikocrysts, 

 that is, as host minerals possessing poikilitic fabric, and with very 

 irregular outlines, except when in contact with the monticellite as 

 noted above. It is often as much as 2 mm. in length and it is usually 

 found that several of these irregular masses are in contact with one 

 another and form roughly a single individual. In these cases, the 

 contact is very uneven, and usually the two grains are in close contact 

 without any intervening minerals. The contact would be indiscern- 

 ible if the two individuals happened to be in optical continuity, and 

 it is quite possible that some of the larger grains are composed of two 

 individuals which happened to be oriented in the same direction when 

 they crystallized from the magma. As a matter of fact, some grains 

 were found to show slightly different interference figures at either 

 end, although, judging from their cleavage, pleochroism, and extinc- 

 tion, no deformation could be observed. If these were originally 

 two distinct crystals which grew together in this way, the exact 

 contact could not be found. 



The poikilitic variety contains grains of the iron and titanium 

 accessory minerals as well as laths, tabular crystals of monticellite, 

 and corroded grains of olivine. 



