164 THE CANADIAN NATURALIST. [June, 



or intimate mixture of orthoclase and quartz, rendered porphyritic 

 by the presence of grains or crystals of orthoclase, of quartz, or 

 of both of these minerals together. The occurrence of this rock 

 at Grenville, where it forms dykes in the syenite of that region, 

 has just been noticed. The fine-grained petrosilicious base of this 

 rock varies in color from dark green to various shades of red, 

 purple, and black ; these diflferences probably depending upon the 

 degree of oxydation of the contained iron. Throughout this paste 

 are disseminated well-defined crystals of a rose-red or flesh-red 

 feldspar apparently orthoclase, sometimes very abundant; and less 

 frequently small grains of nearly colorless translucent quartz. An 

 analysis was made of a characteristic variety of the rock, the base 

 of which was greenish-black, jasper-like, conchoidal in fracture, 

 and feebly translucent on the edges, with a somewhat waxy lustre. 

 The hardness was nearly equal to that of quartz, and the specific 

 gravity 2.62. A few distinct crystals of red orthoclase, and some 

 grains of quartz, were present. The base, freed as much as possi- 

 ble from these, gave as follows : 



I 



Silica 72.20 



Alumina 12.50 



Peroxyd of iron 3.70 



Lime 90 



Potash 3.88 



Soda 5.30 



Volatile 60 



99.08 



The oxygen ratio of the alkalies and alumina is 2.02 : 5.84, or 

 nearly 1:3. The alumina requires 43.80 parts of silica to form 

 with the alkalies 65.48 parts of a feldspar having the ratios 1:3: 

 12, which are those of orthoclase and albite. There will then 

 remain 28.4 parts of silica. This, with the exception of a small 

 amount which is probably united with the oxyd of iron and lime, 

 may be regarded as uncombined. The porphyries of this region 

 receive a high polish, and are sometimes very beautiful. 



Syenite. — The syenite of this region consists of orthoclase, 

 usually flesh-red in color, and grayish vitreous quartz, with a small 

 portion of blackish-green hornblende, which is sometimes almost 

 or altogether wanting, and is occasionally accompanied with a 

 little mica. The orthoclase is often nearly compact, but more gen- 



