NORMAL AND BASIC SYENITES 479 



The rock is a feldspathic one, averaging 75 per cent of this mineral, 

 which is chiefly microperthite, although a small amount of oligoclase 

 is always present in addition. Pyroxenes are next in importance, both 

 angite and bronzite (or hypersthene) being present and exhibiting char- 

 acteristic intergrowths. Quartz and magnetite are always present, horn- 

 blende often, and zircon, apatite, biotite, titanite, garnet, pyrite, and 

 allanite are more or less frequent accessories. The rock is essentially a 

 feldspar-pyroxene combination, with accessory quartz and magnetite, the 

 feldspar being orthoclase in part and plagioclase (albite to oligoclase) 

 in part, in perthitic intergrowth. 



It is mostly a quite even grained rock, having been rather thoroughly 

 granulated during a time of general metamorphism in the region during 

 the Precambrian. Occasional small feldspar augen remain nearly every- 

 where and in some areas become much coarser and more numerous. 

 Though crushed, the normal rock is not especially foliated, owing to its 

 mineralogy. In its general aspect it seems much more metamorphosed 

 than does the older anorthosite, and this is thought to be due to an 

 original finer grain and lack of the huge porphyritic crystals which char- 

 acterize the anorthosite. 



The basic Syenite 



When followed from place to place the syenite proves to be a quite 

 variable rock, becoming both more basic and more acidic than the normal. 

 Both are peripheral changes in the main, though both occur locally within 

 the general mass. The greater basicity is produced simply by increase 

 in the amount of ferro-magnesian minerals, with corresponding decrease 

 in the feldspar and quartz, the extreme phase being a dark colored, heavy, 

 gabbroic looking rock in which the dark minerals substantially equal 

 the feldspar in amount, though it still remains a microperthite-pyroxene 

 rock. The plagioclase constituent of the microperthite is slightly more 

 basic than in the normal rock, but only slightly. Some hornblende is 

 usually present and garnet often appears, occurring as a corrosion rim 

 between magnetite and feldspar, quite as it does in the gabbros, though 

 less abundantly. Quartz practically disappears in the most basic phases, 

 feldspar augen are seldom seen, and the rock is much more distinctly 

 foliated than is the normal variety. 



While there are occasional masses of more basic rock within the normal 

 syenite area, the bulk of it is a border development along the contact 

 with the anorthosite, and to a lesser extent when in contact with the 

 calcareous sediments of the Grenville. Where the border rock is the 

 Laurentian granite gneiss, the syenite becomes more acid rather than 



