OF CENTRAL CANADA PART III. 177 



a distinct line of demarcation between the two. Granite and syenite 

 for example, are placed in the Plutonic series, and trachyte, green- 

 stone, basalt, &c., in the Volcanic division; but certain granitic 

 trachytes connect the granites with the volcanic rocks ; and in like 

 manner, certain greenstones merge on the one hand into syenite, and 

 on the other (the distinction between augite and hornblende, except 

 in a purely mineralogical or crystallographic point of view, being 

 practically of little moment) they pass into augitic lavas. This 

 equally affects the sub-division into Volcanic, Trappean, and Granite 

 rocks, adopted by other observers. I would therefore propose, as an 

 arrangement of convenience, the distribution of our Canadian Erup- 

 tive rocks into the following groups : 1. Granites and Syenites ; 2. 

 Anorthosites ; 3. Traps and Greenstones ; 4. Trachytes ; 5. Obsi- 

 dians ; 6. Lavas. 



1 . Granites : The rocks of this group possess, normally, a crystal- 

 line aspect and strongly-marked granular, structure, the term granite 

 being derived from the latter character. Granites are also especially 

 characterized by the presence of free silica or quartz in a crystalline 

 condition. They occur occasionally in broad, straight veins or dykes, 

 but are most commonly seen in the form of complicated, ramifying 

 veins, or in large irregular masses which have often broken through 

 and tilted up the surrounding rocks. Where a granite mass lies in 

 contact with another rock, the latter will necessarily be the older 

 formation if it be tilted up or otherwise mechanically affected by the 

 granite ; or if it be chemically altered near the points of contact ; or 

 if portions of its substance (in a more or less altered state) be enclosed 

 within the granite mass ; or if the granite run into it in the form of 

 veins (Fig. 93). On the other hand, if the adjacent rock rest in un- 

 disturbed position 

 on the surface of 



the granite, and ex- 

 hibit no chemical 

 alteration, it may FlG - 93 - FlG - 94 



be inferred to be the more recent of the two (Fig. 94). Granitic 

 veins frequently cross or intersect each other : intersected veins be- 

 ing necessarily older than those by which they are intersected. The 

 diagram (Fig. 95) exhibits three veins of different ages. No. 1 is 

 the oldest vein, as it is cut and also displaced or " faulted " by the 

 other two. No. 3, again, is the most recent of the series, as it tra- 

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