﻿GEOLOGY OF THOUSAND ISLANDS REGION 39 



dack syenite. In the most basic variety seen, these dark minerals 

 constitute no more than 15$ of the rock, the remainder being- 

 feldspar with a little quartz. 



Granting the equivalence of the rock with the general augite 

 syenite, its age is rather definitely fixed as one of the great 

 intrusives of the region, younger than the Grenville and the 

 Laurentian granite, and also younger than the anorthosite intru- 

 sion. Since this latter rock is not represented in the district, and 

 the only direct evidence of age seen in connection with the 

 Theresa syenite is that it cuts the Grenville, this additional evi- 

 dence is welcome. 



Alexandria syenite. The intrusive mass of syenite called for 

 convenience by the above name, since nearly the entire mass is 

 in Alexandria township, lies west and north of Redwood, with 

 a major axis of nearly 6 miles, and with a greatest breadth of 

 nearly 2 miles; this on the supposition that but a single intrusion 

 is here represented, as is believed to be the case. It is possible 

 that two intrusions are here in which case the southern one 

 fourth must be separated from the rest. 



Much of the rock is considerably crushed, granulated and 

 recrystallized, converting it into an augen gneiss. The size of 

 the augen, many of which are a half inch long, bespeaks either 

 a very coarse grained rock originally, or a porphyry, the latter 

 being regarded as most probable. These coarse augen gneisses 

 are chiefly peripheral, and mostly at the south end of the mass. 

 Centrally, considerable cores of much less mashed rock remain 

 which, while of medium coarseness of grain, do not approach the 

 coarseness of the augen. The bulk of the rock is an augen 

 gneiss with small augen, and it may be that the coarse augen 

 gneiss at the south should be separated from the remainder ; the 

 two seem, however, to grade into one another, and no evidence 

 that one cut the other was found, except that in a few localities 

 the coarse augen gneiss is cut by dikes of fine grained red granite. 

 These seem rather acid for dikes from the syenite. It is possible 

 that they are stray dikes of Picton granite. 



The least mashed cores show a rock of granitic texture and 

 medium grain, composed chiefly of a reddish feldspar and black 

 hornblende, the latter in sufficient quantity to give some of the 

 rock a strong resemblance to a diorite. These least gneissoid por- 

 tions always show much mashing, when seen in thin section, the 

 feldspars being granulated at their margins, and the hornblendes 

 fraying out into biotite scales. This change increases until 



