﻿GEOLOGY OF THOUSAND ISLANDS REGION l8l 



Picton granite 



The specimen of Picton granite analyzed was selected as an 

 average representative of the rock, and bears out the impression 

 gained in the field that as a whole it is less acid than the Lauren- 

 tian granites where uneontaminated by Grenville material. It 

 seems less quartzose, and always shows considerable hornblende, 

 which is relatively scarce in the granite gneiss. The thin section 

 shows it to be fairly rich in accessory minerals, titanite and apatite 

 especially being frequent and' fairly coarse, the former particularly 

 so. Some pyrite is present, zircon also, little hematite inclusions 

 in the feldspars, and ilmenite or rutile needles in the quartz. A few 

 minute tourmalin crystals also occur. The green hornblende is alter- 

 ing to biotite, and there is additional biotite in the rock as well. For 

 the feldspars, microperthite, microcline, microcline-microperthite 

 and oligoclase are all present in considerable amount, and all with 

 strongly marked characters. A good deal of micropegmatite, some 

 of it quite coarse, is also to be seen. Altogether, in its minor 

 mineralogy, the rock presents considerable contrast to the granite 

 gneiss. 



The norm of the rock is a follows : 



Class i, persalane 

 -89.00 Order 4, britannare 



Rang 2, toscanase 



Or... 



• 33-36^ 



Ab... 



. 26.20 



An.. 



. 8.62 



Qz... 



. 20.82 



Hy.. 



.. 4-48" 



Mt.. 



• 3-48 



11.... 



. 1.52 



Py... 



. 0.15 



Ap.. 



. 1. 01 



Subrang 3, toscanose 



10.64 



It thus falls in the same rock group as the granite gneisses, 

 but is much nearer the border of the group than they are. The 

 greater variety and abundance of the femic and alferric minerals, 

 hornblende, biotite and titanite, would cause the mode to depart 

 somewhat more widely from the norm than in the previous cases, 

 the lime to form titanite being deducted from the anorthite, re- 

 leasing alumina for the biotite and diminishing the quartz per- 

 centage. 



The dike phases of this granite range more acid than this, but 

 with this exception it is thought that the average of the rock com- 

 position is well represented by the analysis. The composition is 



