﻿GEOLOGY OF THOUSAND ISLANDS REGION 41 



Picton granite. This is the latest, most extensive, most inter- 

 esting, and most important of the intrusives of the region. It is 

 named from Picton island (called Robbins island on the map) 

 where it is most extensively quarried. It is, however, best and 

 most extensively exposed on Grindstone island and would have 

 been named after it except for the fact that the whole name was 

 too long, and the term " Grindstone granite " possibly misleading. 

 It is extensively exposed also on the west end of Wellesley 

 island. Abundant dikes of it appear on the mainland of the 

 Alexandria sheet, cutting the Alexandria granite gneiss and the 

 Grenville schists, but the main mass falls short of reaching the 

 shore. It does reach the mainland on the Clayton sheet, how- 

 ever, judging from the exposures of the Precambric inlier up 

 French creek, and may have wide extent here under the Paleo- 

 zoic rocks. Across the border in Canada it seems to have large 

 extent, though it has not yet been differentiated from the 

 Laurentian in mapping. If, however, we are correct in correlat- 

 ing the granite at Kingston with this rock, a bathylith of consider- 

 able extent is implied. 



The general rock is a rather bright red granite of quite coarse 

 grain. It varies much in this respect however, and much of the 

 border rock is of much finer grain, as is also true of the general 

 run of the dikes which radiate out from the mass. To a certain 

 extent this diminution in apparent size of grain is due to mash- 

 ing, but certainly the major part of it is a primary difference. 



Red feldspars (microperthite, microcline and oligoclase) con- 

 stitute 75$ or more -of the rock. Considerable quartz is usually 

 present and is frequently characterized by a slightly bluish cast, 

 which makes a helpful diagnostic feature of the rock. Horn- 

 blende and biotite are sufficiently abundant to show prominent 

 black spots in the otherwise red rock. In the finer grained border 

 varieties and dikes, these black minerals retreat, quartz becomes 

 somewhat more prominent, and the rock appears more acid. 

 The general rock, however, does not impress one as a particularly 

 acid rock for a granite, and this impression is borne out on 

 analysis (given in a later section). 



The rock of the inlier to the south of Clayton, and that at 

 Kingston are correlated with this granite with some reserve. 

 The Kingston rock is a red granite of almost identical appear- 

 ance with this, agrees closely in composition, and the only hesi- 

 tancy felt in the matter is owing to the distance separating the 

 two areas. In all likelihood the rock can be carried across on 



