﻿Notes on Growp of Thousand Islands. 271 



in the granitoid gneiss, and, although much less distinct, 

 is suggestive of the derivation of the latter from a similar 

 granitic rock by movements set up in it. Numerous long 

 and narrow patches of very coarsely crystalline granite or 

 pegmatite are also present in the rock. These are 

 composed of quartz, potash feldspar, black mica and 

 schorl, with occasionally a small amount of a yellowish 

 green mineral having the blowpipe characters of scapolite, 

 a mineral which would hardly be expected among such 

 surroundings. 



Dr. Coleman also mentions the occurrence of a true 

 gneiss as well as of quartzite on this group of islands. 

 These were observed by me on the two islands known as 

 Aubrey and Lemon Islands, where they were cut by the 

 granite. 



Of considerable importance also as serving to correlate 

 these rocks with the Laurentian on the mainland of 

 Ontario and in the Adirondacks, was the discovery of 

 a large exposure of white crystalline limestone which 

 I found on Island No. 18, and which resembles in all 

 "respects that of the Grenville Series. It occurs crossing 

 the south-west corner of the island, being exposed for 

 a distance of twenty-five yards along one shore, and for 

 about ten yards on the other side of the island where it is 

 seen beneath the surface of the water. The rest of the 

 island is composed of gneiss, except that portion occupied 

 by a diabase dyke which is exposed for a width of fifteen 

 yards, traversing the island at its south-west corner and 

 being bounded by the crystalline limestone on one side 

 and by the gneiss on the other. 



At the west end of Island No. 21, also, slabs of coarsely 

 crystalline limestone were found lying upon the beach. 

 Although this rock was not observed in place, the shape 

 of the fragments indicate its occurrence in the immediate 

 vicinity. The island itself is composed of a white 

 weathering gneissic rock with no iron magnesia con- 



