70 NEW YORK STATE MUSEUM 



the larger part. It also outcrops on the smaller islands between 

 Wellesley and Grindstone, including Murray, Picton, and Bluff 

 islands. 



GRINDSTONE ISLAND GRANITE AREA 



Grindstone is an irregular, deeply indented island, about 5 miles 

 long and 2 miles wide, lying nearly midway in the river, directly 

 opposite Clayton. It is included in the Grindstone quadrangle of 

 the United States Geological Survey. The island is low and thinly 

 soiled, though it affords some good grazing and agricultural land. 

 The principal settlement is Thurso on the north shore and near the 

 western end. 



As shown on the geological map by Cushing and Smyth, the red 

 granite occupies all the eastern and northern part of the island, but 

 on the south and west gives way to the older Grenville.and Lauren- 

 tian gneiss series, into which, however, it sends offshoots that in 

 places are of considerable magnitude. It is also not unmixed with 

 these rocks, as inclusions of the Grenville schist and quartzite and 

 of the lighter Laurentian granite are found within the interior of 

 the red granite. These inclusions appear, however, to be arranged 

 in definite belts and are not so generally distributed as to give 

 trouble in quarry operations, if a little care is exercised in the 

 selection of a site. Aside from these larger inclusions the granite 

 shows a fair degree of uniformity. Occasional " knots " of darker 

 color are noticeable in some of the quarries and seem to be in the 

 nature of segregations. 



The principal quarry workings are in the vicinity of Thurso. For 

 the last few years none of the quarries have been actively operated, 

 though some stone is (taken out occasionally on orders for building 

 and monumental work. The period of greatest activity dates back 

 fully fifteen years. In Smock's report of 1888 it is stated that 

 quarries had been opened at more than twenty different places on 

 the island and that three large quarries were then in operation. 



General character and composition. The Grindstone Island 

 granite usually has a coarse texture which is imparted by the abund- 

 ant large feldspar individuals. It has, nevertheless, excellent polish- 

 ing qualities, giving a fine and lustrous surface. The color is bright 

 red for the polished surfaces but lighter on the rock face and very 

 light on hammered work. The stone is therefore suitable for 

 buildings in which a medium color effect is desired and at the same 

 time is well adapted for monumental or interior work. 



The mineral composition of the granite is somewhat variable 



