l6 NEW YORK STATE MUSEUM 



between the two inner granite tongues, is a solid, massive rock 

 which we can interpret only as an intrusion of altered gabbro. It 

 shows two phases, a fine-grained, pepper and salt combination of 

 feldspar and hornblende in about equal quantities ; and a coarser, 

 more blotchy looking rock, of similar composition. This central 

 area has been definitely mapped as gabbro, though its boundaries are 

 very indeterminate. The marginal portions of the mass are more 

 micaceous, much better foliated and less massive, much cut up by 

 granite and quartz veins and dikes, and locally soaked by the 

 granite. Here much of the rock is so exactly like the amphibolites 

 interbedded in the Grenville and considered to be metamorphosed 

 shales, that we are in .doubt how to class them. They have the 

 gabbro on one side of them, and Grenville limestones on the other. 

 All are cut by, and therefore older than, the granite. 



All the granites of the region contain frequent inclusions of the 

 Grenville rocks, and in almost every case these inclusions consist 

 of amphibolite, and inclusions of limestone are never found. 

 Beyond doubt some of these amphibolite inclusions represent 

 altered limestone fragments; but with this exception no amphib- 

 olite was seen within the mapped area which definitely suggested 

 such an origin. 



In the variety of other schists shown in the region, mention may 

 be made of the garnetiferous schist, for the reason that Doctor 

 Martin has found this rock in considerable quantity on the Canton 

 sheet, next east, and has separately mapped the belts of it there 

 found. But two exposures of this rock have been found on 

 Ogdensburg, one near the east margin, which is in direct pro- 

 longation of the belt on the Canton sheet, and the other in a dis- 

 connected patch near the south margin. The rock is a heavy, 

 solid one, chiefly composed of feldspar of greenish-gray color, some 

 quartz, much graphite in small flakes, a small quantity of black 

 minerals, and very numerous purplish-pink garnets, averaging one- 

 quarter of an inch in diameter. 



Rusty gneiss. A very characteristic Grenville rock in all dis- 

 tricts where they are exposed is a peculiar, weak gneiss, which 

 weathers rapidly to a rusty-looking rock of a peculiar yellowish 

 shade. It is usually a highly quartzose rock, holds much pyrite, 

 whose decomposition is chiefy responsible for the peculiar appear- 

 ance of the weathered rock, contains considerable graphite, as do 

 many of the Grenville schists, and in addition certain rarer minerals, 

 such as sillimanite. Thin bands of this gneiss are found in many 



