102 NEW YORK STATE MUSEUM 



syenite areas adjoin, each showing these peripheral phases, it is^ 

 an impossible matter sharply to delimit their boundaries in map- 

 ping. 



In some cases hypersthene, augite'and garnet, with or without 

 biotite and hornblende, constitute such a large portion of these 

 peripheral rocks that they become true gabbros or norites in com- 

 position, so that one can pass from anorthosite to gabbro in going 

 from the center to the exterior of a single intrusive mass. 



Wherever anorthosite is the surface rock, exposures abound. 

 As indicated on the accompanying map, a large area in the south- 

 eastern part of the county is wholly occupied by this rock. It is 

 cut by numerous dikes, later to be considered, but these are almost 

 without exception of no great size. It appears in a series of east 

 and west ridges, which attain high altitudes in the extreme south- 

 east with Mt Seward and Ampersand mount as the culminating, 

 peaks; in the lake belt the relief is but slight, the majority of the 

 ridges not being more than one or two hundred feet above the 

 water surface, with the comparatively low peaks of Boot Bay and- 

 Long Pond mountains representing the highest points; to the west 

 of the lake belt the extent of the rock is not great, but the ridges 

 are higher, culminating in St Regis, Jenkins and Ore-bed moun- 

 tains. Abundant exposures occur about the Saranac and St Regis ; 

 lakes and the host of ponds in the lake belt, the trend of the ridges 

 being at right angles to the trend of the larger lakes, and result- 

 ing in the production of their extremely irregular shore lines. 

 Exposures are so much more abundant, than where the gneisses or 

 the Grenville rocks are at the surface, that the passage from the 

 one to the other may be inferred from this alone where outcrops 

 are not forthcoming. 



The northern boundary of the anorthosite is very plain, unmis- 

 takable anorthositic rocks adjoining closely rocks of totally 

 different character. On the south and southwest this is not the 

 case, the rock fading out into the augen gneiss already described. 

 This change is well shown by the exposures along the Racquette- 

 river going either west or south from the big bend at Axton, where- 

 anorthosite-gabbro is exposed. Going west, the last of the augen* 



