GEOLOGY OF THE BLUE MOUNTAIN QUADRANGLE 1 3 



ward extension of the large Grenville area in the vicinity of 

 Newcomb. 



In the extreme northeastern corner of the map an area of less 

 than a square mile shows few outcrops, these being chiefly of 

 hornblende gneiss. 



The inclusions large enough to be separately mapped north of 

 Sprague pond, 2 miles south-southwest of Sprague pond, west of 

 Grassy pond, west of Minnow pond, and west of Buck mountain 

 consist of hornblende gneiss. The small mass on the side of 

 Stephens pond consists of white feldspar and pyroxene gneiss with 

 some limestone. The small area on the road i mile west of Fishing 

 brook crossing in contact with, and more or less shot through by, 

 pink granite consists of hornblende gneiss and nearly white feldspar- 

 quartz gneiss containing some pyrite and pink garnets. 



Three small areas of Grenville within the mixed gneisses are 

 respectively on the eastern shore of Long lake one-third of a mile 

 south of the northern map limit; on the western shore of the lake 

 directly opposite the last named area ; and on the western shore of 

 Long lake a little south of west of Long Lake village. The first 

 two. areas named show large exposures of white sillimanite gneiss 

 and gray gneisses containing red garnets and pyrite, while the 

 second named area shows large outcrops of a greenish gray almost 

 syenitic looking gneiss with specks of pyrite. 



Description of Grenville types. For purposes of comparison 

 with other areas with the idea of possibly working out certain of 

 the broader structural and stratigraphic relations of the Grenville 

 series in the Adirondack region, the more important types of Gren- 

 ville of the quadrangle are here described somewhat in detail. 



Crystalline limestone. The southern half of the quadrangle, in 

 common with the Newxomb, the southern half of the Schroon Lake, 

 the North Creek, and the Thirteenth Lake quadrangles, shows rather 

 extensive development of Grenville limestone, decidedly more so 

 in fact than the southeastern, southern, and southwestern border 

 portions of the Adirondacks. Throughout all the area just men- 

 tioned much of the limestone is very similar, being thoroughly 

 crystalline, very calcitic, usually graphitic, mostly closely associated 

 with hornblende, hornblende-garnet or pyroxene gneisses, and of 

 great thickness — several thousand feet at least. 



Sometimes the limestone ledges scarcely show stratification, but 

 usually the original bedding surfaces are marked by layers in which 

 the dark minerals are more abundant (see plate 4). Because of 



