10 NEW YORK STATE MUSEUM 



is considerably less than usual in the Adirondacks, there being only 

 about 14 square miles shown as such on the accompanying geologic 

 map. Besides this there must be added a few square miles of Gren- 

 ville represented in the mixed gneiss areas and as nonmappable 

 scattering inclusions throughout the great intrusives. The Grenville 

 is scattered over the quadrangle in masses ranging in size from very 

 small inclusions to a belt over 9 miles long. Almost invariably these 

 inclusions or belts lie parallel to the foliation of the inclosing igneous 

 rocks, with the foliation of the Grenville and inclosing rocks also 

 practically always parallel. 



The largest Grenville area, extending from Wallace mountain 

 to Loomis pond, is remarkable because of its narrowness combined 

 with so great a width. It strikes due east and west 9^ miles, or 

 three-fourths of the distance across the quadrangle, while its width 

 scarcely exceeds i mile at any place. Its westward extension beyond 

 the map limits is not known. There are many good outcrops 

 throughout the area, but more especially on Wallace, Three Ponds, 

 and Silver Lake mountains and from Loomis pond eastward for 

 2 miles. In general the rocks are of two types, namely, gray to 

 nearly white, feldspar-quartz-garnet gneisses and quartzites, the 

 gray gneisses sometimes carrying sillimanite or graphite. All these 

 rock types are more or less interbedded. In thin-section the typical 

 light gneiss shows the following mineral percentages : orthoclase 

 62: microperthite 10; oligoclase 5; quartz 15; garnet (clear pink) 

 18; biotite i: and small amounts of zircon and titanite. The 

 graphitic gneisses are similar except for lower feldspar and higher 

 garnet content, and a few per cent each of sillimanite and graphite 

 with usually a little pyrite. The typical quartzites contain 85 to 

 90 per cent quartz ; a few per cent each of hypersthene and biotite ; 

 and smaller amounts of magnetite, sillimanite, and apatite. Quart- 

 zites are best seen in exposures on the north face of Silver Lake 

 mountain, the top of Three Ponds mountain, and at the west base 

 of Wallace mountain. Great ledges of the light, feldspar-garnet 

 gneisses constitute the upper two-thirds of Wallace mountain, par- 

 ticularly on the west side, where the beds lie nearly horizontal and 

 rest directly upon quartzite. Other good ledges of the feldspar- 

 garnet rocks occur at The Notch and on the north shore of Loomis 

 pond. The best outcrops of the graphitic rocks were found near the 

 north base of Silver Lake mountain. Throughout this long Gren- 

 ville area the dips are southward usually 40 to 50 degrees, though 

 In Wallace mountain it is only 10 degrees. Thus a thickness of at 

 least a few thousand feet of Grenville must be shown. It should 



