GEOLOGY OF THE LAKE PLACID QUADRANGLE 47 



gray labradorites. This is very clearly a transition zone of typical 

 Keene gneiss produced by the assimilation of Whiteface anortho- 

 site by syenite magma. On the little hill just south of the center 

 of the area, several outcrops of quite typical Keene gneiss contain 

 bands or lenslike inclusions of Whiteface anorthosite, the Keene 

 gneiss magma, moving from a lowier level where it was formed, 

 evidently having penetrated or caught up inclusions of unchanged 

 Whiteface anorthosite at the higher level. The Keene gneiss here 

 contains many tiny red garnets, and the labradorite phenocrysts 

 are very conspicuous on the weathered surfaces. 



A little area, shown on the map i^ miles west of Upper Jay, 

 is thought to be Keene gneiss, but the matrix of this rock is finer 

 grained and more gneissoid than usual. 



Sentinel range area. This long, narrow area extends east-west 

 across the middle of the Sentinel range. . It is about 4 miles long 

 and nowhere over one-fourth of a mile wide. It is all in a rough, 

 densely wooded country, but a good many outcrops make the 

 mapping fairly satisfactory. Perhaps the most instructive ledges 

 are on the little hill i mile northeast of Malcom pond. The top of 

 this hill is quite typical Marcy anorthosite. On the southern side 

 the rocks are variable, being mostly fine to medium grained, 

 gneissoid and gabbroic in appearance with some closely involved 

 basic syenitelike rock containing a few small, scattering labradorite 

 phenocrysts, this latter being presumably Keene gneiss. Near the 

 top of the hill, on the west side, the rock is coarser grained with 

 few dark minerals, and this appears to be quite like typical Keene 

 gneiss. All the types mentioned grade into one another. 



On the hillside one-half of a mile southeast of the hill just 

 described, there are outcrops of a moderately coarse-grained, 

 rather gabbroic rock with some labradorite phenocrysts. Its 

 mineralogical composition, given as no. 3 of table 3, shows that it 

 should be classed as Keene gneiss with strong anorthosite affinities. 



Good exposures of Keene gneiss may be seen in other portions 

 of the areas mapped, particularly for a mile eastward from the 

 summit of Sentinel range, where the typical rock forms a wide 

 transition zone between the Marcy anorthosite on the north and 

 the quartz syenite on the south. 



Sunrise notch area. This, the largest area of Keene gneiss 

 within the quadrangle, is about 3V2 miles long and from one-half 

 to two-thirds of a mile wide. It is a rock distinctly intermediate 

 between Whiteface anorthosite and syenite. Most of the outcrops 

 are quite typica,!, Keene gneiss, though usually not strongly foliated. 



