GEOLOGY OF THE LAKE PLACID QUADRANGLE 53 



and pyroxene gneiss which have been cut to pieces by intrusions of 

 VVhiteface anorthosite, the Grenville and the anorthosite nearly 

 always being clearly recognizable as such. Very distinct small 

 inclusions of Grenville gneiss in the anorthositei were noted in 

 various ledges, particularly along the road between i and 2 miles 

 south of Upper Jay; by the road i}i. miles southwest of Upper 

 Jay ; and 2^ miles due north of Keene in the western corner of the 

 large area. 



Another type of mixed gneiss from the above-mentioned area is 

 of particular interest. This rock shows in good outcrops west of 

 the river between 2 and 3 miles north of Keene; in the quarry by 

 the road 3^4 miles north of Keene ; and by, or close to, the road 

 between one-half and three-fourths of a mile north of the quarry 

 (see map). The best place to study this rock is in and around the 

 quarry. In the quarry the rocks are Grenville hornblende and 

 pyroxene gneisses, more or less intimately involved with White- 

 face anorthosite. Much of the rock is a true injection gneiss, the 

 Grenville gneiss having been so intimately penetrated by the anor- 

 thosite magma that the small hornblende and pyroxene crystals 

 were mostly separated from each other and enveloped in the molten 

 mass parallel to the magmatic currents, thus giving to the resulting 

 gray, medium-grained rock a clearly defined foliation. Some por- 

 tions of this rock are richer in dark minerals than others, and some 

 portions show lenselike masses or bunches of dark minerals as dis- 

 tinct inclusions i or 2 inches long which were enveloped in the 

 magma without being broken up. This rock contains occasional 

 light bluish gray labradorite crystals up to an inch in length, and 

 numerous tiny grains of titanite. A thin section of this injection 

 gneiss reveals the following mineral percentages : andesine to labra- 

 dorite feldspar, 58 ; green monoclinic pyroxene, 30 ; green horn- 

 blende, 10; titanite, ij^ ; biotite in tiny flakes, ^; and very little 

 zircon. Another slide is similar, but it has several per cent of 

 quartz in one narrow band. Locally, where the anorthosite greatly 

 predominates, the rock is much coarser grained and the dark 

 minerals are more irregularly arranged so that the foliation is not 

 so pronounced. It is very evident that Grenville gneiss has here 

 been more or less intimately injected by Whiteface anorthosite 

 magma, but there is no indication whatever of actual digestion or 

 assimilation of the Grenville by the magma, the crystals and frag- 

 ments of the Grenville always showing sharp contacts against the 

 enveloping anorthosite. 



