RELATION OF ANORTHOSITE TO GRENVILLE SERIES 421 



to pieces by intrusions of Whiteface anorthosite, the Grenville and the 

 anorthosite nearly always being clearly recognizable as such. Very dis- 

 tinct small inclusions of Grenville gneiss in the anorthosite were noted 

 in various ledges, particularly along the road between 1 and 2 miles south 

 of Upper Jay; by the road I14 miles southwest of Upper Jay, and 21/4 

 miles due north of Keene, in the western corner of the large area. 



Another type of mixed gneisses from the large area above mentioned 

 is of particular interest. The best place to study these rocks is in and 

 around the quarry by the road 3^ miles north of Keene. In the quarry 

 the rocks are Grenville hornblende and pyroxene gneisses more or less 

 intimately involved with nearly pure plagioclase Whiteface anorthosite. 

 Much of the rock is a true injection gneiss, the Grenville having been so 

 intimately penetrated by the anorthosite magma that the small horn- 

 blende 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 folia- 

 tion. Some portions of this rock are richer in dark minerals than others, 

 and some portions show lenslike masses or bunches of the dark minerals 

 as distinct inclusions, about 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 

 grains of titanite. A thin section reveals the following mineral percent- 

 ages : Andesine to labradorite, 58 ; green monoclinic pyroxene, 30 ; green 

 hornblende, 10; titanite, II/2, and a very little zircon. Another slide 

 shows several per cent quartz in a narrow band. Locally, where the anor- 

 thosite 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 the Grenville of this locality has 

 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 fragments of Grenville 

 always showing sharp contacts against the enveloping anorthosite. 



The area, 2 miles long, on the southern end of Wilmington Mountain, 

 contains mostly Whiteface anorthosite, but nearly every outcrop has in 

 it so many inclusions of Grenville, chiefly green pyroxene gneiss, that it 

 has seemed advisable to map this as an area of mixed rocks. Injection 

 gneisses like those above noted were not observed. 



Along the road just east of Franklin Falls there are some instructive 

 ledges of Grenville and Whiteface anorthosite mixed gneisses. One phase 

 of this rock is white, medium grained Whiteface anorthosite (practically 

 all plagioclase) containing approximately 20 per cent irregular lenslike 



