GEOLOGY OF THE WEST POINT OUADRAls^GLE, NEW YORK 5I 



Type f. A garnetiferous gneiss. 



Medium to fine-grained foliated quartz-feldspar-biotite rock, 

 spotted with large half-inch garnet-biotite patches, intergrown with 

 sillimanite needles. 



The essential minerals are coarse red-brown biotite, garnet and 

 plagioclase. Accessories are magnetite, zoisite, microcline, yellow 

 brown hornblende. The biotite and plagioclase grains are bent. The 

 biotite is oriented and intersutial. The garnet is confined to large 

 spot of very coarse-grained garnet and biotite, which is the same 

 as that in the groundmass. (See accompanying photomicrograph, 

 plate 6.) 



Type g. Graphitic amphibolite. 



A dark-green, coarse-grained foliated rock, composed almost 

 entirely of chloritized uralite and graphite. The foliation is pro- 

 duced by bands of different widths which apparently were originally 

 amphibole and pyroxene. 



Some of the Grenville rocks are not so abnormal in composition 

 as those noted above. Most of them, however, are schists of some 

 description unless they are heavily impregnated with minerals of 

 magmatic source. Such a schist is shown in the photomicrographs 

 of no. 511 (plate 7) which is a mixed schist. 



Impregnation and injection and absorption products occurring in 

 the same formation show a still greater variation such, for example, 

 as that exhibited by the photomicrograph of no. 512 (plate 8) 

 which has some of the earmarks of igneous addition. As a matter 

 of fact, it is probably chiefly igneous representing some extreme of 

 differentiation or selective injection whose source is believed to be 

 the so-called Pochuck diorite intrusive member. 



Those portions which are gneisses rather than schists are less 

 striking in microscopic features, but all tend to exhibit suggestive 

 content, such as the photomicrograph of no. 301 (plate 9) which 

 is a sillimanite gneiss, or the hornblende-biotite gneiss represented 

 by no. 501 w^hich is probably impregnated with Pochuck diorite. 

 (Plate 10.) 



The older igneous series. Pochuck diorite. It is judged that 

 the oldest igneous representative distinguishable in the area is 

 essentially a diorite. In this quadrangle it is practically always 

 intimately associated with streaked and banded varieties of rock 

 of the character belonging to Grenville metamorphics. It nearly 

 always has the appearance of a gneiss and the composition, although 

 very variable, includes hornblende, pyroxene and plagioclase feld- 



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