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NEW YORK STATE MUSEUM 



appearance in the ledge and hand specimen. The thin section 

 shows quartz, orthoclase, microcHne and plagioclase. A few small 

 and scattered flakes of muscovite, which is probably a primary min- 

 eral, are present. Microcline is abundant. There is a tendency to 

 microperthitic intergrowth of plagioclase and orthoclase. It has 

 the earmarks of a plutonic rock and bears little evidence of 

 giieiissoid structure, so that if it is of Precambric age it must be 

 thought of as having escaped any pronounced foliation. This seems 

 remarkable, considering the prominence of foliation in the gneissic 

 series. The effects of dynamic metamorphism are chiefly in the 

 form of strain shadows in the quartzes. 



Glenham gneiss. The prevailing and characteristic surface rock 

 of the Glenham belt is a granitic gneiss. It appears to be an altered 

 derivative of other gneisses which are entirely similar to those of 

 the Highlands, and which are exposed in places within the belt. 



The surface gneiss is 

 foliated in certain por- 

 tions, while in others it 

 is massive. There are 

 minor variations in tex- 

 ture and in mineralogy 

 which depend upon both 

 an ancient and a more 

 recent alteration. These 

 varieties grade into one 

 another. The gneiss is 

 usually red from dis- 

 seminated iron stains 

 and over much of the 

 belt is deeply chlorit- 

 ized. 



The thin section 

 shows abundant quartz 

 with orthoclase, microcline, plagioclase, and biotite altered to chlor- 

 ite. Magnetite is abundant and zircons are occasional. 



Occasionally the rock consists of feldspar and quartz with very 

 little or no mica. 



Fig. 7 Glenham gneiss. Actual size 3 mm. Q, quartz; 

 M, microcline; P, plagioclase; Cb, chlorite after biotite, 

 carrying magnetite 



OUTCROP OF THE FISHKILL MOUNTAIN GNEISSES 



Matteawan. Gneisses which can be readily traced into those of 

 the Fishkill mountains outcrop near their base in the eastern part 



