GEOLOGY OF THE POUGHKEEPSIE QUADRANGLE 



17 



condition indicates that the alteration is an ancient character. Fig- 

 tire 4 gives a sketch of a thin section of typical hornblende gneiss. 



Micaceous gneisses. 

 These may be passed 

 over briefly. Except 

 that biotite plays the 

 role of hornblende, they 

 are very similar in their 

 mineralogy. In some 



Pig. s Sketch of'a micaceous gneiss. Actual size 3 mm 

 Q, quartz; 0, orthoclase; P, plagioclase; B, biotite 



ciated with a mineral 

 whose identity is lost or 

 obscured. The thin sec- 

 tions often suggest that 

 the prominent biotite is 

 secondary and in these 

 cases the outlines of an- 

 other mineral, possibly . 

 hornblende, may be 

 faintly traced. In these 

 instances it is possible that the biotitic gneiss was first a hornblende 

 rock and that it was subjected to more than usual alteration before 

 recrystallization. 



Microcline is rather 

 abundant. Biotite oc- 

 curs abundantly as a 

 " primary " mineral in- 

 dependent of horn- 

 blende. Sometimes 

 these gneisses show 

 much quartz and are 

 finegrained, strongly 



suggesting altered sedi- 

 ments. 



Shenandoah moun- 

 tain granite. A coarse, 

 white granite made up 

 almost entirely of 



quartz and feldspar was ^^S- ^ Shenandoah mountain granite. Actual size 3 mm. 



O. quartz; 0. orthoclase; P, plagioclase; M, microcline; 



noted on Shenandoah ^^'^'> muscovite 



mountain at the summit of the steep northwestern slope, along the, 



road from the East Hook to Hortontown. It is very massive in 



