GEOLOGY OF THE POUGHKEEPSIE QUADRANGLE 



17 



Pig. s Sketch ofa micaceous gneiss. Actual size 3 mm. 

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



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 

 cases magnetite is asso- 

 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- 

 mients. 



Shenandoah moun- 

 tain granite. A coarse, 

 white granite made up 

 almost entirely of 

 quartz and feldspar was 

 noted on Shenandoah 

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

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



Shenandoah mountain granite. Actual size 3 mm. 

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

 Mu, muscovite 



