GEOLOGY OF THE POUGHKEEPSIE QUADRANGLE 



Fig. 2 Bald hill granite gneiss. Actual size 3 mm. Q, 

 quartz; O, orthoclase ; P, plagioclase; H, hornblende; 

 black, magnetite 



tion of the usual variety shows quartz in large and small anhedrons. 



Orthoclase and plagioclase are abundant, with the former slightly 



in excess. There is 

 some microcline and 

 hornblende is plentiful. 

 Irregular grains of 

 magnetite are frequent. 

 There are a few scat- 

 tered zircons. 



In some instances, 

 even where the hand 

 specimen appears rather 

 massive, the thin sec- 

 tion shows a stringer- 

 like arrangement of the 

 hornblende ( see figure 

 2). The magnetite is 

 often hydrated, giving 

 surface exposures a 

 rusty color. 

 The principal variation is a rock of coarser texture, with the 



mineralogy of a diorite. It shows hornblende, abundant plagioclase 



and a very little quartz (see figure 3). 

 In one case where the 



rock was extremely 



fresh the magnetite 



formed a perfect pseu 



domorph after the am- 



phibole and was abund- 

 ant in the section, while 



the hornblende was 



greatly bleached. 



There is utter lack of 



evidence to show that 



the rock has undergone 



a complete change from 



an earlier condition. It 



would seem that, so far 



as the rock has just been 



discussed as to miner- 

 alogy and texture, we are dealing with primary features. On the 



whole, the sections indicate a rock of plutonic habit which took on a 



gneissic character and underv/ent certain other changes at the time 



Fig. 3 

 size 



Diorite variation of the Bald hill gneiss. Actual 

 ; mm. P, plagioclase; H, hornblende; Q, quartz 



