GEOLOGY OF THE POUGHKEEPSIE QUADRANGLE 



23 



formed, west of the barn of Irving Knapp, thick masses, resem- 

 bhng both the gneisses and the quartzite in their mineralogy, dip to 

 the north at a moderate angle. Farther along the road to the east 

 of the house, ledges more closely resembling the quartzite were 

 found. The woods and thick covering of drift, however, greatly 

 obscure everything to and for a short distance beyond the west 

 road into the mountains. South of the Carey farm, between the 

 brook and the road, the quartzite was found grading downward 

 into a hybrid rock. 



The Hook district. South of the quartzite slope, back of the 

 farm of Garrett Smith, the thick woods obscure the succession in 

 the gneisses and good outcrops are scattered. The outcrops in the 

 field southwest of Alonzo Smith's house (see plate 4) on the east 

 road into the mountains and in the neighboring woods, are micaceous 

 gneisses. Within the small space of the outcrop shown in the plate 

 the gneiss passes from a rather coarse rock with quartz stringers 

 through one with finer laminations into a purpHsh rock with still 

 finer laminations. 



A comparison of the thin sections of these varieties shows a 

 similarity as to essential " primary " minerals with biotite as the 

 ferromagnesian constituent. The feldspar is chiefly plagioclase. 



Quartz is abundant. 

 The degree of alteration 

 of the primary minerals 

 varies much. It is severe 

 both in the feldspars 

 and the biotite, but 

 shows itself chiefly in 

 the latter. In the 

 coarser gneiss the biotite 

 is only slightly altered, 

 while in the finely lami- 

 nated purplish rock it is 

 represented by masses 

 of magnetite and a great 

 abundance of finely 

 granular material, prob- 

 ably sericite, with only 

 occasional traces of the boundaries of the original mineral (see figure 

 9). The second variety mentioned shows a gradation between the 



Fig. 9 Altered micaceous gneiss from the Hook district. 

 Actual size 3 mm. Q, quartz; P, plagioclase; black, 

 magnetite from biotite 



