GEOLOGY OF THE POUGHKEEPSIE QUADRANGLE 3 1 



These sections, except perhaps, those of the Bald hill granite 

 gneiss and the Shenandoah mountain granite, do not afford any 

 convincing evidence of the original character of the gneisses. They 

 give some support to the inference made as to their alteration and 

 afford some ideas of the age of different characters in the rocks.^ 

 In instances, they bear out the character as seen in the hand speci- 

 men and in the outcrop. In other cases, on account of the coarse- 

 ness of the rock, they entirely fail to show the megascopic structural 

 features. 



There are no striking variations in the kinds of "primary " min- 

 erals present, except in the ferromagnesian, although the propor- 

 tions vary. Quartz is usually present, frequently in large anhed- 

 rons only, but oftener both as large and smaller ones. Sometimes 

 it is absent from the section or quite insignificant. Plagioclase is 

 universal, often with orthoclase, but occasionally alone in types 

 with much ferromagnesian content and little or no quartz. Ortho- 

 clase is occasionally in apparent excess of plagioclase and micro- 

 cHne is frequent. Biotite often appears alone as a primary constitu- 

 ent, being clearly of the same age as the other essential minerals. 

 Hornblende often occurs alone in the same relationships. Biotite 

 sometimes occurs with hornblende, but then often suggests a sec- 

 ondary character from its distribution and subordinate amount. 



Magnetite is abundant and is evidently secondary. It occurs 

 chiefly in irregular grains in bunches or as dust masses in or near the 

 ferromagnesians, or scattered about the section within the feldspars 

 and along fractures. It is occasionally pseudomorphic after the 

 ferromagnesian. The latter are plainly very ferruginous in char- 

 acter. Zircons are numerous and widely distributed. Titanite ap- 

 parently occurs as leucoxene about the magnetite at times. Chlorite 

 is abundant, often replacing all or most of the ferromagnesians in 

 the section, but this mineral is associated with the gneisses which, 

 in the hand specimen, betray an ancient alteration. Muscovite or 

 sericite occur only as secondary minerals in the feldspar, except 

 possibly in the Shenandoah mountain granite. 



The textural features present some variations, but they do not 

 as a rule help much in deciding the question of whether the rock 

 is sedimentary or igneous in origin. Very often the arrangement 

 is very similar to that in plutonic rocks of the granitic or dioritic 

 types and the modifications shown might readily be explained as 

 due to conditions imposed on a magma. Other gneisses, either from 

 a more granular character or from the abundance of the ferro- 

 magnesian mineral, suggest altered sedimentary types. But these 



