GEOLOGY OF THE POUGHKEEPSIE QUADRANGLE 21 



and Bald hills, along the road descending from the reservoir, the 

 granitic hornblende rock is often very dark in color, which cor- 

 responds with a greater freshness in the rock. 



The presence of occasional micaceous variations has been noted. 

 They are apparently confined to the more finely laminated portions 

 of the gneiss and there is reason for thinking that the mica is 

 secondary. The thin sections show abundant disseminated mag- 

 netite which has become hydrated in many places, giving surface 

 exposures a rusty color. 



The homogeneous character of the Bald hill granite gneiss is 

 noteworthy. In areal extent, it covers about eleven square miles 

 east of the Hudson. The general igneous character of the rock is 

 very impressive. The varieties that have been described would 

 appear to be explainable as normal variations from a common 

 magma. 



This rock is certainly of Precambric age. By its form and 

 isolation it does not appear to have the character of a basal mem- 

 ber. I have been unable to discover any other type which could 

 reasonably be referred to this gneiss. If a basal formation, it 

 should be of more frequent occurrence in these greatly eroded rocks. 

 It therefore does not appear to be older than the other gneisses. 

 All evidence of a possible unconformity would have been com- 

 pletely obliterated. 



If contemporaneous with the other gneisses, on the assumption 

 that they are sedimentary and that it is igneous and having the 

 character of a sill, it should then occur also in other places to the 

 east. It might be a laccolith, in which case it might have furnished 

 the initial bulge at the time of folding. The more strongly banded 

 character of the gneiss along the margin and the somewhat massive 

 central portions might permit the interpretation of anticlinal 

 structure. 



The pronounced alignment which this granite has with the other 

 gneisses favors the view that it was thrust up into the gneisses at 

 the time of their folding. All possible exomorphic and endomorphic 

 effects v/ould have been neutralized by the agencies of regional 

 metamorphism. 



In addition to its other characters, the thickness of this formation 

 is opposed to the idea that it is of sedimentary origin. 



The Mount Honness spur. A short distance east of the Cold 

 Spring road in the hollow between this spur and Bald hill the rock 

 resembles the Bald hill gneiss. In some places it is granitelike, 



