GEOLOGY OF THE POUGHKEEPSIE QUADRANGLE 2^ 



At Fowler's kaolin mine, east of Shenandoah, a rock was found 

 beneath the kaolin deposits that was almost identical with the Shen- 

 andoah mountain granite, though coarser in texture. The decom- 

 posed rock, from which the kaolin was derived, is usually coarse, 

 showing quartz chunks the size of a walnut in a mass of altered 

 feldspar. Probably the kaolin is the product of the disintegration 

 of a pegmatitic granite. The clay beds are apparently not very 

 extensive, although their exact extent is obscured by glacial deposits 

 along the slope. If the kaolin is thought of as the decomposition 

 product of an arkosic, conglomeratic quartzite, it is difficult to 

 account for the granitoid texture of certain specimens examined 

 and the perfect resemblance which they have to the Shenandoah 

 mountain granite. The quartz chunks are not rounded as one 

 would expect in a conglomerate. A careful search failed to reveal 

 the quartzite in the neighborhood. 



The structural features suggest that certain gneisses of this 

 mass probably are faulted portions of the Shenandoah spur. Their 

 resemblance might, of course, be explained as due to repetition. 



At Hortontown, near the quadrangle boundary, there were noted 

 certain gneisses which had an almost unmistakable sedimentary ap- 

 pearance. Though firmly crystalline, the quartzes frequently show 

 a granular character on the fresh surface of the hand specimen, and 

 the thin interlocking and dovetailing Hght and dark bands and fine 

 texture indicate an impure sediment. There is nothing about such 

 varieties that points to an altered igneous rock. 



The gneisses of the eastern mass were examined in their outcrops 

 along the base of the northwestern slope, along the mountain roads 

 and to some extent along the wooded summit. It did not prove 

 possible to assemble them into an orderly series. They present 

 irregular repetitions of hornblendic and micaceous gneisses with 

 some few minor variations. The micaceous gneisses were the more 

 abundant. 



No decidedly massive types were noted. The thin sections are not 

 conclusive as to the early condition of these gneisses, although in 

 many cases they hint at altered sediments or ancient derivatives. 



THE GNEISS INLIERS 



The Glenham belt. The southern extremity of this belt is a few 

 yards northwest of the dam at Groveville. Above the dam it forms 

 the west wall of the gorge of Fishkill creek as far as Glenham. 

 Northeastward it may be followed distinctly as a narrow belt as 



