Annual report . es maybest be indicated in discussing their outcrops. The thin sectionsmay be reviewed as a whole later. Possible ancient surface altera-tions must always be carried in mind. Bald hill granite gneiss. This rock is prominently developedwithin and south of the quadrangle. There is great uniformity inits general color, mineralogy and texture. It shows a few variations,but as a whole is remarkably homogeneous. In outcrops it is com-monly drab colored and granitelike in appearance. The thin sec- GEOLOGY OF THE POUGHKEEPSIE QUADRANGLE tion of the usual variety shows quartz in large a
Annual report . es maybest be indicated in discussing their outcrops. The thin sectionsmay be reviewed as a whole later. Possible ancient surface altera-tions must always be carried in mind. Bald hill granite gneiss. This rock is prominently developedwithin and south of the quadrangle. There is great uniformity inits general color, mineralogy and texture. It shows a few variations,but as a whole is remarkably homogeneous. In outcrops it is com-monly drab colored and granitelike in appearance. The thin sec- GEOLOGY OF THE POUGHKEEPSIE QUADRANGLE tion of the usual variety shows quartz in large and small and plagioclase are abundant, with the former slightly in excess. There issome microcline andhornblende is grains ofmagnetite are frequentThere are a few scat-tered zircons. In some instances,even where the handspecimen appears rathermassive, the thin sec-tion shows a stringer-like arrangement of thehornblende (see figure2). The magnetite isoften hydrated, giving. Fig. 2 Bald hill granite gneiss. Actual size 3 mm. Q,quartz; O, orthoclase; P, plagioclase; H, hornblende;black, magnetite surface exposures arusty color. The principal variation is a rock of coarser texture, with themineralogy of a diorite. It shows hornblende, abundant plagioclaseand a very little quartz (see figure 3). In one case where therock was extremelyfresh the magnetiteformed a perfect pseu-domorph after the am-phibole and was abund-ant in the section, whilethe hornblende wasgreatly bleached. There is utter lack ofevidence to show thatthe rock has undergonea complete change froman earlier condition. Itwould seem that, so faras the rock has just beendiscussed as to miner-alogy and texture, we are dealing with primary features. On thewhole, the sections indicate a rock of plutonic habit which took on agneissic character and underwent certain other changes at the time
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