Annual report . Fig. 4 Sketch of a hornblende gneiss. Actual size 3 , quartz; P, plagioclase; H, hornblende; black, mag-netite GEOLOGY OF THE POUGHKEEPSIE QUADRANGLE 17 condition indicates that the alteration is an ancient character. Fig-tire 4 gives a sketch of a thin section of typical hornblende gneiss. Micaceous may be passedover briefly. Exceptthat biotite plays therole of hornblende, theyare very similar in theirmineralogy. In some. Pig. s Sketch ofa micaceous gneiss. Actual size 3 , quartz; O, orthoclase; P, plagioclase; B, biotite ciated with a mineralwhose ident
Annual report . Fig. 4 Sketch of a hornblende gneiss. Actual size 3 , quartz; P, plagioclase; H, hornblende; black, mag-netite GEOLOGY OF THE POUGHKEEPSIE QUADRANGLE 17 condition indicates that the alteration is an ancient character. Fig-tire 4 gives a sketch of a thin section of typical hornblende gneiss. Micaceous may be passedover briefly. Exceptthat biotite plays therole of hornblende, theyare very similar in theirmineralogy. In some. Pig. s Sketch ofa micaceous gneiss. Actual size 3 , quartz; O, orthoclase; P, plagioclase; B, biotite ciated with a mineralwhose identity is lost orobscured. The thin sec-tions often suggest thatthe prominent biotite issecondary and in thesecases the outlines of an-other mineral, , may befaintly traced. In theseinstances it is possible that the biotitic gneiss was first a hornblenderock and that it was subjected to more than usual alteration beforerecrystallization. Microcline is ratherabundant. Biotite oc-curs abundantly as a primary mineral in-dependent of horn-blende. Sometimesthese gneisses showmuch quartz and arefinegrained, strongly suggesting altered sedi-ments. Shenandoah moun-tain granite. A coarse,white granite made upalmost entirely ofquartz and feldspar wasnoted on Shenandoahmountain at the summit of the steep northwestern slope, along theroad from the East Hook to Hortontown. It is very massive in
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Keywords: ., bookcentury1900, bookdecade1900, booksubjectscience, bookyear1902
