Annual report . 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
Annual report . 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. Fig. 3size Diorite variation of the Bald hill gneiss. Actual! mm. P, plagioclase; H, hornblende; Q, quartz i6 NEW YORK STATE MUSEUM of its formation. The gneissic character is best regarded as primary,justifying the use of the term gneissoid granite to qualify the name The restlessness of the magma at the time the minerals wereforming seems to find expression in the stringerlike arrangementof the hornblendes and in parallelly arranged pellets of quartz occur-ring in the feldspars, which do not appear to be secondary and oflater introduction. These features, with the rounded character andsmaller size of some of the grains and the absence of micropegma-titic intergrowth, point to conditions hampering crystal thin sections also show certain dynamic effects of later date,in common with all the gneisses of these mountains, in the form ofstrain phenomena of different kinds. There are one or two instancesof comparative freedom from such in which the quartz always givessharp, de
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Keywords: ., bookcentury1900, bookdecade1900, booksubjectscience, bookyear1902
