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NEW YORK STATE MUSEUM 



of its formation. The gneissic character is best regarded as primary, 

 justifying the use of the term gneissoid granite to quahfy the name 

 granite gneiss. 



The restlessness of the magma at the time the minerals were 

 forming seems to find expression in the stringerlike arrangement 

 of the hornblendes and in parallelly arranged pellets of quartz occur- 

 ring in the feldspars, which do not appear to be secondary and of 

 later introduction. These features, with the rounded character and 

 smaller size of some of the grains and the absence of micropegma- 

 titic intergrowth, point to conditions hampering crystal formation. 

 The thin sections also show certain dynamic effects of later date, 

 in common with all the gneisses of these mountains,, in the form of 

 strain phenomena of different kinds. There are one or two instances 

 of comparative freedom from such in which the quartz always gives 

 sharp, decisive extinction and in which prominent cracks and bent 

 lamellae are absent. 



Hornblende gneisses. The outcrops of these rocks are much 

 alike and the thin sections which have been examined agree very 

 closely. Exposures are dark in color. The essential minerals are 

 chiefly plagioclase and hornblende, with some quartz and a little 

 orthoclase. Magnetite is rather common as irregularly-shaped par- 

 ticles, or as dustings. 

 Zircons are occasional. 

 Some sections show 

 biotite in addition to 

 hornblende, but the for- 

 mer is decidedly subor- 

 dinate and usually has 

 every appearance of 

 being secondary. It ap- 

 parently belongs to that 

 period of metamorph- 

 ism which more usually 

 found expression in 

 strain phenomena of 

 different kinds but 

 which sometimes re- 

 sulted in new minerals 

 among the " primary " 

 ones, especially in those cases where the rock had previously been 

 exposed to unusual alteration. The feldspars also frequently show 

 evidence of former decay. The indurated and general compact 



Fig. 4 Sketch of a hornblende gneiss. Actual size 3 mm. 

 Q, quartz; P, plagioclase; H, hornblende; black, mag- 

 netite 



