GNEISSES IN THE HIGHLANDS OF NEW JERSEY 



599 



Microscopic examination of thin sections of the various types 

 shows that the purest granite is made up almost entirely of micro- 

 cline (having always a little microperthitic intergrowth) an acid 

 plagioclase very near albite, and quartz. In such a type dark 

 minerals may be almost lacking but when present they consist 

 of aggregates of chlorite and epidote whose character bears evidence 

 of a derivation from original biotite. Indications of a sHght amount 



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Fig. 4 



of deformation are often shown by strain shadows in the quartz 

 and the curving of twinning lamellae in the feldspars, and in some 

 cases this has gone so far as to produce cracks, along which epidote, 

 chlorite, quartz, and calcite have been deposited. It is very 

 evident, however, that nothing resembling granulation or mashing 

 has occurred. In the darker bands the distinctive feature is the 

 large amount of biotite or hornblende present. In addition quartz 

 is an important constituent, as well as microcline and acid plagio- 

 clase. Magnetite is also quite common, together with such second- 

 ary products as chlorite, epidote, and sericite in rather minor 



