26 



NEW YORK STATE MUSEUM 



The rocks are certainly only porphyritic phases of the granite or 

 syenite. Thus the only considerable mass of syenite porphyry is 

 that of GrofF mountain which, toward the west, becomes more 

 quartzose and less porphyritic and grades through granitic syenite 

 into granite, while toward the south it merely loses its porphyritic 

 texture and passes into normal syenite. Whitman mountain con- 

 sists of typical granite porphyry which grades southward, especially 

 along Devorse creek, into granite and northward into the granite 

 of the West hill area. From i to 2 miles northeast of Elbow 

 mountain, the small mass of granite porphyry shows a perfect 

 transition westward into pink granite. Similar gradations have been 

 observed in other porphyry areas. 



Some of the best exposures of typical granite porphyry occur 

 along Dewey creek, Devorse creek, the Sacandaga river in the gorge 

 just below Auger fiats, and on the hill just east of Echo lake. 



Examples of granite and granite and syenite porphyries 











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Ol.-An. 5 



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Ol.-An. 25 

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Mixed gneisses 



Six areas of mixed gneisses are shown on the geologic map. 

 Grenville gneisses are more or less prominently represented in all 

 these areas but they are so thoroughly cut up by, and involved with, 

 the great intrusives as to preclude the possibility of separately 

 mapping them. In many places outside these mixed gneiss areas 

 there are locally developed rocks of somewhat mixed gneiss appear- 

 ance, but in all such cases either the intrusive or the Grenville 

 very distinctly predominates. Many times there occur rocks of 

 intermediate character between the Grenville and the granites or 

 syenites and there is no escape from the conclusion that many at 

 least of such intermediate-looking rocks have been produced by 

 magmatic assimilation. Admittedly, however, there are within these 



