24 NEW YORK STATE MUSEUM 



of Grenville gneisses, with these rocks, in turn, grading into pure 

 biotite-garnet and quartzitic Grenville gneisses at the base of the 

 mountain. Thus we have a perfect transition from the gray, gran- 

 itic rock into the granite porphyry, on the one hand, and into the 

 Grenville on the other so that there appears to be no escape from 

 the idea that these gray, granitic rocks were formed by actual 

 fusion and incorporation of more or less of the Grenville into the 

 granite porphyry magma. The presence of the inclusions does not 

 necessarily oppose this view because they may well enough simply 

 represent fragments of Grenville which were caught in the granite 

 magma just before consolidation or when the temperature was not 

 high enough actually to melt the fragments. Gray granitic rocks 

 of apparently the same origin are common throughout this mixed 

 gneiss area. 



Another interesting mixed gneiss area is the one just north of 

 the village of Horicon. In the vicinity of the quarry, at the base 

 of the mountain, the rock is very typical granite porphyry which 

 contains a few long, narrow, sharply defined, Grenville gneiss in- 

 clusions. Going up the mountain side from the quarry, the granite 

 porphyry, which at times (in patches or wide bands) appears typi- 

 cal, is intimately associated with Grenville. This Grenville occurs 

 as large and small inclusions, often sharply defined and nearly al- 

 ways drawn out parallel to the foliation. The included rocks are 

 chiefly banded biotitic, hornblendic, and quartzitic gneisses often 

 in bands from 20 to 30 feet wide. Toward the top of the moun- 

 tain the rock is mostly like the gray granitic rock already de- 

 scribed as occurring at the top of Prospect mountain, and the 

 inclusions are fewer and not so sharply defined. Here again this 

 granitic gneiss appears to be an assimilation product, while farther 

 down the mountain side the temperature seems not to have been 

 high enough to cause any considerable melting or assimilation of 

 the included gneisses. 



In the large, mixed gneiss area south of Henderson mountain 

 there are many fine illustrations of very intimately associated Gren- 

 ville and gray granitic rocks, the Grenville often having been more 

 or less melted into the granites. The granites predominate and 

 some of them at least are thought to be assimilation products. 

 Such phenomena are well exhibited from Igerna southwestward to 

 the river. 



In the mixed gneiss area which borders the Chase-Kelm moun- 

 tain granite porphyry mass on the west, the prevailing rock is a 



