ASSIMILATION OF THE SYENITE-GRANITE MAGMA 259 



they may well enough simply represent fragments of G-renvilte which 

 were caught in the granite magma just before consolidation, or when the 

 temperature was not high enough to actually melt the fragments. Gray 

 granitic rocks of apparently the same origin are common throughout this 

 mixed gneiss area, which occupies about 2 square miles. 



Another interesting mixed gneiss area is the one just north of the vil- 

 lage of Horicon^^ (North Creek quadrangle). In the vicinity of the 

 quarry at the base of the mountain the rock is very typical granite por- 

 phyry, which contains a few long, narrow, sharply defined Grenville 

 gneiss inclusions. Going up the mountain side from the quarry the 

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

 typical, is intimately associated with Grenville. This Grenville occurs 

 as large and small inclusions, often sharply defined and nearly always 

 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 mountain the rock is mostly 

 like the gray granitic rock already described as occurring at the top if 

 Prospect Mountain, arid the inclusions are fewer and not so sharply de- 

 fined. 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 assimila- 

 tion of the included gneisses. 



. Excellent exposures in an open field between Blue Mountain Lake vil- 

 lage and Crystal Lake (Blue Mountain quadrangle) afford a practical 

 demonstration of the very intimate relations of granitic and Grenville 

 gneisses with intermediate rocks due to more or less assimilation. The 

 granites are pinkish to grayish and rather variable. In certain outcrops 

 Grenville gray, biotitic or dark garnetiferous or pyroxenic gneisses may 

 be seen to grade into the igneous rock with no visible contacts. In a few 

 cases contacts are fairly sharp. Most of the exposures, however, consist 

 of rocks of intermediate character, clearly the products of assimilation. 



An area of what is regarded as a basic (gabbroic) phase of normal sye- 

 nite extends from Speculator Mountain to Indian Head Mountain (Lake 

 Pleasant quadrangle), showing a length of 5 miles and a maximum: 

 width of 1% miles. The most typical rock contains 75 to 80 per cent 

 oligoclase to labradorite; 6 to 9 per cent pyroxene (usually augite) ; 3 to 

 10 per cent hornblende ; 1 to 4 per cent magnetite ; to 5 per cent garnet^ 

 to 2 per cent biotite, and usually slight amounts of zoisite or apatite. 

 In some places the rock looks very gabbroic, and in others much like cer- 



«» W. J. Miller : N. Y. State Mus. Bull. 170, 1914, p. 24. 



