258 W. J. MILLER MAGMATIG DIFFERENTIATION AND ASSIMILATION 



A Grenville inclusion, one-half mile long and well exposed in the bed 

 of Elbow Creek, 1% miles north of Wells (Lake Pleasant quadrangle), 

 consists chiefly of dark hornblende-garnet gneiss. Its borders are clearly 

 fused by the inclosing normal syenite giving rise to peculiar looking rocks 

 of distinctly intermediate character. 



On the eastern shore of the lake, 1% miles northeast of Long Lake 

 village (Blue Mountain quadrangle), a pink granite very distinctly 

 grades into a wide band of biotite- quartz Grenville gneiss. 



Magmatic assimilation on larger scales, however, is to be best observed 

 in areas of so-called ^^mixed gneisses." These are really areas of Gren- 

 ville which have been all cut to pieces, and in some cases more or less 

 fused by the intrusive magmas. In some areas true Grenville rocks pre- 

 dominate ; in others true igneous rocks prevail ; while in still others the 

 most common rock appears to be of intermediate character due to an 

 actual melting and incorporation of Grenville sediments by the intru- 

 sives. Except along fault lines, these mixed gneisses everywhere grade 

 into either true Grenville or syenite or granite, and the drawing of 

 boundary lines is largely a matter of personal judgment. 



On such a larger scale, good examples of rocks of intermediate charac- 

 ter make up much of the mixed gneiss area which lies just east of Ches- 

 tertown^^ (North Creek quadrangle). Thus the whole top of Prospect 

 Mountain consists of gray, fine grained, very massive rock which has the 

 composition of a biotite granite. This rock is pretty homogeneous except 

 for occasional patches or stringers of gray Grenville gneisses which are 

 fused into the mass. Passing southward and southwestward down the 

 mountain side, this rock grades perfectly into a medium grained, bio- 

 tite granite which contains very few Grenville inclusions, and this rock, 

 in turn, grades perfectly into the typical biotite granite porphyry at 

 the base of the mountain. PaBsing westward down the mountain side, 

 however, the fine-grained granitic rock at the top gradually becomes 

 coarser grained and contains more numerous and more clearly defined 

 inclusions 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, granitic 

 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 



25 W. J. Miller : N. Y. State Mus. Bull. 170, 1914, pp. 23-24. 



