GEOLOGY OF THE BLUE MOUNTAIN QUADRANGLE 3 1 



Mixed Gneisses 



General statements. Under this caption are included chiefly 

 Grenville rocks which are cut to pieces by, and more or less closely 

 involved with, various facies of the syenite-granite body. Some- 

 times the Grenville, and sometimes the igneous, rocks prevail. Of 

 the igneous rocks in most of the areas, granite appears to pre- 

 dominate. In general it may be said that these mixed gneisses 

 include : long, narrow masses of Grenville and syenite or granite 

 distinctly recognizable as such but showing rapid alterations at 

 right angles to the strike of the foliation; bodies of syenite or granite 

 containing numerous sharply defined lenslike inclusions of Gren- 

 ville ; Grenville rocks intimately shot through by igneous rocks after 

 the fashion of so-called " lit-par-lit " injection; bodies of syenite 

 or granite containing a multitude of small, lenslike Grenville inclu- 

 sions whose borders commonly grade into the inclosing rock so that 

 the Grenville is often difficult to recognize as such; rocks which 

 are intermediate in character between the Grenville and igneous 

 rocks and which have clearly been produced by magmatic assimila- 

 tion; and still other rocks of variable types and puzzling character 

 whose origins are admittedly -rather uncertain. The small scale 

 of the map, together with the usual lack of sufficient outcrops has, 

 for most part, rendered inadvisable any attempt to map separately 

 very small masses of Grenville or igneous rocks recognizable as 

 such within these mixed gneiss areas. It is not uncommon to find 

 small exposures with a so-called " mixed gneiss " aspect within the 

 great syenite-granite intrusive body, but here too it would be unsat- 

 isfactory to attempt any consistent separate delimitation of such 

 rocks. 



Area south to southwest of Indian Lake village. This area 

 of about 2^ square miles shows many features characteristic of a 

 typical mixed gneiss area. 



Along the river and in the fields east of Indian lake dam, there 

 are numerous outcrops of granite and Grenville rocks (even includ- 

 ing limestone) which are clearly recognizable as such, while certain 

 other outcrops consist of more or less intimately associated granite 

 and Grenville rocks where the distinctive characters are not so 

 evident. 



Very instructive exposures occur along the road i mile a little 

 west of south of Indian Lake village. Going southward the first 

 outcrop beyond the pink granite is well-banded pyroxene (cocco- 

 lite), Grenville gneiss, most of which contains considerable feldspar 



