GEOLOGY OF THE NORTH CREEK QUADR-\NGLE 23 



MIXED GNEISSES 



In the areas mapped as mixed gneisses, the rocks are more or 

 less intimate associations of the various Grenville, syenite, gran- 

 ite, and granite porphyry gneisses. They are really areas of Gren- 

 ville which have been all cut to pieces, and in some cases appar- 

 ently partially fused, by the great igneous intrusives. In some areas 

 true Grenville rocks predominate ; in others true igneous rocks pre- 

 vail ; 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 molten intrusions. Except along fault 

 lines, these mixed rocks everywhere grade into either true Gren- 

 ville or syenite or granite and the drawing of boundary lines is 

 largely a matter of personal judgment. Any attempt to separate 

 the various members of these mixed gneiss areas would be un- 

 satisfactory because of the general insufficiency of outcrops and 

 the small scale of the map. 



There are many places within the quadrangle where, as a result 

 of more or less perfect assimilation, rocks of intermediate com- 

 position occur on both small and large scales. One and one-third 

 miles northeast of Kelm mountain and near the map edge there 

 are fine illustrations of dark Grenville garnet gneiss inclusions in 

 the granite porphyry, the inclusions usually grading perfectly 

 through zones of a few feet into the granite. The intermediate 

 rock is coarse-grained, very garnetiferous, and not so porphyritic 

 as the true granite porphyry. Similar cases of local assimilation 

 by granite porphyry, granite and syenite have been observed at 

 other places within the quadrangle. 



On a large scale, perhaps the best examples of rocks of inter- 

 mediate character make up most of the mixed gneiss area just 

 east of Chestertown. 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 quite homogeneous 

 except for occasional patches or stringers of gray Grenville gneisses 

 which are fused into the mass. Passing southward and southwest- 

 ward down the mountain side, this rock grades perfectly into a 

 medium-grained, gray, biotite 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. 

 Passing 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 



