GEOLOGY OF LAKE PLEASANT QUADRANGLE 27 



areas certain peculiar rocks whose origins have not been demon- 

 strated. 



The mixed gneiss area between West hill and Mount Dunham 

 shows, in its northern part, Grenville hornblendic, quartzitic, and 

 biotitic gneisses intimately associated with syenite to granitic syenite, 

 the latter being literally all shot through the Grenville, giving rise 

 to typical mixed gneisses with numerous streaks, inclusions, and 

 bands of Grenville. In its southern part this igneous rock is 

 variable gray to pinkish granite with Grenville gneisses more or 

 less thoroughly fused in. 



Very similar phenomena characterize the larger mixed gneiss area 

 which extends eastward from the confluence of the two branches of 

 the Sacandaga river. Dark-gray, hornblende-garnet or biotitic 

 gneisses are all cut to pieces and often considerably fused by 

 granitic syenite and pink granite. There are excellent exposures 

 along the river one-quarter of a mile above and one-half of a mile 

 below the mouth of West Branch Sacandaga river, and also above 

 the road east of Colombe brook. 



In the Little Cathead mountain mixed gneiss area, which is the 

 largest of the quadrangle, Grenville gneisses are all cut to pieces 

 by gray granitic to syenitic gneisses, but in general there is com- 

 paratively little evidence for assimilation. The Grenville gneisses 

 of the whole area, except the northern portion toward Cathead 

 mountain, are well banded and rich in quartz, feldspar, biotite, and 

 garnet. Toward Cathead mountain the Grenville appears to be 

 mostly either quartzite or gray to white feldspar gneiss. Excellent 

 outcrops occur one-half of a mile north-northeast of Benson Center ; 

 on the east face of Little Cathead mountain ; on Cathead mountain ; 

 and above the road about a mile northeast of Benson P. O. 



A poorly defined area just north of Moose mountain shows much 

 Grenville garnet-bearing quartzitic gneiss frequently cut through 

 by granitic syenite. 



In the small area south of Meco lake mostly light gray Grenville 

 gneisses are verly closely involved or fused together with syenitic 

 to granitic gneisses. Almost continuous ledges occur in the river 

 bed. One big outcrop looks like basic syenite which in thin-section 

 is shown to be a quartz diorite, and this rock is fairly certainly an 

 assimilation product. 



The small area in the extreme southwestern corner of the map 

 shows few exposures, but the rocks appear to be mostly dark-gray 

 Grenville gneisses and pink granite to granite porphyry intimately 

 associated. 



