GEOLOGY OF LAKE PLEASANT QUADRANGLE 1 3 



In the Grenville mass, which is really a distinct inclusion in the 

 syenite, just south of Elbow mountain, the well-bedded rocks are 

 chiefly hornblende-feldspar-garnet gneisses together with a little 

 quartzite, white feldspar gneiss and a one-foot bed of white lime- 

 stone, this last lying within the hornblende gneiss. There is rather 

 clear evidence for the fusion, and at least partial assimilation, of 

 the borders of this inclusion giving rise to massive rocks of inter- 

 mediate appearance and dioritic composition. These intermediate 

 rocks at times look like basic syenites. 



Just southeast of the summit of Dunham mountain two small 

 inclusions of biotite-garnet gneisses are shown on the map. These 

 are more or less assimilated by the syenite and are representative 

 of a number of smaller inclusions in this vicinity. 



The large inclusion extending across the top of West Hill is a 

 nearly black hornblende-feldspar-garnet gneiss or amphibolite with 

 garnets only occasionally present. Three smaller inclusions lie south 

 and southeast of this and consist of white, feldspar-quartz-biotite- 

 garnet gneisses distinctly interbedded. All four of these inclusions 

 are very clear-cut with good exposures in open fields. 



A small inclusion one-half of a mile south of the mouth of 

 Colombe brook show^s a sharp contact with syenite along the road. 

 The rocks are well banded, light to dark gray, feldspar to biotite 

 gneisses usually with garnets. 



Anorthosite-gabbro 



In the extreme northeastern corner of the quadrangle, an area 

 of about I square mile is mapped as anorthosite-gabbro. Beyond 

 the map limits the extent of this rock has not been determined, 

 though the nearest ledges to the south and west show syenite of 

 rather normal character. It is admittedly possible that this so-called 

 anorthosite-gabbro mass is only a very basic phase of the syenite, 

 though in one or two essentials it is different from any such rocks 

 below described. In mineralogical composition and outward ap- 

 pearance, however, it is decidedly like the characteristic anorthosite 

 or anorthosite-gabbro of the great Essex county area, and hence 

 It is rather confidently classed with that rock. In thin-section the 

 typical rock shows : 88 per cent oligoclase to labradorite, mostly the 

 latter; 3 per cent augite; 2 per cent each of hypersthene and horn- 

 blende; i^ per cent garnet; i per cent each of biotite, magnetite 

 (or ilmenite), and quartz; and a little zoisite and apatite. The 

 •quartz is probably of secondary origin. Thus the minerals other 



