GEOLOGY OF THE BROADALBIN QUADRANGLE 9 



and closely involved with thin-bedded, gray, feldspathic and quartz- 

 itic gneisses. The only outcrop actually observed is in the bed of 

 Cadman creek just below the bridge three- fourths of a mile north- 

 northwest of Barkersville. Although other small occurrences may 

 have escaped notice, it is certain that the limestone is present only 

 in small amount. 



2 Quartzite. The rock from one and one-quarter miles northeast 

 of Batchellerville is perhaps the most typical and is coarse grained, 

 light brown to almost white, and made up of nearly pure quartz 

 with occasional very thin layers containing small flakes of badly 

 decomposed mica. The rock is foliated, highly granulated and often 

 stained with iron oxid. A thin section shows^ 96 per cent quartz; 

 2 per cent orthoclase ; i per cent biotite mostly changed to chlorite ; 

 and small amounts of epidote, zircon, apatite and zoisite. The epi- 

 dote occurs in fine euhedral prismatic crystals which are sometimes 

 distinctly twinned and show a pleochroism from greenish yellow 

 to reddish brown to lavender blue. A thin section from north of 

 Northville shows a total absence of feldspar. In some cases, as one 

 mile northwest of Mosherville, the quartzite is quite feldspathic 

 and filled with graphite flakes. 



3 A dark to pinkish gray, distinctly banded gneiss rich in garnet, 

 sillimanite and pyroxene. Under the microscope the rock shows 

 20 per cent orthoclase; 5 per cent plagioclase (chiefly andesine) ; 

 25 per cent garnet; 20 per cent quartz; 10 per cent pyroxene — pale 

 brown, euhedral, monoclinic crystals — probably augite ; 10 per cent 

 sillimanite in long slender prisms ; 5 per cent biotite ; 2 per cent 

 magnetite; i per cent pyrite and chalcopyrite ; and i per cent each 

 of epidote and graphite. The garnets are as large as one-quarter 

 of an inch across, of very clear amethystine color, and distributed 

 through the whole rock. The biotite is largely concentrated in dis- 

 tinct layers. This rock outcrops finely at Glenwild. Rocks very 

 similar to these except for lack of pyroxene are prominently ex- 

 posed two and one-half miles northwest of Cranberry Creek and 

 two miles northeast of Northville. 



4 A very straight, light and dark banded, highly feldspathic 

 gneiss which in thin section shows 60 per cent orthoclase, microcline, 

 and microperthite in about equal amounts together with a little 

 plagioclase; 20 per cent quartz; 10 per cent biotite; 5 per cent silli- 

 manite; 5 per cent garnet of amethyst color; and a little zircon. 



1 Only a close approximation to the mineral composition (volumetric pro- 

 portions) is intended in this and the following- sections of Precambric 

 rocks. 



