GEOLOGY OF THE NORTH CREEK QUADRANGLE I3 



F eldspar-biotite-garnet gneisses There are a number of rather 

 distinct facies of these gneisses which show all sorts of gradations 

 from one to another. One common facies is a fine to medium- 

 grained gneiss in dark gray and nearly white alternating bands. 

 The biotite is wholly confined to the dark layers, while small scat- 

 tering garnets appear in both. Such rock is common in the gneiss 

 areas, being especially abundant on the mountain side east and 

 south of Valentine pond and northeast of Fuller pond. 



Another facies is medium to coarse-grained and not so per- 

 fectly banded. It is best shown in the small mountain i^/^ miles 

 north of \^alentine pond. 



A third facies is a fine to rather coarse-grained, gray to dark 

 gray rock, clearly gneissoid, usually banded and with numerous 

 pink to amethyst garnets up to five millimeters across. Such rocks 

 are very common in the Grenville gneiss areas as, for example, in 

 the quarry near the southeastern end of Loon lake and at the west- 

 ern base of Prospect mountain. 



Hornblende-feldspar gneiss. The most common facies of these 

 rocks is a fine to medium-grained, dark gray gneiss almost wholly 

 devoid of garnets. It is very gneissoid and amphibolite-like but 

 not at all banded. It is closely associated with limestone, some- 

 times with thin layers of that rock interbedded. The whole ridge 

 extending for five miles southeastward from North Creek is prac- 

 tically made up of this rock. 



Another but similar looking gneiss contains orthoclase, plagi- 

 oclase, hornblende, and hypersthene together with small amounts 

 of magnetite and graphite. This is a much more locally developed 

 gneiss as, for example, immediately under the limestone at the Nat- 

 ural Bridge. 



Feldspar-quart:: gneisses. These are the white or very light gray 

 gneisses of the district. Perhaps the most typical examples are 

 found in excellent exposures along the road near the quarry at 

 the southeastern end of Loon lake. This is a fine to medium 

 grained, very light gray gneiss with some tiny biotite flakes and 

 small brown garnets scattered through the mass. A slide shows 

 about 80 per cent of orthoclase, microcline, and microperthite in 

 nearly equal amounts; 13 per cent quartz; together with small 

 amounts of plagioclase, biotite and garnet. This light gneiss is 

 in thin to thick beds and repeatedly interbedded with biotite-garnet 

 gneisses. A very similar light gneiss. l)ut with grajihite flakos, 

 occurs a mile farther northward along the same road. 



