REPORT OF THE DIRECTOR I916 3 I 



a great mass of banded differentiated syenitic rocks, varying in a 

 very broad way, from a normal augite syenite with more basic 

 bands, through a more siUceous red hornblende grano-syenite 

 (interrupted by bands of a more basic or more siliceous character) 

 to a hornblende biotite granite which sends off numerous dikes. 

 Occasional dikes of hyperite with an ophitic texture cut the 

 syenites. 



The central band, about 4 miles wide, comprises a belt of Gren- 

 ville gneisses and marble intruded by sheets and bosses (for the 

 most part parallel to the cleavage or bedding) of gabbro, syenite 

 and granite, the latter usually a coarse porphyritic type. The 

 gneisses are the usual biotite, pyroxene, garnet, sillimanite and 

 rusty gneisses characteristic of this formation. South of Lake 

 Bonaparte the Grenville gneisses are intruded by an elliptical 

 shaped mass of fine-grained red granite. 



The northwestern band is a rock complex consisting of (i) an 

 elongated intruded mass of fine-grained granite partially overlain 

 and completely surrounded by a narrow belt of garnet gneiss with 

 thin beds of intercalated marble; (2) two masses of gray quartz 

 biotite gneiss intruded by sheets of porphyritic granite and more 

 or less abundant pegmatite veins parallel to the cleavage, one 

 mass wrapping around the granite-garnet gneiss belt on the 

 southeast and northeast, the other intruded by a small mass of 

 porphyritic granite and forming the whole northwest corner of 

 the district; (3) a narrow tongue of marble and white quartzite 

 beds overlapping on this quadrangle from the Gouverneur district. 

 The garnet gneiss is a mixed rock formed by intimate injection 

 of Grenville gneiss by pegmatite veins. On the northwest side of 

 the granite mass it possesses a low undulatory dip in general about 

 30° northwest, while on the southeast side of the granite it dips 

 vertical to steep northwest indicating the possibility of an asym- 

 metrical fold slightly overturned toward the southeast. The 

 quartz biotite gneiss is presumably igneous, but of doubtful origin. 

 It contains inclusions or residuals of gabbro but is granitic in com- 

 position. Like the garnet gneiss, the northwest area dips low north- 

 west while that on the southeast side of the granite-garnet gneiss 

 belt dips steep northwest beneath the garnet gneiss. 



Indications are that the mode of intrusion of the porphyritic 

 granite has been dominantly by pushing in along cleavage or bed- 

 ding planes, and to a minor extent by brecciation. In one case, 

 the gabbro similarly forms an almost continuous belt about 14 



