THE PRECAAIBRIAN ROCKS OF THE CANTON QUADRANGLE 65 



tectonic relationships, and has preferred to map the individual out- 

 crops separately, particularly in the Grenville, rather than attempt 

 what must at best be an extremely uncertain correlation. 



GRANITE GNEISS 

 (" Laurentian " of authors) 



Granite gneiss has its most widespread development in the west- 

 ern half of the sheet, but scattered outcrops in the eastern part 

 indicate the presence there of a considerable body of granite of in- 

 definite outline underlying the drift. In the present paper the dis- 

 cussions and descriptions are limited to those masses occurring in 

 the western and southern portions of the quadrangle, for elsewhere 

 outcrops are not sufficiently abundant to exhibit significant relation- 

 ships. 



In texture and mineralogical constitution the granites are remark- 

 ably uniform over wide areas. Of fine to medium grain, almost 

 universally gneissoid, but locally nearly massive, of ruddy pink 

 to light gray color, they are perhaps the most attractive and char- 

 acteristic Precambrian formation in the quadrangle. The more 

 massive phases, of almost saccharoidal translucency, have in places 

 been used for building stone. The rock is distinguished by the 

 low amount, indeed the almost total absence, of ferromagnesian 

 constituents, and the color values are therefore due to the predomi- 

 nance of quartz together with the red or gray feldspar. 



That the pinkish red color is not merely a discoloration caused 

 by surficial weathering is shown by its presence in the freshest 

 possible rock obtained in recently opened road-metal quarries. Thin 

 sections show that this is due to the impregnation of the light- 

 colored components of the gneiss with minute veinlets of hematite, 

 which occupies the cleavage and capillary spaces between the crys- 

 tals. The proper color of these gneisses would therefore be white 

 or light gray, were it not for the late impregnation of the formation 

 as a whole with a minute quantity of red iron ore. The general in- 

 terstitial character of the mineral shows that it is not of the normal 

 order of sequence in the crystallization of the rock magma, but is of 

 secondary origin, perhaps derived by hydatogenesis from primary 

 iron ore early differentiated from the original magma and segre- 

 gated at great depth. Professor Smyth (1899, pages 488-89) has 

 called attention to the occurrence of iron ore, magnetite, at Old 

 Forge, Herkimer county. New York, apparently also due to the 

 activity of heated circulating waters during a late stage in the crys- 

 tallization of the rock. 



