THE PRECAMBRIAN ROCKS OF THE CANTON QUADRANGLE 8l 



In the western half of the sheet most of the pegmatite material 

 is confined to the occurrences or class of occurrences named. In 

 the Pierrepont sigmoid area, however, pegmatite is not found as 

 well-defined dikes, either conformable or nonconformable with their 

 country rock, but as irregular tongues and branching streaks, 

 parallel, in a general way, to the foliation, and at times thoroughly 

 permeating and saturating the country rock. This phenomenon 

 happens more particularly in the areas of garnet gneiss (see plates 

 7, upper figure, and 14, lower figure), although the eastern and 

 northwestern portions of the sigmoid are perhaps more affected in 

 this respect than the central, where the gneiss is often comparatively 

 free for broad areas from such thorough soaking with granitic 

 juice (see plate 5, lower figure). 



That the pegmatitic material is in fact foreign matter injected 

 into the garnet gneiss and not a more coarsely recrystallized varia- 

 tion of the latter formation is shown by the progressive manner in 

 which it is seen to envelop the garnets. Plates 5, lower figure, 7, 

 upper figure, and 15, upper figure, taken consecutively indicate 

 the successive steps in this process. Plate 5, lower figure, is the 

 normal gneiss in which the lighter streaks are of more feldspathic 

 material, possibly representing a minimum amount of injected peg- 

 matite. Though these more often than not surround patches or 

 skeletal crystal growths of garnet, there are some such patches which 

 still repose in their natural unacidified matrix. In plate 7, upper 

 figure, there is a greater abundance of connected streaks of peg- 

 matite, which is seen in the act of inclosing a few of the garnets 

 and isolating them from their former ground mass. In plate 15, 

 upper figure, a large quantity of the quartzo-feldspathic material 

 has been introduced locally, and has not only incorporated similar 

 components of the matrix into itself and actually inclosed the 

 garnets, but has given the molecules of the latter mineral oppor- 

 tunity to segregate and develop into abnormally large clusters, 

 resulting in a pseudo-garnetiferous pegmatite. 



A second natural, but numerically small, group of pegmatites, 

 which is based on mineral content and on age and structural rela- 

 tions, is found in the scattered tourmaline-bearing dikes, including 

 with these a few apatite-muscovite-bearing dikes which undoubtedly 

 belong to the same period of activity. The best locality for observ- 

 ing the relations between this and the preceding simple type of 

 pegmatites is the 600-foot knoll one and one-tenth miles northeast 

 of the fork in the road north of North Russell. The country rock 



