36 WILLIAM J. MILLER 



therefore, which could have been present in very appreciable 

 quantity during the crystallization of these pegmatites was water. 

 These pegmatites rarely if ever show graphic intergrowths. 



In many locahties from lo to 50 per cent of the body of the 

 granite consists of the earlier pegmatite either in the form of narrow 

 parallel bands from a few feet to many feet long, causing the rock 

 to look like a coarse-banded gneiss, or as rather distinct lenses 

 up to several feet long; or in the form of "eyes" or very short, 

 thick, more or less rounded or elliptical lenses usually not more 

 than a few inches long and from one-half to two or three inches 

 thick. The long, narrow bands almost invariably grade into the 

 granite on either side, while the lenses and eyes generally show 

 moderately sharp contacts against the granite. The mode of 

 occurrence of such eyes of pegmatite is fairly well illustrated by 

 Figs. 2 and 3, the granite containing the eyes, usually exhibiting 

 rather wavy flow-structure foliation about them. From the field 

 relations it is quite certain that the eyes and short, rounded lenses 

 represent long lenses or layers of pegmatite which formed before 

 the final consolidation of the inclosing granite, and which were 

 pulled apart and strung out parallel to the magmatic currents 

 which were deflected around them. Their mode of occurrence 

 and lack of sharp contacts strongly suggest that they formed 

 during a late stage of magma consolidation, and that they are 

 not so old as the oldest silexite masses, which are now represented 

 as sharply defined inclusions. 



M. E. Wilson,^ in a description of the gneisses of the Laureptian 

 highlands, notes apparently similar lenses of pegmatite and says 

 that a "lenticular character is particularly evident in the case 

 of the pegmatite which commonly occurs in a succession of lenses 

 around which the foliation of the surrounding gneiss bends." 



Those pegmatites which occur as long, narrow, irregular bands 

 or strips parallel to the granite foliation, and which grade into the 

 granite, no doubt represent the final or nearly final solidification 

 of those portions of the granite in which they occur, and hence 

 they are practically in situ and grade into the granite. Figure 

 I shows a single lens of such pegmatite. 



^ M. E. Wilson, Amer. Jour. Sci., XXXVI (1913), 115. 



