94 NEW YORK STATE MUSEUM 



which have been mechanically impressed upon them, namely, their 

 structural and tectonic relationships. 



Schistosity is an almost universal characteristic of the Precam- 

 brian rocks ; it is developed in the younger of the post-Grenville 

 intrusions as well as in the older metamorphic sediments, and in only 

 a few cases have any of the formations escaped showing the effects 

 of deep-seated compression and accompanying recrystallization. At 

 best, the more massive phases of the granite and gabbro are never 

 entirely free from a trace of foliation, and only the vitreous quartz- 

 ites and crystalline limestones present a perfectly massive appear- 

 ance. In the case of the quartzites this is probably on account of 

 the fact that in addition to their own toughness they were cush- 

 ioned, as it were, by the surrounding limestone, which by virtue of 

 its plastic yielding to pressure, protected the quartzites from exces- 

 sive stress ; in the case of the limestones, the strong tendency of 

 these rocks to recrystallize under sufficient pressure, has resulted in 

 the obliteration of whatever foliation they may have possessed at an 

 earlier stage in their history. \Mth the remaining Grenville rocks, 

 however, schistosity is marked, and is intensified ordinarily by its 

 parallelism to the bedding planes. It is commonly of the type pro- 

 duced by a recrystalHzation under lateral pressure, but a cataclastic 

 schistosity superimposed upon the former is known. At certain 

 points, as in the mixed rock on the eastern border of the sigmoid, 

 this is very clearly shown (see plates 12, lower figure, and 17, lower 

 figure). Although the schistosity is generally parallel with the 

 bedding or banding of a given rock, there are numerous instances 

 where this is not so, but the angle between the two intersecting 

 planes is as high as 90 degrees. This interesting relationship is 

 described farther on in this bulletin (see pages 97-101). 



Pitch, as the term is ordinarily apphed to foliated rocks, is the 

 direction of elongation of mineral groups or individuals in the plane 

 of schistosity. In the nature of the case the two structures named 

 are intimately related to each other in a genetic sense, the develop- 

 ment of pitch being the final stage in the compression of rocks al- 

 ready foliated. Where schistosity or an equivalent crumpling is 

 strongly developed, pitch is prominent and most easily determined. 

 Almost without exception, over the entire Precambrian area exam- 

 ined, the foliation dips toward the west or northwest ; consequently 

 where present, the pitch of mineral streaks in all the igneous 

 gneisses, and of the axes of the major folds and minute crumplings 

 in the sediments and injection zones, is in approximately the same 

 direction, with a dip varying from 20 to 50 degrees. 



