344 NEW YORK STATE MUSEUM 



destroyed by the later inetamorphism, or else the two pressures 

 came from the same direction, and the later foliation was super- 

 imposed on the earlier, this being much the more probable of the 

 two. • 



It has previously been stated that the character of the meta- 

 morphism which these rocks have undergone is indicative that 

 they must have been deeply buried at the time of metamorphism. 

 The igneous rocks specially are rocks of the most massive and 

 resistant sort; yet over a large part of the region their con- 

 stituent crystals have been broken up into a mass of granular 

 fragments, accompanied by much recrystallization. The rock 

 masses hiave also been shortened in the direction of greatest pres- 

 sure and extended in the plane at right angles to this, with the 

 production of foliation in this plane ; and all this has taken place 

 under such great load that no permanent cracks could form, all 

 breaks being closied up by welding as soon as formed, so that the 

 rocks have in general not been weakened and have often been 

 made stronger by the process. The depth below the surface at 

 which permanent cracks can not exist is considerable and more- 

 over varies with the nature of the rock concerned, being greatest 

 for strong, massive igneous rocks of this character. Yet during 

 metamorfthism these rocks were at that depth. The change in 

 shape has been effected by actual movement of the rock particles, 

 so that the rock must have been sufficiently loaded to be plastic. 

 Large feldspar crystals have been bent through considerable 

 angles without breakage. Quartzes have been drawn out into 

 long lenses and spindles. A multitude of phenomena showing 

 stretching of the rock, accompanied by actual flowage of the 

 material, could be cited, yet the strength of the rock has not been 

 impaired. The rocks have been under a pressure whose amount 

 exceeded their ultimate strength, and under a load sufficient to 

 cause welding up of all cracks. The exact depth of burial neces- 

 sary to bring about these conditions in rocks of this sort is uncer- 

 tain, but a depth of 5 miles is probably not more than a mile or 

 two wide of the mark in either direction. 



As near as can be judged from the small and scattered Gren- 

 ville exposures in the heart of the district, their foliation is 

 everywhere parallel to the original bedding. It is also true 

 that, over much Oif the district, the foliation dips are compara- 



