FOLIATION IN THE PRE-CAMBRIAN OF NEW YORK 597 



to develop the distinct foliation, is it not remarkable that the strati- 

 fication surfaces have. never been obliterated and cleavage developed 

 instead, and also that the stratification and foliation are always 

 parallel? Now, the stratification of the highly crystalline Gren- 

 ville is remarkably well preserved. Also, unless we assume intense 

 isoclinal folding, so that mineral elongation could everywhere have 

 taken place at right angles to the direction of lateral pressure, the 

 parallelism of stratification and foliation cannot be accounted for 

 by crystallization under severe lateral pressure. We have already 

 shown, not only that there is no positive evidence for such isoclinal 

 folding, but also that there is much positive evidence against any 

 more than the tilting, or, at most, very moderate folding, of the 

 Grenville on large scales. 



Again, if the foliation of the Grenville were essentially a dynamic 

 process — that is to say, the result of regional compression after the 

 great igneous intrusions — why should the Grenville be notably less 

 foliated and granulated than the intrusives? (See below.) 



We are thus forced to the only alternative conclusion, namely, 

 that the Grenville foliation was developed during the crystalliza- 

 tion of essentially horizontal strata under heavy load of overlying 

 material. Those minerals which cause the foliation were elongated 

 during crystallization under heavy downward pressure where con- 

 ditions of warmth and moisture were also favorable. According 

 to this conception the parallelism of foliation and stratification is 

 precisely what would be expected. It is quite generally assumed 

 that static pressure, that is to say, simple downward pressure, 

 "to the amount exerted in the upper part of the earth's outer crust, 

 appears to have little metamorphic effect." 1 In dealing with the 

 very ancient Grenville, however, it must be remembered that the 

 material now at the surface was once very deeply buried. The 

 thickness of the Grenville series in the Adirondacks is at least some 

 miles and more than likely many miles. Adams and Barlow 2 have 

 recently estimated a thickness of nearly eighteen miles for the 

 Grenville strata in Ontario. It is also definitely known that during 

 pre-Cambrian time the Grenville strata were subjected to tre- 



I L. V. Pirsson, Rocks and Rock Minerals (1909), p. 335. 

 2 Adams and Barlow, Geol. Snrv. Can., Mem. 6, 1910, p. t>2>- 



