FOLIATION IN THE PRE-CAMBRIAN OF NEW YORK 589 



Grenville series generally regarded as highly folded and com- 

 pressed. — It has been quite generally assumed by all (including the 

 writer) who have carried on geological work in the Adirondack 

 region that the Grenville strata have been severely compressed and 

 folded as well as thoroughly metamorphosed and foliated by the 

 compression. A few citations from the more recent publications 

 will illustrate the ideas usually held. "There is abundant proof 

 that the rocks have undergone great compression and have been 

 folded and faulted on an extensive scale." 1 The Grenville rocks 

 "have been greatly compressed and intricately folded and pli- 

 cated." 2 "The old sedimentary rocks have undergone complete 

 recrystallization, entirely obliterating their old textures, and, as 

 a result of severe compression, have had a development of cleavable 

 minerals along certain parallel planes, the mineral particles having 

 a common orientation." 3 "In pre-Potsdam time the pre-Cambric 

 sediments had been tremendously folded and faulted and intruded 

 at great depths." 4 "After the intrusions the whole region was sub- 

 jected to intense compression and metamorphism when the gneissic 

 or foliated structure of all the rocks was developed." 5 



An alternative hypothesis. — That the Adirondack Grenville 

 strata are more or less folded is admitted at the outset, but, in the 

 light of recent studies, the writer doubts the interpretation of the 

 folded, tilted, and foliated structures as due to intense lateral 

 compression. Certain evident features of the Grenville strata and 

 related intrusives are directly opposed to this interpretation, while 

 all of the structural features may be much more satisfactorily 

 explained in another way. Thus it is conceived that the originally 

 horizontal, or at most only very moderately folded, Grenville strata 

 were much broken up and tilted in masses great and small, and in 

 other cases actually domed, by the irregular up welling of the great 

 bodies of magma (especially syenite-granite) under only very 

 moderate lateral pressure. This alternative explanation will be 



1 D. H. Newland, New York State Mus. Bull., No. in, 1908, p. 20. 



3 H. P. Cushing, ibid., No. 145, 1910, p. 9. 



3 Ibid., No. 95, 1905, p. 400. 



4 1. H. Ogilvie, ibid., No. 96, 1905, p. 478. 



5 W. J. Miller, ibid., No. 170, 1914, p. 77. 



