66 NEW YORK STATE MUSEUM 



preserved. Parallelism of Grenville foliation and stratification 

 appears to be universal. The Grenville has, for a long time, been 

 regarded as essentially a result of severe regional compression after 

 the great igneous intrusions had taken place. Recently, however, 

 the writer has seriously questioned this view.^ 



If the Grenville and accompanying great intrusives had been sub- 

 jected to compression severe enough to develop the foliation, is it 

 not remarkable that the stratification surfaces have never been 

 obliterated and cleavage developed instead, and also that stratifica- 

 tion and foliation are always parallel ? Also, unless we assume 

 intense isoclinal folding, so that mineral elongation could every- 

 where have taken place essentially at right angles to the direction 

 of lateral pressure, the parallelism of stratification and foliation 

 can not be accounted for by crystallization under severe lateral 

 pressure. But the Grenville strata were never highly folded. We 

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

 the Grenville foliation was developed during the crystallization of 

 essentially horizontal strata under a heavy load of overlying material, 

 or, in other words, under conditions of static metamorphism. Those 

 minerals which cause the foliation were elongated during crystal- 

 lization under the heavy load of overlying material. According 

 to this view, the parallelism of foliation and stratification is pre- 

 cisely what would be expected. This also explains the important 

 fact that the Grenville rocks are notably less foliated and granu- 

 lated than the great intrusives, particularly the syenite-granite 

 series. 



Foliation of the anorthosite and syenite-granite series.- By far 

 most of the great intrusive rocks exhibit more or less well-developed 

 foliation, ranging from very faintly gneissoid to very distinctly 

 gneissoid, the structure usually being accentuated by the roughly 

 parallel arrangement of the dark-colored minerals. Some masses, 

 particularly of the Marcy anorthosite, are practically devoid of 

 foliation. Granulation of minerals, especially feldspar, is common, 

 the more highly foliated rocks generally being most granulated. 

 The writer considers these intrusives to be so-called " primary 

 gneisses " whose foliation was developed as a sort of magmatic 

 flow-structure under moderate compression rather than by severe 

 lateral pressure brought to bear upon them after the cooling of 

 the magmas. 



^Jour. Geo!., 24:5196-600. 1916. 



2 For a rather full treatment of this subject, see the writer's paper 

 in Jour. Geo!., 24:600-16. 1916. 



