FOLIATION IN THE PRE-CAM BRIAN OF NEW YORK .613 



grained gabbroic types. As regards amount of granulation of 

 feldspar, it is probably not very different in anorthosite and syenite- 

 granite. The gabbroic, well-foliated, and granulated facies are 

 developed on a grand scale around the borders of the great anortho- 

 site area, but similar types are often encountered irregularly dis- 

 tributed throughout the area. Large feldspar individuals, usually 

 unaltered rounded or lens-like cores of crystals, quite typically 

 stand out prominently in a finer-grained, generally well-granulated, 

 groundmass. In spite of much granulation, it seems certain that 

 the typical original rock (before thorough consolidation) was char- 

 acterized by a coarse porphyritic texture. 



Cause of the foliation and granulation. — The foliation and granu- 

 lation of the anorthosite has been explained as due to the same 

 severe compression of the region which is supposed to have caused 

 similar phenomena in the syenite-granite series. According to 

 this view, the more general lack of anorthosite foliation is considered 

 to be due to lesser effect of the regional pressure toward the interior 

 of the great intrusive body than around its borders. Also it is 

 thought that coarseness of original grain and general lack of min- 

 erals, especially dark minerals, other than feldspar have militated 

 against such complete granulation and foliation of the rock as 

 characterizes the syenite -granite series. 



Regarding severe compression after the magma consolidation as 

 the prime cause of the foliation and granulation is, however, open 

 to many of the same objections already discussed in connection with 

 the syenite-granite series. It is the writer's belief that an insur- 

 mountable objection to the severe-compression idea lies in the fact 

 that there are so many sudden variations in degree of foliation and 

 granulation, and in strike of foliation, throughout the great anortho- 

 site area. Thus, well within the area, the writer has repeatedly 

 seen very gneissoid gabbroic facies — both coarse and medium- 

 grained — in close proximity to gabbroic facies of similar grain with 

 little or no foliation. All types of anorthosite also often exhibit 

 varying degrees of granulation in close proximity. If they were 

 caused by regional compression, why are so many portions highly 

 foliated or granulated when others close by are unaffected ? Also, 

 if regional compression were the cause of the foliation, how are 



