460 W. J. MILLER ADLROXDACK AXORTHOSITE 



outlined, and I believe thev have, the failure to find the equivalent of the 

 rather pure plagioclase anorthosite as effusive rocks is explained. 



ORIGIX OF VARIATIOX^S IX THE AXORTHOSITE 



The great body of ^larcy anorthosite is by no means a homogeneous 

 mass. There are many zones or belts and irregular-shaped portions, some 

 distinctly more gabbroid and others distinctly more highly feldspathic 

 than the typical Marcy anorthosite here and there throughout the mass. 

 !Many of these show relatively wide gradation zones into the typical rock, 

 while others are more sharply separated. There are also many degrees of 

 foliation and granulation and differences in coarseness of grain. These 

 variations of the anorthosite and their significance have been somewhat 

 fully discussed in a preceding chapter of this paper, the present purpose 

 being to very briefly state two conceptions of their origin in the light of 

 the hypothesis of the origin of the anorthosite presented in this chapter. 



The view of the origin of such variations which best harmonizes with 

 the field facts may be briefly stated as follows : During the crystallization 

 of the anorthosite magma (formed by the process outlined above) there 

 was local differentiation in the upper portion of the magma reservoir, 

 whereby many portions relatively richer in femic constituents separated 

 from the much larger portions relatively poor in femic constituents. The 

 more femic portions, which contained more liquid, and hence were freer 

 to flow, were in many cases more or less shifted by movements during a 

 late stage of magma consolidation to form the crude bands or zones often 

 well foliated and rather sharply separated from the purer anorthosite. 

 Those belts or zones of more gabbroid anorthosite which gradually pass 

 over into purer anorthosite probably represent differentiates essentially 

 in situ. That there must have been notable movements during a late 

 stage of solidification of the anorthosite magma is abundantly proved by 

 the magmatic flow-structure foliation, more especially in the gabbroid 

 zones, but not uncommonly in the typical Marcy anorthosite. and even in 

 some portions of the nearly pure plagioclase anorthosite. In my opinion, 

 these late stage magmatic movements caused much, or all, of the notable 

 granulation of the anorthosite. But this granulation is by no means true 

 only of the anorthosite. The syenite-granite series, and more especially 

 the later gabbros, usually exhibit high degrees of granulation due to the 

 same cause. 



According to another view, we might imagine the more gabbroid zones 

 to have resulted from upward shifting of small portions of more gabbroid 

 magma from lower levels into the higher levels of purer plagioclase anor- 

 thosite. This implies upward shifting through at least thousands of feet 



