450 W. J. MILLER ADIRONDACK ANORTHOSITE 



not the inclusions of anorthosite in the Keene gneiss (see above) strongly 

 support my view that the Keene gneiss, in some places at least, moved 

 upward as a true magma ? 



Finally, while there are such good reasons for believing that in many 

 cases the Keene gneiss magma formed at lov^er levels and rose to higher 

 levels in true intrusive fashion, it is also true that in many places, par- 

 ticularly in the Long Lake quadrangle, there appear to be no evident 

 irruptive contacts of Keene gneiss against anorthosite, but rather a gra- 

 dation. In such cases the assimilation or digestion of anorthosite by 

 syenite or granite magma is conceived to have taken place with little or 

 no movement of the resulting hybrid magma. This seems also to be essen- 

 tially true of the long, narrow area of Keene gneiss bertween syenite and 

 Marcy anorthosite across the Sentinel Range, in the Lake Placid quad- 

 rangle. 



BOWEN'S SUaGESTION OF POSSIBLE ORIGIN OF SOME KEENE GNEISS BY 



ASSIMILATION 



To argue that because there is a transition rock (Keene gneiss) does 

 not prove Bowen's hypothesis of a stratiform arrangement with syenite- 

 granite above and anorthosite below, the transition rock being a differen- 

 tiate in situ between the two. This utterly ignores the possibility of the 

 origin of the Keene gneiss by magmatic assimilation. In his second 

 paper, Bowen emphasizes the point that much of the syenite magma, 

 which he conceives to have been formed just under the upper gabbroid 

 border phase of the great intrusive, could have intruded and partially 

 assimilated the border phase, thus giving rise to some rocks intermediate 

 between syenite and gabbroid anorthosite, such as those described by 

 Gushing. Now, w^hile it is significant that Bowen admits the possible 

 origin of some such rocks by magmatic assimilation, the plain fact is that 

 no syenite ever formed as a rock intermediate between true anorthosite 

 and the gabbroid border phase, as already proved, so Bowen's hypothesis 

 to account for certain rocks intermediate between syenite and gabbroid 

 anorthosite is wholly out of the question. 



SIGNIFICANCE OF THE DISTRIBUTION OF FEMIC MINERALS 



According to Bowen, a sheetlike arrangement of syenite-granite, , anor- 

 thosite, and pyroxene or peridotite developed, due to settling of crystals 

 in an original gabbroid magma, the femic minerals having very largely 

 gone to the bottom, except probably from the very uppermost chilled 

 border facies. Thus the syenite should be notably poorer in femic min- 

 erals than the anorthosite, and also the transition rock which, according 



