462 W. J. MILLER ADIRONDACK AXORTHOSITE 



anorthosite are also due to the fact that only the upper and outer portions 

 of the laccolithic gabbroid magma effectively attacked the country rock. 



13. The failure to find dikes of typical coarse grained anorthosite in 

 the Grenville is because the rather thick border facies protected the Gren- 

 ville against attacks by the coarse anorthosite, which, having developed 

 from a residual magma, was either too viscous or too much crystallized 

 to be sufficiently free-flowing. 



14. Anorthosite, often highly feldspathic, is in places intimately in- 

 volved with Grenville, giving rise to mixed gneisses quite comparable to 

 the well known Grenville and syenite-granite mixed gneisses. 



15. The variable gneissoid structure of the anorthosite was produced 

 essentially as a magmatic flow-structure foliation. 



16. The commonly occurring belts, zones, and very irregular masses of 

 anorthosite-gabbro, and even gabbro, throughout the great body of the 

 anorthosite probably represent local differentiates, some of which were 

 more or less shifted by magmatic flowage and others not. 



17. The more gabbroid portions of the anorthosite show more notable 

 development of foliation because they longest retained very considerable 

 percentages of liquid, but even the typical, coarse, nearly pure plagioclase 

 anorthosite locally exhibits distinct foliation. 



18. The notable granulation of the anorthosite was produced by move- 

 ments in the magma during a late stage of its consolidation. 



19. The relative absence of Grenville strata from the anorthosite area, 

 particularly its southv^estern half, as compared with its common occur- 

 rence throughout the syenite-granite areas, is explained, first, by the fact 

 that the laccolithic anorthosite intrusion caused the Grenville to be mostly 

 lifted or domed over its back rather than deeply penetrated, broken up, 

 or engulfed as during the batholithic intrusion of the more highly fluid 

 syenite-granite magma, and, second, probably because deeper erosion over 

 the southwestern part of the anorthosite area caused complete, or almost 

 complete, removal of the Grenville. 



20. The general absence of the syenite-granite series from the anor- 

 thosite area, particularly its southwestern half, is best explained as due 

 to great resistance of this the thickest portion of the relatively homoge- 

 neous anorthosite laccolithic body to the intrusion of the syenite-granite 

 magma, as compared with the much slighter resistance offered by the 

 surrounding Grenville strata. The notable absence of stocks of the later 

 gabbro support this view. 



