ORIGIN BY DIFFERENTIATION . 461 



of congealing magma, which seems improbable under the conditions of 

 the intrusion, differentiation, and consolidation of the magma. Also the 

 very perfect gradations of many gabbroid zones into purer anorthosite are 

 not so satisfactorily explained on this view. 



Summary of Conclusions 



1. The Adirondack anorthosite is a great laccolithic intrusive body, not 

 very much greater in diameter than its present area of outcrop at the 

 time of its intrusion. 



2. The anorthosite is by no means a mass of practically pure plagio- 

 clase, the average rock (not including the marginal facies) containing 

 fully 10 per cent femic minerals as well as smaller amounts of other sig- 

 nificant constituents. 



3. The anorthosite is distinctly separate from and older than the great 

 syenite-granite series of the Adirondack region. 



4. Many small dikes and small to large tongues of syenite and granite 

 were forced into the anorthosite as offshoots from the distinctly later 

 syenite-granite magma. 



5. The anorthosite differentiated practically in situ from an intruded 

 gabbroid magma. 



6. A chilled gabbroid border facies developed both as an outer and an 

 upper margin of the anorthosite. 



7. The anorthosite ■ crystallized from the upper or residual portion of 

 the magma during and after the sinking of many of the femic constitu- 

 ents. 



8. The anorthosite practically as such was, to a very considerable degree 

 at least, actually molten. 



9. There was little or no development of syenite or granite as direct 

 differentiates of the anorthosite, particularly between the chilled border 

 and the typical coarse grained anorthosite. 



10. True transition rocks (Keene gneiss) between the anorthosite and 

 the syenite-granite series were produced locally by magmatic assimilation 

 of still hot, but not molten, anorthosite by the syenite or granite magma. 



11. The border facies of the anorthosite not uncommonly sent dikes 

 into, engulfed, or even locally injected portions of the country rocks 

 (Grenville series), because this border facies represents that portion of 

 the original gabbroid magma which came into contact with the country 

 rock. 



12. The rather common occurrence of inclusions of Grenville in the 

 border facies and their practical absence frora the typical coarse grained 



