508 .. . M. P.CUSHING 



parent-mass below ground. The other known outliers are all small, 

 are all composed of anorthosite gabbro, lie within a distance of 10 

 miles from the anorthosite boundary, and are either demonstrable 

 or probable inclusions in the syenite or else are dikelike or pluglike 

 offshoots from the main mass. So far as I know the evidence, they 

 do not at all require belief in the greater extent of the anorthosite 

 mass underground. 



Conclusion. — While, therefore, I am quite in accord with 

 Dr. Bowen in the belief that the gabbro, anorthosite, syenite, and 

 (in part) granite bodies of the Adirondacks are all differentiates 

 from a common parent-magma and are closely akin in age, I do 

 not believe that the present surface exposures can be successfully 

 explained as constituting one great igneous body. The anorthosite 

 mass arose to its present position as a gabbro magma, developed a 

 chilled border, differentiated with production of anorthosite and 

 quite possibly overlying syenite, and solidified. The overlying 

 syenite has since been eroded away, except for the possibility that 

 the Placid and Keene inliers may represent portions of it. In so 

 far I can follow Bowen without trouble. But to account for the 

 outlying syenite and granite bodies away from the anorthosite I 

 think that we must resort to the conception of at least one other 

 body of magma, probably of several others, which went through a 

 similar differentiation well below the present surface and from whose 

 upper parts bodies of molten syenite were pushed upward. Some 

 of these came up along the margins of the solidified anorthosite 

 mass while it was still hot and produced the contact relations which 

 we find today. 



It is not safe to say, at the present time, that the floor of the 

 entire Adirondack region is constituted of representatives of this 

 one igneous group. It is quite true that the Grenville remnants 

 in the region always rest on igneous rocks which bear an intrusive 

 relation to them. But in my view there are considerable masses 

 of orthogneiss present much older than the rocks of the anorthosite- 

 syenite group; in places the Grenville rests on these, and it is not 

 at all certain that they rest on the younger intrusives; and in many 

 parts of the region these rocks border the anorthosite, as, for 

 example, along Cold River on the Long Lake quadrangle. The 



