50 NEW YORK STATE MUSEUM 



resulted from pretty thorough assimilation of Whiteface anorthosite 

 by syenite or granite magma. Still farther north on Washburn 

 ridge very typical Keene gneiss is well exposed. 



Many fine ledges of very typical Keene gneiss occur on the 

 eastern face of Bailey hill, no. 33 of table 3 representing a thin 

 section of this rock. 



In the smaller of the two largest areas of Keene gneiss, exposures 

 are generally rather scarce except on the ridge north of Rogers 

 pond where the rock toward the south contains relatively few 

 large labradorites and suggests a gradation into granite, while 

 toward the north the large labradorites are common and the rock 

 appears to grade into the Marcy anorthosite. 



An area about i l / 2 miles long of mostly Keene gneiss occupies 

 approximately one-half of a square mile west, south and south- 

 east of the mouth of Hewitt pond brook (see map). A number of 

 good exposures show the rock to be somewhat variable, but it is 

 unusually rich in hornblende and never contains phenocrysts of 

 labradorite. In the field the rock looks much like a gabbroid facies 

 of Whiteface anorthosite, but thin sections (nos. 46 and 48 of table 

 3) and the field relations cause it to be rather confidently classed as 

 Keene gneiss. 



Conclusion as to the origin of the Keene gneiss. Enough 

 examples have been described to prove that the Keene gneiss of 

 the Schroon Lake quadrangle has developed on small and large 

 scales by assimilation of anorthosite by granite and syenite magmas. 

 If we adopt Bowen's hypothesis, this Keene gneiss must be regarded 

 as having developed by differentiation in situ between an overlying 

 sheet of syenite-granite and underlying anorthosite. If one 

 admits, as the writer does not, that syenite usually may have 

 developed by differentiation in situ close upon the Marcy anortho- 

 site, how can one imagine, in places like in the Schroon Lake 

 quadrangle, a similar development of granite close upon the anor- 

 thosite? It might be argued that the granite magma formed at a 

 higher level and was then forced downward. But, if so, it must 

 have been forced downward through still lower syenitic material. 

 Not only is the field evidence against this view, as already pointed 

 out, but even if we grant it, we are still forced to conclude, by the 

 obvious field facts, that the granite magma produced the transi- 

 tion rock (Keene gneiss) by assimilation of more or less anortho- 



