TRANSITION ROCKS 441 



zone between the intruded and intruding masses, thus accounting for the 

 narrow transition zone between the two in the ledge. 



The foliation of this Keene gneiss is quite certainly an original struc- 

 ture due to magmatic flowage under moderate pressure, and accordingly 

 the marked differences in degree of foliation within this one outcrop are 

 regarded as the result of differential magmatic flowage. 



It is clearly evident that the typical Keene gneiss just described is of 

 very local origin. Evidence of the local origin of other Keene gneiss is 

 presented below. The larger bodies of Keene gneiss are in every way like 

 these smaller ones, and there is good reason for believing, therefore, that 

 all the Keene gneiss, even where present as considerable bodies, is of 

 rather local origin due to assimilative attack of molten syenite or granite 

 on anorthosite. This is, of course, directly opposed to Bowen^s hypothesis, 

 w^hich regards the transition rock (Keene gneiss) as having formed by 

 differentiation in situ between syenite and anorthosite. 



Area near Upper Jay. — In the area of over one-half of a square mile, 

 just east of Upper Jay, there are many good exposures, certain of them of 

 particular interest because they throw important light on the origin and 

 relations of the Keene gneiss. Near the top of the hill, at the northeast- 

 ern border of the area, syenite and Whiteface anorthosite in big exposures 

 are separated by a zone, a few feet wide, of basic syenite-like rock with 

 scattering bluish gray labradorites. This is certainly a transition zone of 

 typical Keene gneiss produced by the assimilation of Whiteface anortho- 

 site by syenite magma. On the little hill, just south of the center of the 

 area, several outcrops of typical Keene gneiss contain bands or lenslike 

 inclusions of Whiteface anorthosite, Keene gneiss magma, moving from 

 a lower level where it was formed, evidently having penetrated or caught 

 up small masses of unchanged Whiteface anorthosite at a higher level. 

 The Keene gneiss here contains many tiny red garnets, and the labra- 

 dorites are very conspicuous on the weathered surfaces. 



Sentinel Range area. — This long narrow area extends east-west across 

 the middle of the Sentinel Eange. It is about 4 miles long and nowhere 

 over one-fourth of a mile w^ide. It is all in a rough, densely wooded 

 country, but a good many outcrops make the mapping fairly satisfactory. 

 Perhaps the most instructive ledges are on the little hill 1 mile northeast 

 of Malcom Pond. The top of this hill is quite typical Marcy anorthosite. 

 On the southern side the rocks are variable, being mostly fine to medium 

 grained, gneissoid, and of gabbroid appearance, with some closely involved 

 basic syenite-like rock containing a few small, scattering labradorite phe- 

 nocrysts. Near the top of the hill, on the west side, the rock is coarser 



