506 H. P. CUSHING 



it follows of necessity that the anorthosite is a differentiate in situ 

 from a gabbro intrusion and that the chilled border determines for us 

 the original size of the mass at the depth represented by the present 

 erosion surface — in other words, that this particular anorthosite 

 mass cannot be regarded as spreading out underneath the outlying 

 syenite masses and extending throughout the region. If it be 

 argued that the anorthosite body, while cooling, developed a syenite 

 cover, since removed by erosion, I would state that I think this 

 very probable, and would even go so far as to suggest that the Placid 

 and Keene inliers of syenite may be remnants of this cover. If so, 

 it would be in their vicinity that true transitional rocks between 

 syenite and anorthosite would be most likely to occur. Whether 

 they so originated, or represent plugs of syenite rising through the 

 anorthosite, can be determined only in the field with aid of favorable 

 exposures, if any such exist. But even this gives no aid in explana- 

 tion of the outlying syenite masses. 1 



The syenite-anorthosite boundary across the Long Lake 

 quadrangle suggests intrusive attack of syenite upon anorthosite 

 for its entire length. Along it the syenite develops a basic border 

 of its own, which I have elsewhere endeavored to show is due to 

 assimilation of gabbro and anorthosite gabbro by the molten 

 syenite. 2 In places the syenite thrusts deep salients into the 

 anorthosite, and an excellent sample may be seen on the Long 

 Lake map, coming down to the Raquette River just north of 

 Raquette Falls. It cuts into the anorthosite body to a depth of 

 two miles, cutting out much of the gabbro and anorthosite-gabbro 

 border, though these appear in full width on both sides of the salient. 

 Within it are several inclusions of anorthosite gabbro, five of which 

 are of sufficient size to be delineated on the map. Each inclusion 

 has an aureole of very basic syenite, grading away imperceptibly 

 into the normal rock. These are remnants of the anorthosite- 

 gabbro border which was there before the syenite salient was thrust 

 in, and which has escaped the utter digestion experienced by the 

 remainder. 



1 It should be noted that the transitional antiperthites described by Bowen are 

 from the Placid region {op. cit., pp. 221-22). 



2 Bull. Geol. Soc. Am., XVIII, 477-92; Bull. 115, N.Y. State Mus., pp. 478-82. 



