52 NEW YORK STATE MUSEUM 



exceeding the feldspar in quantity. Some of the feldspar is micro- 

 perthite and some oligoclase-andesine. " The most of the basic 

 syenite, and all of the more gabbroic of it, is in close association 

 with the anorthosite border. . . . Now the syenite is unques- 

 tionably younger than the anorthosite, and the observed relations 

 seem to point to the conclusion that the change (in the syenite) is 

 due to the actual digestion, by the molten syenite, of material from 

 the (anorthosite) gabbro."^ The Keene gneiss of the Lake Placid 

 region differs in being coarser grained, distinctly porphyritic, and 

 not so rich in dark minerals, but both Cushing's basic syenite and 

 the Keene gneiss are intermediate in position and composition 

 between the anorthosite and the syenite-granite series in their 

 respective regions, and the writer believes that Cushing's suggested 

 explanation is the correct one. 



Another rock, earlier described by Gushing^ from a railroad cut 

 nearly 5 miles north of Tupper Lake Junction, is regarded by him 

 as intermediate between the syenite and the anorthosite. Judging 

 by the description, this rock is, in almost all respects, similar to the 

 typical Keene gneiss except that the labradorite phenocrysts are 

 not so large. This work of Gushing, therefore, strongly supports 

 the idea that the Keene gneiss is a magmatic assimilation product. 



Mixed Rocks 



Grenville or amphibolite and Whiteface anorthosite mixed 

 gneisses. Under this caption there are described several areas in 

 which Grenville rocks and probably some ortho^amphibolite have 

 been cut to pieces by, and are more or less intimately associated 

 with, the Whiteface type of anorthosite. In some places the Gren- 

 ville or amphibolite predominates, and in others th^e anorthosite, but 

 the two rocks are too closely associated to be satisfactorily separated 

 on the geologic map. During the writer's recent survey of the Lyon 

 Mountain quadrangle next to the north much gabbro-amphibolite 

 older than the syenite-granite and probably older than the anorthosite 

 was encountered. Quite likely then at least some of the amphibolite 

 of the Lake Placid quadrangle is to be placed in the same category. 



The largest area, over 3 miles long, lies between Keene and 

 Upper Jay. It is traversed by the East Branch Ausable river. For 

 most paLrt, the rocks of this area are Grenville hornblende gneiss 



' X. Y. State Mus. Bui. 115, p. 479. 1907. 



' N. Y. State Mus. Rep't 54, i:r43 and r68. 1902. 



