GEOLOGY OF MOUNT MARCY 33 



The anorthosites as just passed in review come under the variety 

 characterized by blue to very dark, almost black plagioclase, which 

 in the greater number of exposures is to a greater or less degree 

 granulated around the margins of the crystals. They belong to the 

 Marcy type as named by W. J. Miller. We have some evi- 

 dence that in the Mount Marcy quadrangle, as in the Elizabethtown, 

 inclusions of an older consolidated variety have been caught up 

 in a later irruption. On the northern slopes of Baxter mountain 

 such inclusions have been detected by H. L. Ailing and furnish 

 some parallels with the observations recorded in Bulletin 138, pages 

 37-39. On Baxter mountain, however, gneissoid anorthosite is 

 included in one of more massive texture. Apparently an older and 

 probably viscous, cooling mass was given a gneissoid character by 

 frictional drag near the edge. After it had chilled a renewed out- 

 break of still molten matter from the depths, penetrated and included 

 fragments of the older chill. 



The Whiteface type. On the extreme northeast corner of the 

 quadrangle, where the anorthosites of the usual variety are in asso- 

 ciation with the Grenville strata, the plagioclase takes on the white 

 color characteristic of the Whiteface type. The exposures are of 

 such limited extent as compared with the great area of the 

 Marcy type, and the type has been so well recognized as a border 

 phase by the writer ^ and by H. P. Gushing ^ that no further discus- 

 sion is called for at this point. 



From the anorthosites to the syenites there are some passage 

 forms. We note at times in the anorthosite microperthitic develop- 

 ments which are much more characteristic of the syenites. In the 

 syenite rocks in the mountains north of the Cascade lakes we may 

 sometimes see large blue, rectangular labradorites in ledges appar- 

 ently of green syenites. An intermingling of the two rotks is a 

 feature of Pitchoff mountain. The observer is almost at a loss to 

 decide where one ends and the other begins. Beneath the iron bridge 

 which crosses the East branch exactly a mile south of the norrhern 

 edge of the quadrangle, ledges of syenite are again extensively 

 exposed and contain blue labradorite crystals. Either an intermedi- 

 ate magma has led to their development, or else the later syenite has 

 absorbed older anorthosite almost but not quite to extinction. 



Inclusions in the anorthosites. On an earlier page in speaking 

 of the Grenville gneiss, several inclusions in anorthosite were 



iKemp, J. F., N. Y. State Mus. Bui. 138, p. 35-37. 

 2 Gushing, H. P., N. Y. State Mus. Bui. 95, p. 310-12. 



