1 6 NEW YORK STATE MUSEUM 



order to distinguish the granitic syenite from the normal syenite 

 several features, given in the following description, must be 

 considered. 



The rock is not only clearly gneissoid but also distinctly 

 banded. The rock bands are straight and rather persistent and 

 the minerals are commonly arranged with their long axes paral- 

 lel to the foliation. Frequently the '' leaf gneiss " effect is beau- 

 tifully shown because of the flattening out of the quartz and 

 feldspar crystals. The color of the typical rock is red although 

 at times gray bands are present. The size of the grain is rather 

 variable but mostly pretty coarse. At times the quartz and 

 feldspar crystals are almost porphyritic. The average quartz 

 content is noticeably higher than in the normal syenite and com- 

 monly the more v/eathered surfaces show numerous projecting 

 quartz crystals; often the rock might well be called granite. 

 I'he granitic syenite also lacks the homogeneity of the ordinary 

 syenite. In the larger exposures of the typical red quartzose 

 gneisses there are occasional bands of gray, less quartzose 

 gneisses very similar to the normal syenite. 



Another noteworthy feature is the presence of long, narrow 

 patches or inclusions much like those sparingly present in the 

 ordinary syenite, but they are here much larger and more numer- 

 ous. They are always drawn out perfectly parallel to the folia- 

 tion planes but they are seldom more than 30 or 40 feet in 

 length. They are composed of about equal parts of badly de- 

 composed basic plagioclase and brown hornblende with which 

 are associated thin layers of almost pure biotite. These biotite 

 layers often give the rock a decidedly schistose appearance. 

 The presence of these basic patches has doubtless aided in the 

 production of the distinct banding of the granitic gneiss during 

 the process of dynamic metamorphism. In this connection the 

 writer has read the recent paper^ by Professor Adams which 

 deals with the origin of the amphibolites in the Glamorgan gran- 

 ite of Ontario, Canada. He says: ''Here the limestone (Gren- 

 ville), toward the granitic contact, passes gradually over into 

 amphibolite, the latter being undoubtedly produced by the alter- 

 ation of the former. . . The granite, furthermore, not only 

 penetrates the (limestone) series, but floats off masses of the 

 altered rock which in the form of bands, streaks, and isolated 

 shreds are seen thickly scattered through the granite in the 



^Joiir. Geol. 1909. 17:8. 



