1 8 NEW YORK STATE MUSEUM 



ularly then more rapidly increases in gneissoid structure till at 

 a distance of from 3 to 5 miles from the limestone a region 

 of red hornblende gneiss is reached. . . West of Natural 

 Bridge in Jefferson county the syenite unquestionably passes 

 over into a very perfect red gneiss. . . Both in the field 

 and under the microscope the gradual change of the rock can 

 be followed through every step." The same sort of a change 

 has been observed by Gushing in the Tupper Lake syenite.^ 



Syenite-Grenville mixed gneisses 

 A considerable portion of the known Precambric rock area 

 has been represented upon the geologic map as being made up 

 of various mixed gneisses. These gneisses include a great many 

 rock varieties which make up a heterogeneous mass. After a 

 careful study of these gneisses the w^riter is fully convinced that, 

 for most part at least, they represent a more, or less intricate 

 mixture of the syenite and Grenville rocks. Some of these 

 gneisses are admittedly of uncertain origin. Certain small 

 patches within these areas are undoubtedly rather pure Gren- 

 ville, while others seem to be pure syenite, but the small scale 

 of the map does not permit these to be separately shown. Some- 

 times the syenitic and sometimes the Grenville facies predomi- 

 nate. These rocks are everywhere thoroughly gneissoid and 

 they are generally well banded except in some of the more syen- 

 itic masses. The " leaf gneiss " structure is at times well developed. 

 It has already been stated that the syenite is intrusive into 

 and younger than the Grenville and that the Grenville areas 

 must be regarded as large inclusions. A study of the syenite- 

 Grenville mixed gneisses furnishes convincing evidence of the 

 same kind. Actual inclusions of undoubted Grenville may occa- 

 sionally be seen within the syenitic masses. Such inclusions 

 may be seen in the vicinity of Lyons Falls; ^ mile north and t 

 mile east of Port Leyden; ij^ miles above the mouth of Miller 

 brook; i mile above the mouth of Fall brook, etc. As already 

 suggested, the writer is of the opinion that the Grenville was 

 often either partially or wholly incorporated into the syenite 

 by fusion when the latter came up in a molten condition. Vari- 

 ous rock types formed in this way would depend partly upon 

 the degree of metamorphism and partly upon the character of 

 the Grenville. The masses of rather syenitic looking rocks which 



' N. Y. State Mus. Bui. 95, p.323. 



