106 NEW YORK STATE MUSEUM 



siderable content of the black, ferro-magnesian silicates. In the 

 more quartzose rocks these are present in only slight quantity, 

 and instead of foliation a linear structure appears, due to the 

 drawn out, spindle form which the quartz assumes. 



These rocks rapidly undergo a color change from green to 

 brown on exposure to the weather, though greenish nodules may 

 frequently be found in the brown rocks. This color change may 

 occur without perceptibly impairing the freshness of the constitu- 

 ent minerals. Usually however the hypersthene (or bronzite), 

 and often too the augite are found to be more or less decayed in 

 the brown rocks. These brown gneisses cover a wide area in 

 Franklin county, and, while in part they are easily recognizable 

 as belonging with the syenites, in other part they are puzzling, 

 and may or may not belong here. It thus becomes a very difficult 

 matter to determine the precise limits of the main intrusions and 

 in the case of smaller patches to determine at all whether the 

 rocks belong in this group. Furthermore, the ground has not 

 been exhaustively studied, so that the limits of these rocks as 

 shown on the map are in a high degree provisional. The main 

 areas are indicated, but there is question as to their precise ex- 

 tent. In general doubtful varieties occurring in close association 

 with undoubted syenites have been classed with them; those with 

 no such association have not been so mapped. 



There is much local variation in structure and composition in 

 these rocks, reminding one of the similar variations which are so 

 characteristic of the gabbro group. This local variation hampers 

 the endeavor to determine whether differentiation has taken place 

 in the main intrusions so that no certainty has been reached in 

 regard to it. It is however believed that such has been the case 

 and that the associated granites will be found more massed 

 toward the centers of the intrusions. All intermediate stages be- 

 tween granite and syenite may be found so that there is evidently 

 a passage of one rock into the other. These granites are either 

 red or green in color, and while very quartzose and poor in dark 

 silicates, these are the same as appear in the syenites, and the 

 quartz has the same spindle form. 



