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NEW YORK STATE MUSEUM 



changed by metamorphism, it is possible that the rocks above 

 referred to as igneous may not be igneous at all, while others not 

 referred to as igneous may in reality be igneous. Nevertheless 

 most of the rocks of the series appear to be of undoubtable sedi- 

 mentary origin. 



Syenite gneiss. A considerable portion of the region mapped 

 as Precambric is made up of a rock which shows every indication 

 of being truly igneous in origin. Also its areal distribution and 

 relation to the Grenville rocks [see below] clearly indicate that it is 

 intrusive in character. Study shows this rock to be a syenite 

 gneiss. It is from fine to medium grained and shows the typical 

 granitoid texture. The weathered surface of all exposures is of a 

 light brown color, while the real color of the fresh rock is greenish 

 gray. Because of the depth of the weathering the fresh rock is 

 usually difficult to obtain. As compared with the other Precam- 

 bric rocks this syenite presents a remarkably uniform structure 

 and composition throughout. Near the contact with the adjacent 

 syenite- Grenville complex (below described) the rock is usually 

 somewhat finer grained and not so typical in character. 



Of the two areas mapped, the larger is very irregular in shape 

 and occupies much of the northeastern portion of the district. 

 This mass is mostly heavily wooded and the exposures are often 

 few in number and unsatisfactory. However, many large expos- 

 ures have been found, especially in the vicinity of Northwood, 

 North Wilmurt, Reeds Mill and on the Little Black creek near 

 the county line. The smaller area lies in the town of Forest- 

 port extending northeastward from Forestport station for several 

 miles and on either side of the railroad. Although much of the 

 area is sand covered, excellent exposures may be seen at Woodhull. 

 Meekerville and in the stream beds southeast, west and southwest 

 of Anos Siding. 



The syenite clearly exhibits a gneissic structure although it is 

 not distinctly banded. The concentration of dark colored minerals 

 such as biotite and hornblende along certain lines serves to 

 accentuate the gneissic structure. These lines are wavy and as a 

 rule do not extend far without interruption. All the outcrops 

 show the gneissic structure although it is not always evident in 

 the hand specimen. 



Under the microscope the mineralogical composition is shown to 

 be feldspar — including anorthoclase, microperthite, and acid 

 plagioclasc — quartz, hornblende, biotite, augite, enstatite (or 

 bronzite), magnetite, apatite, zircon, titanite. 



