ROGERS, GEOLOGY OF THE CORTLANDT SERIES 03 



of course, with the demand, but it probably greatly exceeds in value the 

 emery mined to the south. 



Syenite. 



Syenite constitutes a member whose areal importance is small and 

 whose existence even is not well defined. Three small patches were 

 found, two on the borders and one in the interior of the district. The 

 patch to the north lies between biotite norite and Manhattan schist, as 

 does also the small area on the southern border ; the interior patch is sur- 

 rounded by hornblende norite. 



In all of these areas, the rock is heavy and dark gray, appearing much 

 more basic than is actually the case. The rock in the northern patch is 

 very fine grained and micaceous (biotitie). although the mica has no 

 regular orientation. Some of the feldspar is evidently glassy oligoclase, 

 while much of it is faintly pinkish. The thin section confirms these ob- 

 servations; the feldspar is about half oligoclase, slightly kaolinized, and 

 half orthoclase in larger and more irregular grains, somewhat more 

 altered than the triclinic feldspar. Slight recrystallization and wavy ex- 

 tinction are apparent. Several small grains of quartz, giving unmistaka- 

 ble interference figures, and evidently original, are present. Biotite, 

 typically developed, is practically the only dark mineral to be seen; green 

 augite is present in very small quantity. Apatite is abundant in fairly 

 large crystals, and ilmenite is present in small grains. 



In the southern patch, the rock is also fine grained, gray and mica- 

 ceous. The specimen sectioned was taken from a small quarry at the 

 juncture of the two roads and looks entirely fresh. In thin section, it 

 resembles the rock described above, except that the orthoclase is much 

 more abundant and that there is more quartz. The triclinic feldspar is 

 oligoclase, as above. There is also a moderate amount of deep green 

 hornblende and a little pyrite. The amount of alteration, however, is 

 surprising: the feldspar is somewhat sericitized, and there is a moderate 

 amount of chlorite and epidote and considerable calcite in small irregular 

 patches. This rock is evidently much like the rest, although it is a more 

 typical syenite. 



In the interior patch, the rock is similar to the other megascopically, 

 except that there is less biotite. In thin section, it appears that the 

 orthoclase is predominant over the oligoclase but that what quartz there 

 is is secondary. Hornblende is the chief ferromagnesian mineral, but 

 alteration to scaly chlorite, starting from the center of the crystal, is very 

 common. The biotite is typical but somewhat altered and is usually 

 found clustering around the hornblende laths. Pyrrhotite is present in 

 small quantity and magnetite, ilmenite and apatite in moderate amount. 



