PETTKK, MANHATTAN SCHIfiT OF NEW YORK 215 



tendency of minerals rich in OIL to alter to those lower in OH and finally 

 to those free from it entirely. Chlorite will be replaced by biotite; zoisite 

 and epidote by plagioclase, and nmscovite, by orthoclase and microcline- 



In the case of the Manhattan schist, it has already been seen that 

 the biotite-quartz-feldspar varieties predominate, although nmscovite is 

 usually also present and is frequently an important constituent. These 

 evidently represent the final stages to which metamorphism will proceed 

 in this zone. With the exception of the area of schist to the north of 

 Croton-on-the-Hudson and that in the vicinity of Peekskill far enough 

 away from the Cortlandt intrusions to be out of range of very much influ- 

 ence from their contact metamorphic effects, the schists of the region 

 under discussion have all undergone about the same degree of metamor- 

 phism. 



In the case of the schist just north of Croton Village, it is quite evident 

 that the fine matrix of muscovite, biotite, quartz and iron oxide in which 

 the coarser biotite flakes are imbedded, if recrystallization had proceeded 

 to a further stage, would have been converted into a much coarser mass 

 consisting of larger biotite crystals, feldspars and quartz, with possibly 

 some garnet and only a little muscovite. Further north of the same area 

 of schist where metamorphism has been somewhat more intense, feldspar 

 does become quite prominent. Staurolite and garnet also become quite 

 abundant constituents of the schist here. The garnet, as seen from the 

 petrographic description of the schist from widely distributed outcrops, is 

 a quite common constituent of these rocks. Staurolite, on the other hand, 

 is quite rare, this being the only place south of the Highlands where it 

 was found in any abundance. Apparently \Wth the more severe meta- 

 morphism which took place to the east and south, it was converted into 

 other minerals. AAHiat has been said in regard to the schist just north of 

 Croton-on-the-Hudson also applies to the schist occurring near the south- 

 east corner of the town of Peekskill along the road to Yorktown Heights. 



As may be seen from the petrographic description, the minerals present 

 in the schist are quartz, orthoclase, plagioclase (ranging from oligoclase 

 to labradorite), biotite, muscovite, garnet, staurolite, sillimanite, cyanite, 

 magnetite, pyrite, apatite, zircon, zoisite and tourmaline. Of these, all 

 but the tourmaline have probably resulted from the recrj^stallization of 

 constituents already present in the original sediments before recrystalliza- 

 tion took place. N'one of these minerals contain components which would 

 not occur in such a formation as the one from w^hich the schist was de- 

 rived. The presence of the tourmaline on the other hand is probably due 

 to the introduction of a least a portion of its constituents, especially the 

 boron by emanations which accompanied tlie pegmatitic intrusions re- 

 ferred to later. 



