FETTKE, MANHATTAN SCHIST OF NEW YORK 209 



present. The plagioclase shows extinction angles up to 9° in sections at 

 right angles to the albite lamellae and is evidently a variety of oligoclase. 

 The garnet occurs in irregular grains at times full of quartz inclusions. 

 Thin seams of very micaceous type are often interbedded with this 

 quartzitic variety of schist. These are usually very much crenulated and 

 contorted, while the quartzitic variety does not show these minor folds. 

 This micaceous type consists largely of muscovite and biotite, with small 

 amounts of quartz and a little garnet. The mica flakes curve around the 

 garnet. 



Toward the northeast, the most northerly outcrops of Manhattan schist 

 occur in the vicinity of Brewster in southeastern Putnam County. Schists 

 and limestones belonging to the Manhattan-Inwood series are well ex- 

 posed in a cut about two miles east of Brewster along the New York and 

 New England Eailroad. The schist is rather coarsely crystalline and has 

 a distinctly foliated structure. In thin section under the microscope, it is 

 seen to consist principally of feldspar, biotite and quartz, together with a 

 little tremolite, pyrite and an occasional rounded zircon grain. The 

 feldspar is mostly plagioclase giving extinction angles up to 24° in sec- 

 tions at right angles to the albite lamellae. It is probably an acid labra- 

 dorite. Some orthoclase is also present, since many of the feldspars are 

 unstriated and optically negative. The biotite shows marked pleochroism 

 from light yellowish to deep reddish brown. The rock has undergone 

 considerable strain. Most of the feldspar shows strain shadows and wedge 

 twins are common. Mortar structure is also developed in the case of some 

 of the feldspar grains, which are frequently surrounded by a border of 

 finely granular material. 



About one mile south of Brewster along the road to Croton Falls a 

 quartzite phase of the schist is well developed. The rock here is made up 

 of quartz, biotite, feldspar and muscovite, with quartz greatly in excess of 

 the other constituents. Occasionally a light pink garnet is also present. 

 The quartz is quite free from inclusions. It and the feldspar occur in 

 interlocking grains usually elongated parallel to the foliation. The bio- 

 tite is a dark greenish brown, highly pleochroic variety. More micaceous 

 and feldspathic phases of the schist are also present at this locality, but 

 the quartzitic type predominates. On the whole, however, the schist in 

 the vicinity of Brewster is very closely similar to that occurring farther 

 south. 



The relation of the Manhattan schist to the underlying limestone is 

 well shown in the excavation at present being made for the new Kensico 

 reservoir dam at Valhalla ahout two miles north of White Plains in south- 

 ern Westchester County. The Fordham gneiss, Inwood limestone, and 

 Manhattan schist occur in their normal order of succession here, the 



