FETTKE, MANHATTAN SCHIST OF NEW YORK 205 



The schist in the vicinity of New Rochelle and northeast of that point 

 becomes for the most part veiy feldspathie in composition and takes on a 

 gneissoid structure. A thin section from a specimen collected east of 

 Pelhamville shows a medium-grained crystalline texture and foliated 

 structure. The principal minerals are feldspar and quartz in allotrio- 

 morphic, interlocking grains, together with smaller amounts of biotite 

 and muscovite. A little apatite is present as needle-like inclusions in the 

 feldspar and quartz. An occasional grain of zoisite, a little magnetite 

 and a few rounded grains of zircon also occur. The feldspar which is the 

 most abundant mineral present consists of both orthoclase and plagioclase. 

 The plagioclase is an andesine variety, being optically positive and show- 

 ing extinction angles up to 10° in sections at right angles to the albite 

 lamella. The biotite occurs in small flakes whose basal sections are in the 

 plane of foliation. It shows marked pleochroism from light brownish 

 yellow to deep brown. 



Farther northeast, in the vicinity of Eye, most of the Manhattan schist 

 formation becomes very quartzose in composition. Alternating with the 

 thicker beds of quartzitic schist are thinner seams which are more mica- 

 ceous and hence show foliation to a much more marked degree. The 

 quartzitic schist has a light gray color and a medium-grained texture. 

 Examination under the microscope shows that it is made up largely of 

 irregular interlocking grains of quartz and minor amounts of feldspar, 

 mostly plagioclase of an oligoclase-albite variety, giving extinction angles 

 up to 8° in sections at right angles to the albite lamellae and being opti- 

 cally positive. Some biotite of a deep greenish brown variety and a little 

 muscovite are also present. A few minute rounded grains of zircon may 

 be seen. 



A gneissoid to schistose quartz-mica-feldspar rock probably belonging 

 to the Manhattan schist formation occurs in the east central portion of 

 Westchester County. It has been considered a part of this formation by 

 F. J. H. MerrilP^ in mapping the lower Hudson sheet for the New York 

 State Survey and also by Edson S. Bastin,^^ who examined the pegmatites 

 occurring in it at Bedford Village. Lea M. Luquer and Heinrich Eies,^' 

 who have also made a study of the area, on the other hand consider it a 

 part of the Fordham. The writei*^s studies in this region were not suiB- 

 ciently detailed to allow him to make a positive statement, but it seems 

 most likely from the position of these rocks with respect to surrounding 

 limestone belts, outcrops of which occur occasionally and which are 



i» Geologic map of New York. Lower Hudson Sheet. N. Y. State Mus. 



20 Bull. 315, U. S. Geol. Surv., pp. 344-399. 1906. 



21 '*The 'Augen' gneiss area, pegmatite veins and diorite dikes at Bedford, N. Y." Am. 

 Geol., Vol. XVIII, pp. 239-261. 1896. 



