FETTKE, MANHATTAN SCHIk^T OF NEW YORK 203 



PP^TROLOGY 



The Manhattan schist as typically developed on Manhattan Island con- 

 sists chiefly of a dark coarsely crystalline mica schist. In a hand speci- 

 men biotite, muscovite, quartz, feldspar and some garnet can usually be 

 recognized. The relative amounts of these different minerals in a par- 

 ticular specimen will vary greatly from place to place. In some cases, the 

 micas greatly predominate over the other constituents, and the rock often 

 shows a crenulated structure where it has undergone intense folding and 

 crumbling. Often considerable amounts of feldspar are present, but in 

 other cases, this constituent is almost entirely absent. Garnet is also more 

 abundant in one place than another. In occasional seams, the rock. is 

 made up largely of quartz and feldspar with only a little mica in small 

 flakes. The rock takes on a gray color and is less coarsely cr3^stalline, the 

 structure becoming gneissoid rather than schistose. Some of these grade 

 almost into a quartzite, as the amount of feldspar present grows less. On 

 Manhattan Island, however, the micaceous varieties are greatly in excess 

 over the others. 



A thin section made from a typical specimen of the micaceous variety 

 taken from the site of the Journalism Building of Columbia University, 

 at the southeast corner of West 116th Street and Broadway, shows under 

 the microscope a coarsely crystalline texture and marked foliated struc- 

 ture (PI. XIII, Fig. 1). The chief minerals present are biotite, musco- 

 vite, feldspar, garnet and a little quartz. Magnetite is fairly abundant 

 and small amounts of pyrite are also present. Several grains of staurolite 

 have been noticed. The biotite is a dark greenish-brown, intensely ple- 

 ochroic variety. It is practically always oriented with its basal plane in 

 the plane of schistosity, to w^hich cause the foliation of the schist is prin- 

 cipally due. Muscovite is not nearly as prominent as the biotite. It is 

 often intergrown with it and shows a similar orientation in the plane of 

 foliation. The space between the micas is occupied by the feldspar and 

 quartz. The outlines of these minerals are quite irregular and they are 

 closely interlocked. They are usually quite free from inclusions. Plagio- 

 clase is the most abundant feldspar present, although some orthoclase also 

 occurs. The plagioclase is optically ])ositive and belongs to the andesine 

 variety. It has a maximum extinction angle of 20° in sections cut per- 

 pendicular to the albite lamellae. The garnet is a light pink variety oc- 

 curring usually in idiomorphic crystals which reach a diameter of 1.4 

 millimeters. Analysis 1 quoted in a later paragraph gives the chemical 

 composition of this specimen. 



