FETTKE, MANHATTAN SCHIST OF NEW YORK 



221 



In a hand specimen, the rock shows a very gneissoid structure and 

 medium coarse crystalline texture. The principal minerals present are 

 biotite, hornblende, feldspar and quartz, with the ferromagnesian min- 

 erals occurring in such large amounts as to give the rock a dark color. 

 Occasionally ^^augen'^ of feldspar are a prominent feature in the rock. 

 These sometimes reach a length of one inch with a width of one-quarter 

 inch. 



A thin section of a specimen from north of Larchmont in Westchester 

 County, when examined under the microscope, shows a medium coarse 

 crystalline texture and foliated structure due to the more or less parallel 

 orientation of the biotite and hornblende. The section consists largely 

 of feldspar, hornblende, biotite and quartz. Magnetite, titanite and apa- 

 tite are present as accessory constituents. The feldspar is largely plagio- 

 clase, which gives extinction angles up to 26° in sections at right angles 

 to the albite lamellae and is optically positive. It is evidently an acid 

 labradorite. Some orthoclase is also present, much of the feldspar being 

 unstriated. A micrographic intergrowth of feldspar with quartz is occa- 

 sionally developed. Undulatory extinction in the feldspar and quartz is 

 of common occurrence. 



Another thin section made from a specimen from Greenwich, Connecti- 

 cut, shows practically the same structure and texture (PL XII, Fig. 5). 

 The mineral composition varies only slightly. Biotite is present in excess 

 of hornblende. In addition to the plagioclase and orthoclase, some micro- 

 cline also occurs, i^n analysis of this rock is given in the next paragraph. 



The following analyses of hornblende schist, epidosite and granodiorite 

 gneiss were made by the writer : 



Analyses of HornNende Schist. Epidosite and Gneiss 





1 



2 



3 



SiOj 



45.90 

 15.58 



2.23 

 10.48 



7.02 

 11.14 



2.47 



1.19 

 .20 

 .06 



**3.7i 



43.52 



16.60 



6.66 



4.79 



3.42 



19.95 



.77 



.25 



.31 



.11 



.41 



3.92 



55.71 

 19.15 



4.52 

 6.42 

 3.55 

 4.56 

 .09 

 .06 



!75 



AI2O3 



FeoO, 



FeO 



Mgo 



CaO 



Na^O 



K2O 



H2O + 



H2O — 



CO2 



TiOj 



Total 



99.98 



100.71 



100.62 



