FETTKE, MANHATTAN SCHIST OF NEW YORK 243 



of the pegmatitic intrusions is shown by its relationship to the other 

 schist and the later pegmatitic intrusions. 



The frequent association of these feldspar "augen" with little veinlets 

 of secondary quartz and pegmatite favors such a hypothesis. The fact 

 that micrographic and perthitic intergrowth are occasionally present in 

 the orthoclase and microcline also points toward such an origin as such 

 intergrowth would hardly be expected in feldspar representing pheno- 

 crysts of a sheared porphyry. The variation in the mineral composition 

 of the matrix can also be explained on this basis, as it would be that of 

 the sheared rock into which the injection took place. 



Occurrence of Zeolites in the Manhattan Schist 



Zeolites are occasionally found lining cavities and small crevices in the 

 Manhattan schist. Among them, thomsonite, natrolite, analcite, chaba- 

 zite, phacolite, harmatome, heulandite and stilbite have been reported 

 from Manhattan Island.*^ 



Specimens of stilbite and chabazite occurring in the Manhattan schist 

 in this manner were given to the writer by Mr. J. R. Healy, assistant 

 engineer with the New York Board of Water Supply. They were ob- 

 tained from Shaft 15 of the Catskill aqueduct at 65th Street and Cen- 

 tral Park West. The crystals of stilbite and chabazite lined the walls 

 of a small open crevice which followed the plane of schistosity of the 

 mica schist for a short distance. The stilbite has a honey-yellow color 

 and has crystallized in sheaf-like and radiated masses. The chabazite is 

 white in color and has a nearly cubic form. It precedes the stilbite in 

 order of crystallization, as the latter sometimes grows on top of it. Little 

 veinlets of pegmatitic material and epidote occur in the schist closely 

 associated with the stilbite and chabazite. 



When examined in thin section under the microscope, it is seen that 

 much of the biotite of the mica schist has been altered to chlorite. The 

 orthoclase of the little pegmatite stringers is also much kaolinized. As- 

 sociated with these pegmatitic stringers but later in origin are veinlets 

 of quartz, which, under the microscope, appear as a fine mosaic of little 

 grains. Veinlets of epidote with a little accompanying calcite are often 

 associated with these quartz stringers and cut them in such a way as to 

 show that they were the last to be introduced. 



The formation of the zeolites probably accompanied the last stages 

 of the pegmatitic intrusions. The zeolites were probably deposited by 



*® B. B. Chamberlix : "The Minerals of New York County." Trans. N. Y. Acad. Sci., 

 Vol. VII, No. 7. 1888. 



