FETTKE, MANHATTAN SCHIST OF NEW YORK 213 



1. Mica feldspar schist from soutlieast corner of Broadway and West 116th 

 Street, New York. 



2. Gray gneissoid variety from Shaft 18, Catsliill Aqueduct, West 42nd 

 Street, near Fifth Avenue, New York. 



3. Staurolite mica schist north of Croton-on-the-Hudson. 



4. Garnetiferous mica schist, Kensico. 



5. Cyanite schist. West 120th Street, east of Amsterdam Avenue, New York. 



An examination of these analyses brings out several interesting fea- 

 tures. In all of these, except No. 2, the MgO is present in excess over the 

 CaO and the KgO over the NaaO. Also the ratio of AI2O3 to the CaO, 

 NagO and KgO exceeds the 1 : 1 ratio in each of these cases. In No. 1, 

 this excess amounts to 70 per cent ; in No. 3, 147 per cent, and in No. 4, 

 181 per cent. In the case of No. 5, no comparison is necessary, as the 

 amounts of KgC^ ^^^ NagO present are practically negligible and CaO is 

 absent entirely, while the rock contains 33.82 per cent of AI2O3. Such a 

 relationship as the above coijld only exist in a rock which was originally 

 of sedimentary origin.^^ The analyses, therefore, give an important clue 

 as to the origin of these schists. 



ORIGIN 



As shown above, the analyses of various types of mica schist belonging 

 to the Manhattan formation, with the possible exception of No. 2, indi- 

 cate a sedimentary origin for this formation. Analysis No. 1 of the typi- 

 cal mica feldspar-quartz schist developed on Manhattan Island corre- 

 sponds to that of a rather argillaceous shale. The same holds true for 

 Nos. 3 and 4. No. 2, on the other hand, which is that of a gray gneissoid 

 variety, as far as chemical composition is concerned, might be either of 

 sedimentary or igneous origin. Its field relation, however, to the associ- 

 ated typical mica schist is such that it can only be interpreted as being of 

 the same origin and merely representing a phase of deposition of some- 

 what different character. The clastic material from which it was derived, 

 originating from the disintegration of an igneous rock of granitic com- 

 position, had probably been less thoroughly decomposed and sorted before 

 deposition took place, therefore giving rise to the deposition of an arkose. 

 It probably represents a coarser phase of deposition than any of the others 

 which originally were undoubtedly fine muds. 



The variation in texture and composition of the schist both vertically 

 and horizontally over large areas also permits of but one interpretation, 

 namely, a sedimentary origin. The occurrence of occasional very quartz- 

 itic beds grading into pure quartzites furnishes further corroboration 

 toward this conclusion. The nature of the contact between the schist and 



2aEDS0N S. Bastin : Jour. Geol., Vol. 17, p. 472. 1909. 



