4o NEW YORK STATE MUSEUM 



This series together with the gneisses below constitutes the bed 

 rock and controls the underground conditions for all of the line 

 south of the ^loodna valley, 50 miles above Xew York. All of the 

 southern aqueduct, and the Xew York city distribution conduits are 

 wholly concerned with these rocks, and two divisions of the northern 

 aqueduct have a large proportion of their work in them. 



It is not wholly clear what age these crystallines represent. It 

 is certain that the underlying gneisses are Grenville and that the 

 metamorphic quartzite, Inwood, Txlanhattan series, is Post-gren- 

 ville. It is possible that these latter are also Precambric. But 

 usage following the correlations of Dana^ and in the absence of 

 as good evidence from any other source has regarded them as the 

 Cambro-Ordovicic crystalline equivalents of the Poughquag-Wap- 

 pinger-Hudson River series of the north side of the Highlands. 

 The writer has elsewhere shown- that the evidence and arguments 

 are not all on one side and that considerable doubt may still be 

 entertained on that point. There is no object in following that 

 argument here or in modifying the treatment here followed of mak- 

 ing them a distinct series. Even if they should prove to be th^ 

 exact equivalents of the Hudson River-Wappinger-Poughquag 

 series the formations are physically so different and rec|uire so 

 different treatment in discussion that they must for our present 

 purpose be regarded as an essentially distinct series. From that 

 standpoint alone the usage here followed is justified. The ]\Ian- 

 hattan schist of AVestchester county as a type dift'ers as much 

 petrographically from the Hudson River formation of the Xew- 

 burgh district as the Catskill formation of Slide mountain dift'ers 

 from the Jameco gravels of Long Island. In a discussion where 

 physical or petrographic character is in control there is no doubt 

 about the advisability of treating the two separately. 



(i) Manhattan schist." This is primarily a recrystallized sedi- 

 ment of silicious type. It occurs as a nearly black or streaked, 

 micaceous, coarsely crystalline, strongly foliated rock. The chief 

 constituents are biotite, muscovite and quartz. Quartz, feldspar, 



1 Dana, J. D. On the Geological Relations of the Limestone belts of 

 Westchester county, X. Y. Am. Jour. Sci. 20:21-32, 194-220, 359-75, 450-56 

 (1880); 21:425-43; 22:103-19, 313-15, 327-35 (18S1). 



2 Berkey, Charles P. " Structural and Stratigraphic Features of the 

 Basal Gneisses of the Highlands." N. Y. State ^lus. Bui. 107 (1907), 

 p. 361-78. 



3 Manhattan schist of Merrill. N. Y. State Mus. 50th An. Rep't, 1:287. 

 Same as " Hudson schist," of X. Y. city folio no. 83. 



