MerriUr—Metamorphic Strata of 8. K New York. 301 



found on Manhattan Island, is evidently a part of the same 

 littoral deposit to which the Fordham gneiss belongs. The 

 Manhattan schists are the only beds now to be fonnd on New 

 rk [aland with the exception of the limestone areas de- 

 scribed and mapped by Professor Dana and the small area of 

 Fordham gneiss at the north end of Seventh avenue. 



If in time this group be correlated with some other which 

 has been previously described, the names here suggested may 

 be unnecessary, but until the question is indisputably settled 

 they are of use in referring to the formation and its sub- 

 division-. 



Intercalated with the Manhattan schists and also with the 

 beds oi the Fordham gneiss we find at a great number of 

 localities, hornblendic and angitic strata of limited thickness, 

 usually only a few feet. In composition these rocks resemble 

 diorites and diabases, and in structure they are granular, but 

 their present well stratified condition renders it difficult to say 

 whether they are originally eruptive rocks or not. Whatever 

 their origin they are now metamorphic rocks and as such may 

 be called amphibolites and pyroxenites according to the ter- 

 minology of Kalkowsky. It is probable that to rocks of this 

 character we are indebted for some of our serpentines, notably 

 that of 60th street uear 10th avenue, Xew York City, for as 

 originally suggested by Dana and lately demonstrated by 

 Gratacap it is derived from the alteration of amphibole, and 

 on 61st street near 11th avenue, a bed of amphibolite occurs, 

 of which the line of strike passes through the well known and 

 interesting serpentine above mentioned and which lies about 

 250 feet southeast. 



Age of the Manhattan Group. — It is not yet in the power 

 of the writer to contribute any positive information on this 

 important question He has not yet found any decisive evi- 

 dence of the age of the rocks in question. All the suggestive 

 evidence, however, favors the view taken by Professor W. W. 

 Mather and subsequently elaborated by Professor J. D. Dana, 

 viz : that the rocks of the Manhattan Group are the metamor- 

 phosed equivalents of the Paleozoic beds of Southern Dutchess 

 County. 



After a careful study of the stratigraphy in the vicinity of 

 Peekskill which seems to be the index of this geological 

 chapter, and at other points along the northern margin of the 

 Manhattan terrane, the writer concludes that if this group is 

 pre-Cambrian, its identity as such has been obscured by a series 

 of stratigraphic vicissitudes so complicated that it is beyond his 

 powers, at present, to conceive them. 



Am. Jouu. Sci.— Third Seriks, Vol. XXXIX, No. 233.— Mat, 1890. 

 26 



