I30 NEW YORK STATE MUSEUM 



very short distance between typical occurrences of Manhattan 

 schist of the southern series and the phylHtes of the Peekskill Valley. 

 Nevertheless the two are strikingly different in petrographic quality. 

 Since the present condition of both seems to be the result of regional 

 metamorphism, it is very difficult for one to believe that such strik- 

 ing differences could be consistent with so short a distance. 



Everyone who is familiar with tliis question has remarked how 

 very different petrographically the two series are, and there is always 

 the strongest inclination to consider them entirely distinct series. As 

 soon as one begins a comparison, however, the difficulty of the whole 

 problem is found to be much greater than was supposed, and a dis- 

 cussion of its essential points covers many other related features. It 

 is such an important matter, however, in this particular area that it 

 is necessary to go into this discussion with some care, and the fol- 

 lowing is a statement of the various points of evidence bearing on the 

 correlation of these two apparently different series. 



It is realized that the problem can not be solved in this quadrangle 

 but that it probably can be solved by a careful round-about study of 

 adjacent territory carried through for this particular purpose. Such 

 a regional study does not fall within the scope of our present enter- 

 prise, but it is felt that a statement of the nature of the problem and 

 the factors belonging to it will clarify the situation in this district 

 and may be of service ultimately toward a complete solution of the 

 matter of correlation. 



Comparison of the two series. The argument can best be fol- 

 lowed by arranging in parallel the two groups of factors believed to 

 be essential and suggestive. First, for convenience, the factors that 

 are considered to show identity of the two series ; second, the factors 

 that indicate independence of the two groups. Incidentally the 

 study may serve as an illustration of the kind of problem usually 

 presented in the correlation of obscure crystalline rocks. 



Principal points of similarity. 



I The two series show great similarity in succession and original 

 character and origin and jn comparative thickness of the mem- 

 bers. The upper member is made up of a great but indeterminate 

 thickness of shales, sandstones, graywackes and slates on the nortih 

 side of the Highlands and a strongly micaceous schist of usually very 

 coarse and thoroughly crystalline habit but similar great thickness 

 on the south side. The former represents the Hudson River shale 

 series and the latter the so-called Manhattan mica schist (also called 

 Hudson schist). 



