124 NEW YORK STATE MUSEUM 



as well as some of the basic intrusives are in reality younger than 

 this metamorphic series, so that if the age of this series could be 

 determined, it would to that degree fix age limits to a certain amount 

 of igneous history. Likewise, since this same series proves to be 

 most intimately associated with the oldest gneiss, the Fordham 

 gneiss in Westchester county and in New York City, where there 

 seems to be conformity between them, the possibility of greater age 

 for this series than is usually assumed is thereby suggested. 



The questions raised, therefore, by the confused situation repre- 

 sented in the last item are: (i) Are some of the igneous members, 

 both acid and basic ones, comparatively young, even late Paleozoic, 

 as would seem to be indicated if the Manhattan-Inwood series is itself 

 Cambro-Ordovician in age?, or (2) Is the Manhattan-Inwood 

 series shown to be very ancient — perhaps a part of the Grenville 

 itself — by the evidence of its roughly conformable relation to the 

 oldest gneisses and by its igneous associations? 



The enormous development of the Manhattan schist with its asso- 

 ciated Inwood limestone south of the Highlands is of course most 

 disconcerting, and if one assumes that they are of Grenville age, they 

 certainly dififer much from the typical Grenville of the Adirondacks 

 and of the central Highlands belt. But they are not very unlike the 

 Grenville of the typical Canadian occurrences. The chief difficulty 

 therefore is not their character, but their sudden appearance, together 

 with the fact that a very similar succession of sediments, the Hudson 

 River slates, Wappinger limestone and Poughquag quartzite, of 

 well-established Cambro-Ordovician age, are developed on even a 

 greater scale only a short distance away. It is therefore somewhat 

 easier to account for the difference in character by some meta- 

 morphic influences, which gave to the series south of the Highlands 

 a greater complexity and more elaborate recrystallization, than it is 

 to find a good structural explanation or reason for a great series 

 like this one being so sharply delimited areally, and the failure to 

 find any overlapping of the two contrasted series. A discussion of 

 the points of the problem, however, in any greater detail is not in 

 place here. (See a further continuation of the discussion on 

 page 128.) 



/ The Precambrian erosion interval. 



Both in the Highlands and in the Adirondacks, long erosion 

 cutting deep into the metamorphic and igneous Precambrian series, 

 preceded the Cambrian subsidence and deposition, and the first 



