366 NEW YORK STATE MUSEUM 



Members of the basal gneiss 



Systematic work from New York city to the northern borders 

 of the Highlands on both sides of the Hudson river, especially 

 in the Tarrytown and West Point quadrangles, reveals no evi- 

 dence of any fundamental break or change in the gneiss forma- 

 tion. " The Fordham gneiss " of the Harlem quadrangle as in- 

 dicated in the New York City Folio 83, United States Geological 

 Survey, is not different in position or significance or general 

 character from the same gneiss series of the Tarrytown quad- 

 rangle, with which it is continuous ; and the writer sees no 

 essential point of difference between these and the basal gneisses 

 of the West Point quadrangle, from which they are separated 

 by only a belt of later limestones and schists occupying the 

 synclinal fold of the lower Croton valley. The same banded 

 types and granitic fades, as well as the same relations to over- 

 lying formations, prevail throughout. But in the northern 

 Highlands it appears that interbedded limestones and quartzites 

 and schistose graphitic beds are common, whereas in the south- 

 ern localities the limestones at least are not so frequently seen. 

 Yet traces of such limestone beds are found as far south as 

 the vicinity of the Jerome Park reservoir in the Harlem quad- 

 rangle. Whether, they really do occur more frequently can not 

 be determined because of the relative abundance of the over- 

 lying formations, which may normally cover the portions that 

 contain these types. The whole area in the north, being eroded 

 down to the gneiss floor, gives much better opportunity to 

 discover the smaller and less resistant members. They are 

 noted at several points and described by W. W. Mather in his 

 Geology of the First District published in 1843. The best develop- 

 ment of these interbedded limestones is along the Hudson river 

 near Fort Montgomery and Highland Station, and near Gar- 

 rison at Arden Point, and at McKeel's Corners northeast of 

 Cold Spring. They have all been noted and mapped before, 

 but have not been interpreted in this way. 



No subdivision of the gneiss formation at present seems 

 possible. There is no natural stratigraphic break. Because of 

 the abundance and regularity of the igneous injections and the 

 close folding and frequent faulting, it is not even clear as to 

 the order of superposition of the constituent members. At a 

 few places, in what seems to be an upper member, because 

 of its connection with overlying formations, the banded black 



