8o NEW YORK STATE MUSEUM 



line schists and limestones represented by the Manhattan-Inwood 

 formations, and their attitude at the boundary with the gneiss in the 

 southeast quarter of this quadrangle suggest a sharp flexure along 

 the margin which may have the same meaning as a fault line. It is 

 a question whether that boundary might not better be represented as 

 a fault. It is not clear, even so, whether it would represent any 

 different age relation from those already discussed. The reason for 

 calling attention to it is the suspicion that this particular deformation 

 may date to a much older period than either of the two just 

 described, and this may account for the sudden disappearance of 

 the Manhattan-Inwood series farther north. It is, in other words, 

 connected with the very confusing problem of the interpretation of 

 the Manhattan-Inwood series. 



Minor structures. In addition to the faults of a major sort, 

 there is an endless variety of crumples, minor faults, drag effects, 

 etc. which are the incidental accompaniment of major deformation. 

 In some cases they are undoubtedly simply secondary and tertiary 

 and minor drag effects. In other cases a similar appearance is 

 created by movements in the magmatic masses, where partially 

 digested slabs of older rocks are distorted or where poorly distributed 

 matters are drawn out and twisted by magmatic movements. These 

 give great variety of appearance to the formations under discussion, 

 but they do not deserve detailed treatment here. 



Folds. One's first experience with the Highlands gneiss struc- 

 tural features, especially the high angle of dip which nearly all 

 formations of the district have, leads one to assume a very profound 

 folding as one of the contributaiy causes. Undoubtedly much fold- 

 ing has taken place and such structural features as belong to folds 

 are especially well developed in the Cambro-Ordovician and in the 

 Manhattan-Inwood series. But it does not appear, after further 

 consideration, that much of the structural detail of the gneisses or 

 of the banded and streaked rocks of mixed origin have been greatly 

 influenced by folding, except such as may date back to Grenville 

 time. 



It is clear, from knowledge of the structural history of the gen- 

 eral region to which this area belongs, that it must have been within 

 reach of the deforming forces of every mountain-making period 

 since Grenville time ; but it may very well be that deformation in 

 the form of folding has not affected these massive members of the 

 Highlands belt as much as the more superficial and less competent 

 overlying sediments. It must be appreciated that the Highlands belt 



