74 ^^EW YORK STATE MUSEUM 



has yet shown igneous transgression of the Cambrian unconformity 

 in this area ahhough it is possible that this does not hold farther to 

 the south and to the east. It certainly does not hold to the south if 

 the Manhattan-Inwood series is Cambro-Ordovician, but reasons for 

 questioning that correlation are given elsewhere in this paper. One 

 of the latest of the great igneous invasions belonging to the Pre- 

 cambrian series is the Storm King granite which is, we believe, the 

 equivalent of the syenite series of the Adirondacks, and it is most 

 prominently developed in the very section where the unconformity 

 is best preserved. Whatever igneous activity there may be of Post- 

 cambrian age must be much later than this. Even the dikes which 

 cut the Storm King granite do not cut the Cambro-Ordovician series. 

 It is our belief that only the Cortlandt series fulfils this last 

 condition. 



It appears, therefore, that the Cambrian unconformity is here a 

 complete break separating both the ancient metamorphics and the 

 whole great series of igneous intrusives from the later Cambro- 

 Ordovician sediments. When one appreciates that these intrusives 

 are large, deep-seated masses of very massive habit and could not 

 possibly have developed near the surface, this unconformity takes 

 on some'hing of its true significance. It represents an erosion inter- 

 val of vast time during which some thousands of feet of overlying 

 rock must have been removed. 



The same unconformity has been found on the west side of Peek- 

 skill creek west of Putnam Valley and the relation is the same, 

 except that the erosion surface has been tilted until it stands verti- 

 cally. But satisfactory exposures are difficult to find. On the east 

 side of the valley, although both the quartzite and underlying granite 

 are well exposed, the structural relation is obscure because the under- 

 lying member is a rather featureless granite instead of a true gneiss. 

 It is very significant, however, on this point, since the granite here is 

 the so-called Reservoir granite. It is clear that the granite does not 

 cut or in any way afifect the adjacent quartzite. Here also the 

 quartzite member runs so straight and true in spite of the fact that 

 it is tilted into almost vertical position that one is impressed with the 

 evident uniformity and monotony of the erosion surface on which 

 it was formed. The similarity in thickness of the quartzite as 

 developed in this valley and on the northern border seems to argue 

 for the same thing. 



A great deal of attention has been given to the question of a pos- 

 sible additional unconformity in this southeastern Ne\7 York region. 



