3^8 NEW YORK STATE MUSEUM 



to Garrison and in the creek valley from Cold Spring to and 

 beyond McKeel Corners, as probably also belonging to the 

 upper members of the gneiss formation. In this case the valley 

 just referred to would represent an eroded syncline and the 

 larger granite intrusions of the mountain ridges on both the 

 northwest and southeast sides would represent eroded anticlines 

 with granitic cores. Although this is a reasonable interpreta- 

 tion no further support to it is at hand, and no subdivision of 

 the gneiss is yet feasible. 



Overlying formations 1 



The formations that come in contact with the normal varieties 

 of basal gneiss series are of six apparently separate types. Two 

 are quartzites, two are limestones, and two are schistose to 

 slaty in general character. The two quartzites have definite 

 sedimentary contacts with the gneiss, though not alike. All 

 of the others when in contact have been forced into that relation 

 by faulting. Of the quartzites one is a rather inconstant bed 

 varying from o to perhaps over 100 feet in thickness, rarely 

 outcropping and in essential relation closely connected with the 

 basal gneiss. This is the rock described as the " Lowerre 

 quartzite " on a preceding page. The other is a very pure 

 quartzite, of a thickness from 300 to 600 feet, and always with 

 unconformable or faulted contact with the gneiss. It is not 

 believed by the writer that these two formations can be equiva- 

 lent. If this be granted, the other four formations * may be 

 divided into two groups, so that each has a definite and constant 

 relation to one of these quartzites, and this together with allow- 

 ance for certain structural features, to be described later, makes 

 many of the seemingly abnormal occurrences of rock type in 

 the region intelligible. These six formations are: 



5 A phyllite or slate (very thick "Hudson River") 



J l It is recognized by the writer that this is a question of great complexity and considerable 

 difference of opinion and interpretation. There is no attempt in this paper or at the present 

 time to review these opinions or discuss their merits. Likewise it is appreciated that the 

 general question of grouping of these overlying formations affects adjacent districts, with some 

 of which the writer is not familiar. 



The reason for discussing local conditions affecting this problem at this time is the more 

 clearly to present the meaning and influence of certain large structures that are not believed 

 to have been given enough prominence and to indicate more fully the true position of the basal 

 gneisses. There is no intention to attempt a broad application of this grouping; but it is 

 believed to be worth while to present the evidence in its favor as it appears in the particular 

 district under discussion. 



To W. W. Mather (i) and thelate Prof. J. D. Dana (2) and Dr F. J. H. Merrill (3) belongs 

 the chief credit for the descriptions, summaries and interpretations that have been published. 



1 Mather, W. W. Geology of New York: Report on First District. 1843. 



2 Dana, J. D. Limestone Belts of Westchester County, N. Y. Am. Jour. Sci. XX, 1880. 



3 Merrill, F. J. H. Geology of the Crystalline Rocks of Southeastern New York. 50th N. Y. 

 State Mus. Rep't 1896. Appendix A. 



