24 NEW YORK STATE MUSEUM 



rangle and in one long downfaulted strip along Peekskill hollow 

 and across the Hudson river to the south margin of the sheet. The 

 amount of territory covered by the three minor groups is not very 

 different one from the other. They are all small and comparatively 

 insignificant in contrast to the dominant series represented by the 

 ancient crystallines of Precambrian age. 



The drift cover on the north margin and in the southeast quarter 

 of the quadrangle obscures the boundaries of these groups so that 

 mapping can not be done accurately, but over most of the quadrangle 

 the major divisions are easily distinguished and are mapped with 

 fair precision. In most cases, however, where the attempt is made 

 to distinguish the minor subdivisions much greater difficulty is 

 experienced. This difficult}^ is measurably increased by the fact 

 that some of these smaller members are found only in those areas 

 already referred to as being obscured by drift. The greatest uncer- 

 tainty of this sort is in the southeast quarter, where certain critical 

 structural relations must exist, l)ut can not l^e seen because of the 

 heavy cover of glacial deposits. 



On other portions of the area, more elevated and rugged, repre- 

 senting the Highlands belt proper, outcrops are numerous and very 

 extensive, and the difficulties emphasized above do not exist, but 

 other greater ones appear. In this case the difficulty arises from 

 the normal obscurity of the structural relations and the great 

 variability of appearance and quality of the formations them- 

 selves. It is the sort of problem that always confronts one in 

 attempting to draw lines of division in a series which seems to have 

 all sorts of gradations and transitions and no sharp boundaries. The 

 best that can be said in the matter of formational subdivision of 

 this ancient series of gneisses and granites is this : Certain dominant 

 types are recognized as fundamentally distinct and all the variations 

 noted in them and between them are understandable as develop- 

 ments from these fundamental imits under the history indicated for 

 the region. (See statement of the petrogenesis of this series, 

 page 29.) 



There is very great petrographic variability in nearly all forma- 

 tions of the quadrangle. This is particularly true uf the more 

 ancient ones and less true of those representing slightly metamor- 

 phosed sediments. But all that have Precambrian history exhibit 

 elaborate petrographic variations, some of which are extremely diffi- 

 cult to interpret. It is this great variability of the members them- 

 selves that has always made it difficult to do geological work in the 



