GEOLOGY OF THE WEST POINT QUADRANGLE, NEW YORK III 



or variants and that they are insignificant in comparison with the 

 principal factors given. Whatever complexity of structure or 

 quality they develop and whatever gradation or relative sharpness 

 of relation they may show in the field they do not in any case mate- 

 rially conflict with the explanation given, nor do they suggest a 

 definite modification of the history and principal stages. 



Precambrian erosion. It would not be possible for the qualities 

 of rock and the structures represented in this district to develop 

 except at great depth. All the phenomena represented in the Pre- 

 cambrian, except that belonging to the original deposition of the sedi- 

 ments themselves, are of a deep-seated type. How deep, no one can 

 tell, but probably many thousands of feet. What the surface ex- 

 pression was like, no one knows. There may have been and probably 

 was volcanic activity, but all of this has been eliminated by the 

 erosion which followed upon the completion of the igneous history. 



Judging from the relations of the rock at the unconformity, 

 between the Highlands series of gneisses and the Poughquag quartz- 

 ite of Cambrian age, the region was worn down to a comparatively 

 uniform surface. This surface is particularly smooth wherever 

 exposed in this quadrangle and it would be allowable to assume that 

 planation by erosion was very perfect and that essentially a peneplain 

 was developed across the irregularities of the Precambrian series. 

 This erosion cut down into the granite thus removing in many places 

 all the overlying rocks and beveling across the representatives of 

 every stage preceding. This may mean that a large amount of 

 deformation accompanied the other transformations of the Precam- 

 brian history, both faulting and folding. But except for prominent 

 shear zones, some of which appear to date back to this period, there 

 is no other evidence beyond that exhibited by the unconformity just 

 referred to. It is clear in any case that an immense length of time 

 subsequent to the last igneous development must be represented in 

 this erosion interval. 



Cambro-Ordovician sedimentation. Whatever the conditioiis 

 were under which erosion of the Precambrian series was completed, 

 it is clear that sedimentation began with a clean swept floor on which 

 remarkably pure quartz sands were developed. There is nowhere 

 any development of conglomerate and nowhere much mixed or arko- 

 sic material such as might reasonably be expected from the erosion 

 of a granite gneiss area. This fact makes one suspicious of the 

 source of the material and the conditions under which these rocks 

 developed. If developed from the gneisses and granites directly, 



