GEOLOGY OF THE POUGHKEEPSIE QUADRANGLE 47 



Summary and conclusions. The relationships among the 

 quartzite, limestone and calcareous shale described above are ex- 

 hibited nowhere else in this quadrangle. 



The field relations of the gneisses and the quartzite indicate that 

 the older rocks have been thrust up into the younger series and 

 that in general their present relative position must be regarded as 

 very different from that which obtained when the Cambric sea over- 

 lapped the older land. It is plain that the quartzite was involved 

 in the thrust movement and, although never violently folded, was 

 yet greatly disturbed by folding in certain places. In many instances 

 the quartzite was moved bodily with the gneisses, so that where it 

 is now present, or was apparently present up to a comparatively 

 recent epoch, it is not contiguous with the limestones of its own 

 epoch, but with later ones on which it has been thrust. 



A not unreasonable restoration of the Precambric floor, which 

 is thus assumed to have been fractured and elevated, would allow 

 a considerable extension of the thick quartzite formation south- 

 ward from its present northern position. The actual evidence for 

 such a former extent consists in the faulted mass at Hortontown, 

 which, since the thrust movement was northwestward, could hardly 

 have had an original position farther northwest, but which might 

 readily have come from the southeast, and in occasional ledges 

 observed in the woods during a reconnoissance south from West 

 Fishkill Hook across the quadrangle boundary. The character of 

 the slope of the quartzite where least disturbed, as in West Fishkill 

 Hook, its thickness and the rather steep southern termination at 

 certain places, indicate a former southward extension. 



The varying strike and dip of this formation is best interpreted 

 as the result of disturbance subsequent to its deposition, rather than 

 to original initial slope. 



In attempting to explain the present valley position of the 

 younger rocks along the northern border of the Highlands, instead 

 of assuming that they were deposited in valleys, we are offered 

 the alternative explanation of down-faulting, and subsequent partial 

 or entire erosion in which the ice sheet may have played an import- 

 ant part. 



The Precambric masses may have stood as islands in the early 

 Paleozoic sea, but the present relationships do not require such an 

 interpretation. 



The disturbance of the quartzite has given it such inclination that 

 it might be regarded as of different geological age at different 

 altitudes. Of this there is no evidence. 



