GEOLOGY OF THE POUGHKEEPSIE QUADRANGLE 1 3 



that produced the ridges. The gneisses clearly show the effects of 

 repeated orogenic disturbances. 



In some places it is clear, from the position and structure of the 

 overlying younger rocks, that most of the features of the gneisses 

 date from Precambric time. Where the relationship of the basal 

 quartzite to the underlying gneiss is most plainly seen, as in the West 

 Fishkill Hook,^ the latter stands at a high angle with a uniformly 

 northeast-southwest strike, while the quartzite dips at a low angle 

 with varying strike. In other places the discordance between the 

 dips and strikes is plainly discernible. The quartzite has been folded 

 relatively little in many places, and never within this quadrangle to 

 the extent shown by the gneisses. Faulting, instead of extreme 

 folding, occurred in connection with Postcambric movements within 

 the gneisses. 



The early crystalline condition of the gneisses would have favored 

 faulting and shearing and would have prevented much later folding 

 within them. It is certain that the isoclinal character is of Pre- 

 cambric age. 



It seems possible, therefore, in a large way, to apportion the 

 structural features of these gneisses as seen in the field among 

 orogenic movements of Precambric and later time. It is quite 

 uncertain how many different disturbances may have occurred in 

 Precambric time and whether all the later structural features are 

 of similar age. 



The lines of foliation, as seen in outcrops, are usually rectilinear. 

 When wavy, they are only slightly so. This latter feature seemed 

 most noticeable on Shenandoah mountain. Crinkling is rare. Two 

 or three instances of it were noted in the Glenham belt. Jointing 

 is common and frequently gives the appearance of thick exfoliation. 



Faults are divisible into two kinds, reversed and normal. It seems 

 most likely that the normal faults followed the compression that 

 produced the thrusts and are therefore of the nature of adjust- 

 ments. All the faults that have been noted appear to belong to the 

 great mountain building process of Ordovicic time which elevated 

 the Paleozoics of the Green mountain belt. This is indicated by the 

 relations which exist between the younger and older rocks and by 

 the fact that the fault lines of the mountains are projected north- 



1 The recesses east and west of the short spur that separates Mount 

 Honness from Shenandoah mountain are respectively known as East 

 and West Fishkill Hook. 



