NEWARK ROCKS OF ROCKLAND COUNTY, N. Y. 45 



mation and cause the repetition of several thousand feet of strata. 

 Many smaller dislocations have been observed in quarries and 

 other sections. In New York state, however, evidence of fault- 

 ing is scanty. The data bearing on this question are as follows: 



At Sparkill, Overpeck creek crosses the trap sheet through a 

 gap eroded nearly to sea level, and probably located along a fault. 

 The evidence favoring this conclusion is found first in the abrupt 

 offset to the west of that part of the ridge lying north of the gap, 

 and second in the greater elevation of the base of the trap north 

 of the gap. The offset measures over 300 yards, which, if the 

 motion were entirely vertical, would mean a throw of over 200 

 feet. The base of the trap has not apparently been raised that 

 amount, and therefore the motion seems to have been a combina- 

 tion of an uplift and a side thrust. 



At Upper Nyack a fault with throw of not less than 10 feet can 

 be seen in a ledge of red shale a little above the river. The fault 

 plane hades 15° west and trends about north. There are no 

 means of telling how much more than 10 feet the throw may be. 



At Trough Hollow the trap ridge appears to be slightly offset, 



as if a fault crossed it diagonally, and the base of the trap rises 



,somewhat abruptly near the stone-crusher at Oosgriff's quarry. 



Both of these phenomena may be due solely to a change in horizon 



of the trap. Faulting, if present, is slight. 



Some observers have seen evidence of faulting in the gaps at 

 Long Clove and Short Clove, but, after careful examination of the 

 ground, I was not convinced that they mark fault lines. Mani- 

 festly the gaps themselves do not necessarily indicate faulting. 

 As already suggested, these gaps with several others of accordant 

 hight are probably parts of a drainage system formed when the 

 land stood somewhat lower with respect to sea level than at 

 present. Their location may have been determined by faults, but 

 I could find no positive evidence that such was the case. All the 

 facts observed can be accounted for without calling in the aid of 

 faulting. Were these gaps along fault lines, the ridge would 

 commonly be offset, though it is conceivable that by a peculiar 

 combination of circumstances faulting might have occurred with- 

 out causing offset in the ridge. 



