GEOLOGY OF THE NEW YORK CITY AQUEDUCT 6l 



faults occur (with northwest-southeast direction across the strike), 

 but so far as is known they are always of minor consequence. In 

 rare instances, the trace of a fault line on the surface describes 

 curious curves, such as that at Cronomer hill above Xewburgh, 

 apparently inconsistent with the chief structural trend, but a study 

 of the whole geologic relation in such cases shows them to be con- 

 nected with the projecting spurs of underlying formations which 

 in any large thrust movement plow their way with some success 

 through the younger overlying, less resistant, strata. They differ 

 in no material way from the ether more simple looking lines. 



Both normal and thrust faults occur, but the thrust type appears 

 to be most common. 



The amount of displacement or throw is extremely variable. The 

 larger faults represent movements of several hundred feet. In 

 rare cases the movement may be as much as 2000 feet. 



The eft'ects may be grouped as follows : ( i ) the appearance of 

 formations out of their normal order, i. e. contacts between forma- 

 tions that do not normally lie next to each other; (2) the produc- 

 tion of escarpments, i. e. steep clift'-bordered ridges; (3) the de- 

 velopment of zones of more or less extensively crushed rock along 

 the principal plane of movement; (4) the determination of loca- 

 tion for stream courses and gulches and valleys that cross the 

 formations. 



All of these effects are more noticeable and better preserved for 

 the later movements than for the earlier ones. ^lany of those 

 dating back to the earliest epoch, affecting only the crystalline rocks 

 of the Highlands, are not readily detected. Most of the breaks 

 have been healed by recrystallization and the contacts are often 

 as close and sound as any other part of the formation. 



But this is not so true of the later epochs — and in them a good 

 deal depends upon the type of rock aft'ected. The more brittle and 

 hard and insoluble types are more likely to still have open seams 

 and unhealed fractures than the softer and more easily molded 

 formations. In some of these, recent water circulation has still 

 further injured the fault zones by introducing rock decay to con- 

 siderable depth. Because of the more ready circulation in them, it 

 is noticeable that some of the extensive decay effects are produced 

 in crystalline rocks that otherwise very successfully resist destruc- 

 tion. On the whole the softer clay shales and slates are less likely 

 to preserve open water channels of this sort than any other forma- 

 tion of the region. 



