GEOLOGY OF THE NEW YORK CITY AQUEDUCT 249 



Condition of the limestone schist contact. The finding of 

 Inwood limestone above the Manhattan schist in hole no. 33 at 

 1 21 St street east of Momingside and the fairly sound condition of 

 both types raises the general question of the condition of contact 

 zones as compared with fault zones. 



There are three important facts to consider bearing on this case : 

 (i) The contact zones are commonly weaker than either formation 

 alone and (2) at this particular point an abnormal relationship is 

 shown by the overturned strata (the limestone lying above), and 

 (3) the fault zones are always weak and extensive^ decayed. 



Because of the abnormal position of the limestone here, lying as 

 it does overturned, a weaker more pervious rock upon a more sub- 

 stantial and less pervious one, it appears to be reasonable enough to 

 find the limestone and schist fairly well preserved, under conditions 

 where a vertical or a normal position would have encouraged decay 

 because permitting a more ready circulation. 



But there is a further conclusion that seems allowable, i. e. the 

 fault or crush zones are more extensively decayed than the simple 

 contact or transition zones. And contrariwise, where an especially 

 extensive decay is encountered, it probably is to be associated with 

 a crush zone due . to fault movement rather than with any other 

 structure. 



A further inference seems allowable from tlie data of these holes. 

 It is probable that these fault zones do not follow the contacts or 

 bedding exactly but cut across at low angles, sometimes coinciding 

 with the contact Hues and sometimes falling wholly within the lime- 

 stone or the schist. 



Great depth of decay at south end of Morningside Park. The 

 finding of approximately 150 feet of decayed rock in hole no. 16 

 and of nearly 200 feet of similar type in hole no. 36, all so rotten 

 that the material came up as a mud, raises a very difficult question 

 as to the conditions that make such extensive decay possible. 

 Hole no. 7 (113th st.) shows extreme decay to elevation -204 feet 

 Hole no. 16 (iioth st.) shows similar condition to elevation -250 



feet 

 Hole no. 36 (io8th st.) shows similar condition to elevation -185 



feet 



These three holes showing similar condition of very deep decay 

 are located almost exactly in Hne. Nothing on either side of this 

 line is in so poor condition. 



Consideration of these conditions can not fail to raise certain 

 questions of interpretation. 



