GEOLOGY OF THE NEW YORK CITY AQUEDUCT 265 



/ Condition of the rock 



That the upper lOO feet of ledge is \ery rotten can not be de- 

 nied, but it is certain that this lower portion of the hole 

 is not in so bad condition as the low saving- of core 

 would lead one to think. The grains are affected by chloritic 

 alteration in such manner that they can not resist much 

 disrupting force. The small diameter of drill used subjects 

 the whole core to enough strain to cause the gradual pul- 

 verization of the rock. This aft'ects both the core that has 

 been cut loose and the hole wall that is further subjected to 

 the thrashing of the drill rods. A larger size core would 

 make a very much more encouraging and fair showing. 



There may be an occasional small seam so badly decayed that 

 it is encouraged to run or cave under such treatment. But 

 there is absolutely no evidence that slumping or caving is 

 common or even likely on any considerable scale. 



The material that partly fills up the hole when the drill i-- 

 pulled up is believed to be in considerable part the settlings 

 of suspended matter which during the agitation of drilling is 

 distributed through the rising column of water. The reduc- 

 tion in volume (lo gallons being fed and only 5^ gallons 

 being recovered) due to rock porosity is favorable to such 

 behavior of the loosened material. 



SUMMARY OF LOCAL GEOLOGY. 



Formations. Only three formations are represented in the 

 rock floor of this section. These are the regular crystallines char- 

 acteristic of all southeastern New York. 



1 ^Manhattan schist 



2 Inwood limestone or dolomite, and 



3 Fordham gneiss, including the Ravens wood granodiorite as a 

 special intrusive member, and an unusually strong development of 

 the interbedded limestones and associated schistose facies. 



These formations have their usual relation — the IManhattan 

 above and youngest, the Inwood intermediate, and the Fordham 

 underneath and older. These simple relations, however, are much 

 complicated by dynamic disturbances of more than usual violence 

 so that the series is thrown into folds so close that the individual 

 beds stand almost on edge. In addition lateral thrusts of that same 

 time or later have broken the strata and faulted them in several 

 places. This complicates the structures still more, and, since the 



