GEOLOGY OF THE NEW YORK CITY AQUEDUCT 213 



drift is sand and gravel that is probably saturated and in such con- 

 dition as to permit easy flow to any lower outlet. It may readily 

 carry 8-10 quarts of water to the cubic foot or about 2 gallons. 

 The area covered by such deposits is about 2500 feet long on the 

 southerly base along the creek and at this margin is approximately 

 150 feet deep. The northerly margin is vaniable and reduces in 

 places to o feet in thickness. It may, however, really represent 

 500,000,000 cubic feet of this gravelly material holding 1,000,- 

 000,000 gallons of water as a nearly permanent supply. 



This overlying material is necessarily a menace of no mean pro- 

 portions. Every crevice or crush zone remaining unhealed will 

 have water and plenty of it, the inflow being limited only by the 

 size of the cracks and their abundance until the reservoir should 

 be drained. There is no hardpan bottom to act as a dam. 



Outside additions to this permanent supply are confined to that 

 received from rain and the stream. The rainfall on the area and 

 immediately available as addition to the underground supply in the 

 lower sands, together with the stream flow, which would probably 

 sink into the sands, if an attempt to drain the underground supply 

 were made, may be expected to furnish additional water at a pos- 

 sible rate of 2500 gallons per minute. How much of all this is 

 available at tunnel level depends wholly upon the openness of 

 structure in the rock. There is nothing else to materially control 

 the permanent and additional supply. 



There is evidence in hole 40 of considerable crushing. That 

 means capacity for water circulation, but how much no one can 

 tell. There is also much rotten rock in the same hole. This means 

 that circulation has been easy and effective, but how much now no 

 one can tell. The single hole (no. 40) in the absence of any other 

 corroborative data is not sufficient to base more elaborate or precise 

 quantitative estimates upon. 



5 Solubility. What is " the nature of the limestone with 

 reference to its resistance to solution?" 



This limestone is, as all limestones are, more easily attacked by 

 circulating water than most other rock types [see Rondout Valley]. 

 The Inwood limestone such as occurs at Bryn Mawr is crystalline, 

 often contains much mica and then is inclined to be foliated In 

 structure, and it prevailingly stands steeply inclined. Because of 

 these features in which It diflfers from the Rondout Valley lime- 

 stones, It Is likely to be more generally afifected by decay along the 

 zones permitting circulation than any of the Rondout Valley types. 



