GEOLOGY OF THE NEW YORK CITY AQUEDUCT 1 39 



this concrete was mixed as rich as i part cement to 5 parts aggre- 

 gate and the work was well done, excessive leakage reaching a total 

 of 1,250,000 imperial gallons per day was developed within a year. 

 It was found that the limestone fragments of the aggregate were 

 corroded forming holes through the lining of the aqueduct and that 

 these holes actually enlarged outward. All this was done under cut 

 and cover conditions with not more than a 6 or 7 foot head on the 

 bottom of the aqueduct. 



In the Rondout valley, the aqueduct will cut no less than 6 lime- 

 stone beds in all cases under great pressure. This fact will in all 

 probability tend to increase the action. But, of course, some of the 

 beds may not yield so readily to solution. Tests made thus far, 

 however, indicate that all are attacked in water. Considering these 

 facts it seems desirable, so far as possible, to avoid the limestone 

 beds wherever rock of greater resistance to solution can be reached, 

 and further it is equally desirable to use a more resistant rock for 

 the lining concrete. So long, however, as the formation is not very 

 pervious so that a new circulation could not be established by the 

 escaping water there would be little harmful effect. 



An average of five analyses of the Thirlmere limestone, different 

 varieties of the same formation, gives the following: 



Insoluble silicious matter 2 . ^^2% 



Alumina and iron oxid Al203H-Fe203 0.276 



Lime, CaO 53 . ^yd 



Magnesia, MgO 390 



Carbonic anhydrid, CO2 42.248 



Total 99 . 362 



Estimated calcium carbonate, CaCOg — 95.85^ 

 The limestone is fossiliferous. 



Suitable analysis of the limestones of the Rondout valley are not 

 recorded in sufficient numbers. But these are a few, as given below. 



At Hudson 

 BECRAFT LIMESTONE. At Rondout At Wilbur (av'ge of 2) 



SiOz 3-87% 7-10% 1.865% 



A1203 1.07 2.50 .818 



FCaOs 1.34 1.65 1. 185 



CaO 54.11 45.32 51-375 



MgO tr tr 2.870 



CO2 40.60 39.10 40.795 



Total 100.99 95.67 98.908 



Corresponding to total calcium carbonate.. 96.62 80.75 91.74 



