212 NEW YORK STATE MUSEUM 



Comparison of Bryn Mawr and shaft 13 

 The following statements embody an opinion on the points raised 

 or suggested in connection with a reference to the New Croton 

 difficulties at shaft 13. Tlie items are therefore treated by compari- 

 son or contrast so far as possible: 



1 Type of rock. The rock explored at the Bryn Mawr siphon 

 is the same formation as that in the Saw Mill valley cut by the 

 New Croton aqueduct, i. e. the Inwood limestone — sometimes 

 called '' Stockbridge dolomite." It is the same also as the other 

 large limestone belts in Westchester county. There are occasional 

 small strips of limestone of another type, but its behavior could not 

 be very different. 



2 Soft material. " Is any material of this sort " (like t-hat in 

 the New Croton tunnel near s'haft 13) " likely to be encountered 

 either in the crushed zone at boring 40 or elsewhere in the lime- 

 stone belt?" 



It is sure to be encountered, especially near hole 40, if that zone 

 is cut shallow. The behavior of the lower portion of this hole is 

 very similar to the described case near shaft 13. The only prob- 

 ability of avoiding it lies in placing the tunnel deep enough to cut 

 more substantial rock. Ttie single hole upon which all this argu- 

 ment is based can scarcely be considered a thorough enough ex- 

 ploration to build up a quantitative statement as to depth or width. 



There is no evidence, either on surface or in the exploration 

 holes, of any other such zone on this line. 



3 Depth and extent. Under the circumstances, the increased 

 depth makes it less probable that so much ground of like behavior 

 would be found. Again, it is not likely that precisely the same 

 conditions would so effectually halt operations or be considered so 

 nearly insurmountable at this time. One of the many serious 

 objections is that the tunnel would have little strength or resist- 

 ance to a bursting pressure. It must be admitted that if caving 

 ground were penetrated it would prove very difficult to handle with 

 the gravel cover at the depths now considered, i. e. 300 feet or 

 more below the surface. 



4 Water. " What are the probabilities in regard to the quan- 

 tity of water to be met in the crushed zone near boring 40? Can 

 any limit be set which it would be extremely improbable that the 

 inflow would exceed, on account of the topography of the country 

 and the nature of the overlying materials?" 



There is likely to be much water. Nearly all of the overlying 



