GEOLOGY OF THE NEW YORK CITY AQUEDUCT I23 



veloping an interlocking tendency [pi. 22, lower figure]. At the 

 same time the fibrous sericitic and chloritic aggregates have developed 

 to such extent as to fill most of the remaining pores, and in many 

 cases the fibrous extensions have actually grown partly around the 

 adjacent quartz grains [pi. 23, lower figure]. The effect has been 

 to develop a silicious binding of unusual toughness. This combina- 

 tion of changes has made a rock that is now remarkably well bound 

 or interlocked for a sedimentary type. 



Durability. First-class stone of the grades indicated above 

 would have as great durability as any stone in the market, except 

 perhaps a true quartzite. With the exception of the almost neg- 

 lectable quantities of pyrite, occasionally found, there is no con- 

 stituent prominently susceptible to decay. The rock as a whole 

 mineralogically is stable and its texture indicates unusual resist- 

 ance to ordinary disintegrating agencies. 



General conclusions 



From the microscopic study it is clear that the variety of rock 

 most fully meeting the demands of heavy exposed construction are 

 the coarser beds and those freest from reed and shale. 



From the field study it is apparent that ledges of suitable char- 

 acter occur occasionally and that at least three such are not far 

 from the Olive Bridge site. 



From additional explorations it is certain that ledges of high 

 grade rock occur, and that the grade varies rapidly in the same 

 bed and that suitable material can be obtained in the immediate 

 vicinity of the Ashokan dam. Xo doubt rock of equally high 

 quality may be obtained at many other localities. 



