GEOLOGY OF THE NEW YORK CITY AQUEDUCT I99 



Such incipient disintegration is common in the more even grained or 

 granular varieties. 



The accompanying photomicrograph [pi. 28] is taken in plain 

 light and shows the outlines of the grains due to this cause. 



4 Dioritic gneiss (Garden quarry). Rock is of medium grain 

 and with a strong tendency to schistose or foliate structure. The 

 dark grains are hornblende and biotite, the light grains are feldspars 

 and quartz. 



The rock is fresh, durable and has 1 o injurious constituents. It 

 is good enough for the use in all respects, but has a dark color and 

 is more strongly foliated than any of the others considered. 



It is evident from these observations that the rocks considered 

 are all of suitable mineralogic character for the purposes of large 

 dam construction. For very large quantities of material, however, 

 it is probable that neither the coarse granite nor the gneissoid 

 granite could be depended upon for uniform supply. The true 

 regular Yonkers gneiss, however, is very abundant, and can be relied 

 upon for indefinite amounts. The dioritic gneiss is also abundant. 

 The immediate region is not capable of furnishing any better rock 

 than those described above. 



Additional tests 



Some instructive tests were made by the Board of Water Supply 

 under the direction of Mr J. L. Davis who has charge of the 

 testing laboratories. A few of these applying to the rocks at 

 Kensico are tabulated below. 



The tests cover : specific gravity, weight per cubic foot, porosity 

 in per cent, ratio' of absorption, per cent water absorbed, ratio of 

 drying 24 and 48 hours, retained water pounds per cubic foot 24 

 and 48 hours. 



In the accompanying tabulation the terms used are subject to the 

 following limitations as to definition: 



1 Ratio of absorption, sometimes called porosity, " is the ratio of 

 the weight of water absorbed to the dry weight of the stone." 



2 Porosity gives " the actual percentage of the stone which is 

 pore space." " The difference between the dry and saturated 

 weights of the sample is multipHed by the specific gravity of the 

 rock and the product added to the dry weight. This gives the 

 weight the specimen would have provided it contained no pore 

 spaces. The difference between the dry and saturated weights 



