GEOLOGY OF THE NEW YORK CITY AQUEDUCT 1 97 



Ferris quarries. The " Old Ferris quarry — is '' Yonkers 

 gneiss " considerably more weathered than the Dinnan. It is con- 

 sidered less promising than the " Xew Ferries " quarry which has 

 been explored by the engineers of the Kensico division. The rock 

 of this quarry site is not all of one quality. There are essentially 

 three varietal facies of the Yonkers gneiss type and relationship. 

 One (a) is essentially a granite. It has a coarse grain and shows 

 almost no f ohate structure. It has a decidedly massive appearance ; 

 but it is not of very great extent. This rock is evidently very 

 closely related to the true Yonkers gneiss into which it passes on 

 all sides through an intennediate variety. 



This intermediate variety (b) has medium size of grain, is only 

 slightly foliated and passes without sharp limitations on the one 

 side into the granite facies and on the other to true normal Yonkers 

 gneiss. It is not so strikmgly massive as the granite, but is more 

 so than the gneiss proper. This rock may be called a gneissoid 

 granite to distinguish it from the other. 



The true Yonkers (c) gneiss surrounds these two special varie- 

 ties. It is of finer grain than either of the others and is more 

 strongly fohate and is strictly a granite gneiss. Varieties (a) and 

 (b) occur as sort of a lens within the Yonkers gneiss. 



The extent of the granite as now uncovered at the site is be- 

 lieved to represent its limits. The prospect of enlarging the area 

 will not meet with much success. It is essentially a local develop- 

 ment connected with the differentiation of the parent magma from 

 which all three varieties were derived. It seems to have been the 

 last of the three to solidify, and it has some of the characteristics 

 of certain pegmatite lenses. . 



Although this is certainly an attractive rock and one against 

 whicli there is little ground for objection, it is reasonably certain 

 that a sufficient quantity of this variety can not be obtained here 

 for the whole proposed use. And the prospects are not good for 

 locating another quarry of the same quality. 



The gneissoid granite (b) is of greater extent, in fact it will be 

 found to encroach on the present area of the granite. It is as good 

 rock and almost as attractive as the granite. 



The regular type of Yonkers gneiss such as that represented in 

 the Dinnan quarry can be obtained in almost unlimited quantity, 

 and, with the splendid showing that it makes in further examina- 

 tion, it has come to be considered the best suited to the purposes 

 of dam construction at Kensico. 



