QUARRY MATERIALS OF NEW YORK 1 29 



feet east of the southern end of the reservoir, in a mixture of 

 igneous and Fordham gneisses ; and the City quarry, on the eastern 

 margin of the reservoir, also in a mixed phase. 



The quarries from which the supply of stone for the dam is 

 being obtained are apparently a new location, considerably south 

 of the others in the Yonkers area. They are based on an exposure 

 of several acres, thinly covered with soil which, when removed, 

 shows glaciated but practically fresh rock at the surface. The first 

 few inches from the surfaces show a slight brownish stain, but no 

 marked decomposition. There are scattered inclusions of micaceous 

 and hornblendic gneisses, the former perhaps derived from the 

 Fordham. For the most part, however, the area consists of Yonkers 

 in quite uniform development, well suited for architectural or 

 general construction purposes. There is some variation of texture 

 which ranges from massive and medium or coarse-grained to finely 

 granular foliated gneiss. The massive type appears in limited 

 quantity. The foliation is in part a result of flowage when the 

 mass was still in a viscous condition. Pegmatitic and aplitic phases 

 of the rock are not infrequent, the two occurring in irregular 

 patches rather than dikes. The pegmatite is distinguished by large 

 red, perthitic feldspars and smoky quartz with more or less graphic 

 intergrowth of the minerals. 



The jointing is widely spaced, as a rule, and no difficulty is found 

 in obtaining blocks of any required size. The stone is quarried by 

 drilling and blasting. The rough blocks are used for cyclopean 

 masonry or are dressed to dimensions, while the finer material goes 

 to the crushing plant which has been erected near the quarries. In 

 the spring of 1913, work was in progress at two places. 



The average product of the quarries may be described as a 

 grayish or brownish gray gneiss of medium to fine texture. The 

 feldspars range from .5 mm to .3 mm in diameter. The composi- 

 tion is that of a normal biotite granite, with microcline as the chief 

 alkali feldspar. The feldspar and quartz are in nearly equidimen- 

 sional grains, closely crowded, but not interpenetrating, as in some 

 of the stronger granites. The even granular type seems to break 

 down more readily under the weather than the irregular grained 

 Yonkers, but at this place there is little evidence of physical dis- 

 integration. 



Physical tests. The Yonkers gneiss from the Dinnan quarry, of 

 probably similar character to the stone in the new quarries, was 

 tested by J. L. Davis, of the New York City Board of Water Sup- 

 ply. Two samples showed: specific gravity 2.64; ratio of absorp- 

 tion .30 per cent and .39 per cent; porosity .87 per cent and 1.01 



