198 NEW YORK STATE MUSEUM 



Petrographic character of the rocks 



This Hne of investigation is confined to four sets of samples. 

 No. I The granite of the New Ferris quarry 



2 The gneissoid granite of the same quarry 



3 The Yonkers gneiss of Dinnan quarry 



4 The dioritic gneiss of Garden quarry 



1 Granite. The rock is coarse grained and well interlocked. 

 The chief constituents are orthoclase, quartz and microcline. 

 There are but small amounts of dark minerals, and there is not 



much decay. 



Both surface material and the drill core were examined. The 

 deeper material showis a little calcite, that may be original, occur- 

 ring in irregular grains. They do not seem to indicate decay. 

 There is a little kaolin alteration of the feldspars, but not to a 

 serious degree. There are no injurious impurities in the rock such 

 as might cause rapid disintegration or discoloration. 



The rock is undoubtedly of good grade as to strength, composi- 

 tion and durability. 



2 Gneissoid granite (Ferris quarry). The rock is of medium 

 grain, containing quartz, the feldspars and a little mica. 



There is very little alteration, and no serious decay or injurious 

 constituents. A small amount of sericite and calcite present are 

 not considered of consequence, and as in the case of the granite, 

 the calcite is believed to be original. 



The grains are intimately interlocked and the rock is certainly 

 of good quality and very similar to the granite proper. 



3 Yonkers gneiss (Dinnan quarry). This rock is fine grained, 

 and is composed of quartz, mica and the feldspars among which 

 microcline is very abundant. 



The condition is good, — very little alteration, close structure, but 

 with a. httle more granular appearance than any of the other types. 



It is a good rock and gives good durability tests. 



On hadly weathered surfaces the Yonkers gneiss breaks up into 

 a granular product like sand long before it decays to earthy matter. 

 This seems to be caused by expansion and contraction of the dif- 

 ferent constituents under changing weather conditions inducing a 

 weakening of the sutures. Sometimes there is very little decay 

 even along these sutures, but as they open slightly they become the 

 channels for moisture and staining solutions. This makes the 

 boundaries of the grains very well marked in weathered specimens. 



