3O NEW YORK STATE MUSEUM 



apatites and some recrystallized quartz. A few ore grains are 

 scattered through the mass. This granite also shows evidences 

 of crushing. 



3 Granite 

 Peekskill, Westchester co. N. Y. 



COLEMAN, BRUCHARD & COLEMAN 

 See plate 1 1 



The stone from this locality has been quarried for use in the 

 construction of the Croton dam. It is a medium to coarse grained, 

 very light stone made up of white feldspar, smoky quartz and 

 muscovite with small amounts of biotite. 



Under the microscope, quartz and feldspar are the more promi- 

 nent minerals, the feldspar being mostly a very acid plagioclase 

 idiomorphic with respect to the orthoclase, of which there is com- 

 paratively little. Some microline and micropegmatite are also 

 present. The feldspars show alteration, mostly to muscovite. 

 They are clouded, usually in the center, although, in some cases, 

 the alteration has been in zones around the outside of the crys- 

 tals, beyond which more feldspar has been deposited. Of the 

 alteration products muscovite alone is recognizable, though kao- 

 linite may also be present. Both muscovite and biotite were 

 seen, the former being the more abundant and the latter showing 

 alteration to epidote in places. Chlorite is an accessory mineral, 

 and apatite crystals are not rare. 



4 Diabase 



Nyack, Rockland co. N. Y. 



MANHATTAN TRAP ROCK CO. 

 See plate 1 2 



This is a fine grained rock of a dark gray color used entirely 

 for road metal and concrete. It is so fine grained that the mineral 

 species can not be easily distinguished with the naked eye, but 

 bright cleavage faces suggest the presence of a plagioclase feldspar. 



Under the microscope the plagioclase was the only feldspar 

 recognized. It is very basic, in part probably bytownite and it 

 occurs in lath shaped crystals having an average length of .5 

 millimeter and an average width of .10 millimeter. Colorless to 

 green augite makes up the greater part of the remainder of the 

 section. This augite, has, in places, altered to hornblende. 

 Magentite and other metallic grains, probably ilmenite, are also 

 present. 



