QUARRY MATERIALS OF NEW YORK 59 



or less quantity, especially plagioclase, which may share importance 

 with the potash feldspar, and by mica, hornblende or, rarely, py- 

 roxene. The potash feldspar is either microcline or orthoclase, the 

 former being the more common. Mica occurs in two forms — 

 the white or transparent muscovite and the black biotite ; usually 

 both are present, but if one alone occurs, it is more often biotite. 

 Hornblende is a rather common ingredient of local granites in 

 which it replaces the mica wholly or in part. Pyroxene, which 

 resembles hornblende in appearance when seen, in the hand specimen, 

 is restricted to a few types which are related to the syenites. 



Besides the more important or essential ingredients named, 

 granites usually contain a number of others in very small amount 

 which may be called accessory constituents. Such are apatite, zir- 

 con, rutile, magnetite, pyrite, fluorite, tourmaline and garnet. There 

 may be also various secondary minerals which have been derived 

 by chemical alteration from some of the original constituents ; thus 

 sericite, kaolin and calcite result from the alteration of feldspar, 

 and chlorite, serpentine, epidote and iron oxides result from the 

 dark iron-magnesia minerals. 



The chemical composition of various local granites will be found 

 under the quarry localities elsewhere in this volume. 



The texture of granite is usually even-grained, with the feld- 

 spar and quartz in particles of nearly the same magnitude. There 

 is no regularity, however, as to the size of the particles in granites 

 from different localities, and there is likely to be more or less 

 variation in that respect in different parts of the same mass. A 

 granite may be said to have a coarse texture if the crystals of 

 quartz or feldspar average over 10 mm or 0.4 inch in diameter; 

 medium if the crystals range between 10 mm and 5 mm; and 

 fine if they are less than 5 mm. In the very fine sorts, the crystals 

 average under 1 mm. The same rule for classifying textures will 

 be applied to the other quarry stones. 



The specific gravity of granite varies from about 2.5 to 2.75. 

 This corresponds to a weight, without allowance for porosity, of 

 from 156 to 172 pounds to the cubic foot. The average weight is 

 about 165 pounds, and a cubic yard in the quarry may be taken 

 roundly as equal to 4500 pounds. 



Granites are white, gray or pink in color, with occasional examples 

 showing a bright or deep red. The feldspar is the main coloring 

 agent, as it predominates over the other ingredients, but the gen- 

 eral color effect is really a combination of the individual colors 

 of the minerals. Muscovite and quartz are colorless or translucent 



