68 New York State Museum 



The more important minerals composing igneous 

 rocks are quartz, the feldspars, the micas and the fer- 

 romagnesian group. Quartz or silica is glassy in ap- 

 pearance and rarely in crystals and can be recognized 

 by its hardness. It will scratch glass but can not be 

 scratched by a knife. The feldspars are silicates of 

 aluminum and an alkali metal (as sodium and potas- 

 sium) or an alkaline earth (as calcium and magne- 

 sium) or both. The feldspars may be roughly divided 

 into orthoclase or potash feldspar and plagioclase 

 feldspars or those with soda or lime or both. The 

 feldspars with potash only are the most acid ; with 

 lime only, the most basic. Potash and potash-soda 

 feldspars are characteristic of acid rocks, or rocks high 

 in silica ; and in color are very apt to show some tinge 

 of red, varying from pale flesh color to strong brick- 

 red or brownish red, a distinct flesh color being most 

 common. The lime and soda-lime feldspars are found 

 in basic rocks and are commonly of a gray color — 

 dark smoky or bluish gray or even black. Feldspar 

 will scratch glass, can not be scratched by a knife and 

 can be scratched by quartz. The micas for general 

 purposes may be divided into light-colored or Musco- 

 vite micas and dark-colored or biotite micas. Musco- 

 vite mica, the common, light mica, is a silicate of po- 

 tassium and aluminium with some hydrogen and is 

 found in acidic rocks where other potash minerals are 

 found, as granites, and in metamorphic schists. Bio- 

 tite or black mica has in addition magnesium and iron 

 with a reduction of silica and aluminum, which makes 

 it a more basic mineral. In the mica-ferromagnesium 

 series, muscovite is placed at the acid end and magnetite 

 (pure iron oxide) at the basic end, and between in 



