122 NEW YOKK STATE MUSEUM 



Besides the above species which are monoclinic, there is an 

 important group of orthorhombic pyroxenes. 

 These are hypersthene, bronzite and enstatite. 

 Of the micas there are many species. The most important 

 rock-making mica is Uotite, a silicate of alumina, potash, iron and 

 magnesia. It is brownish black in color and is abundant in the 

 granites and gneisses. 



Muscovite, a silicate of alumina and potash, is less important 

 as a rock mineral but is valuable commercially for its thin trans- 

 parent plates used in stove doors, etc. 



The hydro-micas, margarodite and damourite, are similar to 

 the true micas in composition but contain water. 



Olivine, or chrysolite, is a silicate of iron and magnesia which 

 occurs usually in small crystals or grains in igneous rocks. It 

 is pale green in color. 



Olivine is of special importance because from it, by decomposi- 

 tion, is derived a large proportion of the serpentine rocks. 



Besides these few minerals which are essential components of 

 rocks and usually by their presence or absence determine the 

 rock species, there are others which are only accessory and while 

 of frequent occurrence do not so invariably affect the name of 

 the rock in which they occur. Such are garnet, zircon and 

 staurolite. 



In addition to the rock-making minerals are those which occur 

 in large masses in other rocks and have a commercial value. 

 Such are corundum, or emery, the ores of iron, e. g. magnetite, 

 hematite, spathic ore; coal, asphalt, halite or rock salt, gypsum, 

 the ore of lead and silver, galenite; the ore of copper and gold, 

 chalcopyrite and graphite or black lead. 



Of rarer occurrence and great commercial value are the gems 

 diamond, ruby, sapphire, emerald, etc. None of these are found 

 in New York. 



