120 NEW YORK STATE MUSEUM 



Minerals 



Minerals are classified by their chemical composition and by 

 the geometric forms which they assume in crystallization, each 

 mineral having a certain range of forms from which it cannot 

 depart.* 1 j | 



These forms are grouped in six systems named as follows: 

 Isometric, Tetragonal, Hexagonal, Orthorhombic, Monoelinic and 

 Triclinic. These systems are characterized by and named in ac- 

 cordance with the number and relation of the axes about which 

 the external geometric faces are developed. In physical relation 

 with these axes are distinct optical properties which can be 

 determined by cutting the minerals in very thin slices and exam- 

 ining these by means of optical instruments. While there are 

 over 700 recognized mineral species, only a small number are 

 important to the geologist as rock making minerals. Of these a 

 few are sometimes found to be the single components of entire 

 rock masses. 



Quartz, the crystalline form of silica, is frequently found in 

 large masses in mineral veins and, in its fragmental form, con- 

 stitutes beds of gravel and sand when loose and, when solidified 

 by cementation, forms conglomerates, sandstones and quartzites. 



Calcite and aragonite are two crystalline forms of carbonate 

 of lime, the former of which is the chief constituent of many 

 great beds of limestone; the latter is usually deposited by water 

 in forms called stalactites, calcareous tufa, travertine, etc. 



Dolomite, the double carbonate of lime and magnesia wholly 

 or in part forms extensive strata of magnesian limestone. 



Kaolinite, the hydrous silicate of alumina, is also a very prom- 

 inent mineral in rock masses. In its pure condition it forms 

 beds of potter's clay, and mingled with various kinds of rock- 

 dust it constitutes extensive strata of clay and shale. 



Of the minerals which mingle in the formation of rocks, the 

 most important are quartz, the feldspars and the magnesia-iron 

 silicates. ', 



a For an elementary discussion of crystallography as well as of mineralogy the 

 reader is referred to Dana's Manual of Lithology and Mineralogy. 



