THE MICROSCOPE IN MINERALOGY AND GEOLOGY. 89 



An apparent distortion may also be produced by an oblique 

 section. 



The following examples may be of service, as showing 

 the value of angular measurement in minerals: 



Quartz. Rhombohedral system. Inclination of two 

 adjoining faces 94 15'. 



Felspar. Monoclinic. Cleavage planes at right angles. 



Albite or soda felspar. Triclinic. Angle 93 36'. 



Mica. Oblique prisms. 



Magnesian mica. Right, rhombic, or hexagonal prisms. 



Garnet. Dodecahedrons or trapezohedrons. 



Idocrase. Square prisms. 



Epidote. Oblique prisms. 



Scapolite. Square and octagonal prisms. 



Andalusite. Prisms of 90 44'. 



Staurotide. Rhombic prisms of 129 20'. 



Tourmaline. Three, six, nine, or twelve-sided prismsl ' 



Topaz. Rhombic prisms of 124 19'. 



Beryl. Six-sided prisms. 



Hornblende. Monoclinic. 124 30'. 



Augite or pyroxene. Monoclinic. 87 5'. 



Calcite or carbonate of lime. Forms various, but 105 

 5' between the cleavage faces. 



Magnesite. Angle 107 29'. 



Dolomite. 106 15'. 



Gypsum. Monoclinic. 



Crystals within Crystals. Many specimens which appear 

 perfectly homogeneous to the naked eye are shown by the 

 microscope to be very complex. The minerals of erupted 

 lavas are often full of minute crystals, leading to very 

 anomalous results of chemical analysis. Some care is 

 needed at times to distinguish such included minerals 

 from cavities filled with fluid. The use of polarized light 

 will sometimes determine this point. 



Cavities in Crystals. Mr. Sorby has shown that the 

 various cavities in minerals containing air, water, glass, 



