10 



MINERALS AND GEOLOGY 



Crystal-axes of three lengths 

 Refraction, double, with 

 two neutral lines or optical 



rThe, Rhombic or Ortho-Rhombic 



Axes at right- 1 Sy * tem ("w^ng ng ht rect .^ n ' 

 J gular pnsms and pyramids, 



rhombic prisms and pyramids, 



One axis ob- 

 lique. 



All the axes 

 oblique. 



V. and combinations of these). 



The Clino-Rhombic or Monoclinic 

 System (including oblique rec- 

 tangular and rhombic com- 

 binations). 



The Triclinic or Anorthic System 

 (including doubly -oblique com- 

 binations). 



The study of these Crystal groups, and that of crystal forms and 

 combinations generally, constitutes the science of Crystallography. 

 To enter into the details of this science would extend our present 

 discussion much beyond its proposed limits and object, the simple 

 determination of commonly occurring minerals ; but it will be advis- 

 able for the student to impress upon his memory the names of the 

 groups in question, with the general aspect of their more common 

 forms and combinations, as given in the following enumeration. 



The Isopolar or Regidar System. This group includes the cube 

 (Fig. 1), the rhombic dodecahedron (Fig. 2), the regular octahedron 

 (Fig. 3), trapezohedrons (Fig. 4), pentagonal dodecahedrons (Fig. 5), 

 &c. Figs. 6 and 6* are combinations of the cube and octahedron ; 





IG. 1. 



FIG. 2. 



FIG. 



FIG. 4. 



FIG. 5. FIG. 6. FIG. 6*. FIG. 7. 



No. 7, a combination of the cube and pentagonal dodecahedron. 

 Native gold, silver, copper, iron pyrites, galena, zinc blende, grey 

 copper ore, red copper ore, magnetic iron ore, spinel, garnet, fluor 

 spar, rock salt, and numerous other minerals, crystallize in this 

 system. 



