THE PROPERTIES OF MINERALS. 673 



Pyramidal-hemihedral type. — The tetragonal bipyrainid. Form liaviug the tetrago- 

 nal (\J) axis only and the C plane of symmetry. 



JVapczohidral-liemihedntl type. — The tetragonal tra])ezohedrou. Form having the 

 tetragonal {1.) and four digonal (O) axes with no ]>lano of symmetry. 



Sphenoldal-hiviihednd type. — The tetragonal scalenohedrou. Form having three 

 digonal ( ) axes and two S planes of symmetry. 



Ihmimorphic-tetariohedral type — The tetragonal pyramid. l''orm iia\ ing bnt one 

 axis, the tetragcnial (D), and no plane of symmetry. 



."^Iihenoiddl-fctdrioliedral type. — The tetragonal bisphenoid. Form having but one 

 axis, a digonal (C), and no plane of symmetry. 



Orthorhomhic system. — Types : Holohedral, hemiliedral, and lieinimor- 

 phic. The most general form possesses tliree planes of symmetry, of 

 which two (S planes) intersect each other at right angles, and a third 

 (C) i^ normal to these. 



There are three axes of symmetry, all digonal (O), which are per- 

 pendicular to the planes of symmetry. 



Examples of the types are described as follows: 



Holohedral type. — The orthorhomhic bipyramid. Form having three digonal (O) 

 axes with one C and two S planes of synmietry. 



Hemihcdral type. — The orthorhomhic bisphenoid. Form having three digonal (O) 

 axes and no plane of symmetry. 



Uemhiiorpliic type. — The orthorhomhic pyramid. Form having one digonal (O) 

 axis and no plane of symmetry. 



Mo)ioclinic system. — Types: Holohedral, hemimorphic, and hemihe- 

 dral. The most general form possesses one plane (S) of symmetry and 

 one digonal (q) axis, which is perpendicular to the plane of symmetry. 



Examples of the types are described as follows: 



Holohedral type. — The monocliuic pyramid. Form having one digonal (O) axis 

 and one S plane of symmetry. 



Hemimorphic type. — The niouocliuic sphenoid. Form having the digonal ('^) axis 

 bnt no plane of symmetry. 



Hemiliedral type. — The monoelinic dome. Form having an S plane of symmetry, 

 but no axis of symmetry. 



Triclinic system. Types: Holohedral and hemihedral. The most 

 general form possesses centrosymmetry only, and has no plane or axis 

 of symmetry. 



Examples of the types are described as follows: 



Holohedral type. — The triclinic pyramid. Form centrosymmetrical, but without 

 either jdaue or axis of symmetry. 



Hemihedral type. — The pedion. Tlie form is completely unsymmetrical. 



COMrOUND CKYSTALS. 



Compound crystals are divided into two classes, according as the 

 several individuals are in reversed or parallel positions with reference 

 to each other; that is, into twin crystals and parallel growths. 



Twin crystals are those in which one or more i^arts, regularly ar- 

 ranged, are in a reversed relation to the other part or ])arts. A twin 

 crystal may be conceived of as two individuals, or parts of the same 

 individual placed in a parallel position, and then a half revolution to 

 NAT MUS 1)7 43 



