SYSTEMS OF CRYSTALLIZATION. 29 
Occasionally, as in the diamond, the planes of crystals are 
convex; and then, of course, the angles will differ from the 
true angle. It is important, in order to meet the difficulties in 
the way of recognizing isometric crystals, to have clearly in the 
mind the precise aspect of an equilateral triangle, which is the 
shape of a face of an octahedron; the form of the rhombic face 
of the dodecahedron ; and the form of the trapezoidal face of a 
trapezohedron.. With these distinctly remembered, isometric 
crystalline forms that are much obscured by distortion, or which 
show only two or three planes of the whole number, will often 
be easily recognized. _ 
Crystals in this system, as well as in the others, often have 
their faces striated, or else rough with points. This is gener- 
ally owing to a tendency in the forming crystal to make two 
different planes at the same time, or rather an 
oscillation between the condition necessary for 
making one plane and that for making another. 
Fig. 63 represents a cube of pyrite with stri- 
ated faces. As the faces of a cube are equal, 
the striations are alike on all. It will be noted 
that the striations of adjoining faces are at right 
angles to one another. ‘The little ridges of the 
striated surfaces are made up of planes of the pentagonal dode- 
eahedron (fig. 49, p. 26), and they arise from an oscillation in 
the crystallizing conditions between that which, if acting alone, 
would make a cube, and that which would make this hemihe- 
dral dodecahedron. Again, in magnetite, oscillations between the 
octahedron and dodecahedron produce the striations in fig. 64. 

64. «65. 

— 

mR 
Le 
AA 
Af 












MAGNETITE. COMMON SALT. 
Octahedral crystals of fluorite often occur with the faces 
made up of evenly projecting solid angles of a cube, giving 
