RHOMBOHEDRAL SECTION OF HEXAGONAL SYSTEM 



49 



sided pyramid. Sometimes there are traces of 

 to the faces of a six-sided pyramid. 

 5. Irregularities of Crystals — 

 Distortions sometimes disguise 

 greatly the real forms of hexagonal 

 crystals by enlarging some planes 

 at the expense of others. This is 

 illustrated in iig. 14, representing 

 the actual form presented by a 

 crystal having the planes shown 

 in fig. 13. Whenever in a prism 

 the prismatic angle is exactly 120° 

 or 150°, the form is almost always 

 of the hexagonal system. 



cleavage parallel 



14. 



2. EHOMBOHEDRAL SECTION. 



1. Descriptions of Forms. — The following figures, 1 tu 17, 

 represent rhorabohedral crystals, and all are of one mineral, cal- 

 cite. They show that the planes of either end of the crystal 

 are in threes, or multiples of threes, and that those above are 

 alternate in position with those below. There is one exception 



FIGURES OP CRYSTALS OF CALCITE. 



to this remark, that of the horizontal or basal plane 0, i*x figs. 

 8, 11, 13. The simple rhombohedral forms include: 



1. Hhombohedrons, figs. 1 to 6. These forms are included 

 under six equal planes, like the cube, but these planes are 

 4 



