1844 Count de Bournon’s Description of 
secondary plane, represented by the dotted lines in Fig. 15, 
would give, if it existed at the same time in the three angles. 
By a longer duration of the act of crystallization, under 
the same modification, the plane corresponding to the trun- 
cated apex of the pyramid (Fig. 18.) becomes progressively 
smaller; the most acute isosceles triangular planes, which 
answer to the secondary ones, encroach on the least acute, all 
which are the primitive planes of the crystal, and the pyramid 
becomes truly triedral at its upper extremity, whilst it remains 
hexaedral at the base, on account of those parts of the three 
planes of the primitive crystal which are still preserved. 
(Fig. 19.) 
By a still more considerable duration of the act of crystal- 
lization, the pyramid would become completely triedral, and 
would not be truncated at its apex I have never met with this 
modification so complete ; but I have seen the variety repre- 
sented in Fig. 1 q, which however, as well as Fig. 18, is very 
uncommon. 
The triedral prism is subject to a fourth modification, which 
takes place at the three edges of one of its two bases or ter- 
minal surfaces only, and replaces each of those edges by a 
plane, much more inclined on the side of the prism on which it 
is placed, than on the terminal surface. (Fig. 20.) I have not 
been able to determine, in these crystals, the angles formed by 
these new planes, either with the sides of the prism, or with the 
terminal surfaces ; but the varieties belonging to this modifica- 
tion, demonstrate that these angles are the same as those which 
the secondary planes of the solid angles make, either with the 
terminal surfaces, or with the edges of the prism on which 
