1 82 Count de Bournon’s Description of 
Very frequently, the triedral prism passes to a tetraedral 
modification, by the simple replacing of one of its edges by a 
plane, equally inclined on the adjacent ones. This plane is 
either very narrow, (Fig. 11.) or of a more considerable width. 
(Fig. 12.) Sometimes the width of the plane is such, that it 
reduces the primitive adjacent planes to extreme narrowness. 
(Fig. 13.) In this last case, the crystal appears under the form 
of a rectangular plate or lamina, having two of its narrow 
opposite sides or planes inclined, in one and the same direction, 
on one of the two broad planes. It sometimes happens, in this 
variety, that the two broad opposite planes approach more or 
less to a square form. (Fig. 14. ) I have also seen some crys- 
tals, in which the two other edges of the prism seemed to have 
likewise very slight secondary planes ; but, when that hap- 
pens, they are always very narrow, especially when compared 
with the secondary plane of the third edge. This modification, 
in its various forms, is the most common one of this species. 
Sometimes one of the solid angles of the triedral prism is 
replaced, on one side only, by a plane that is much inclined to 
the edge of the prism on which it is situated ; but the crystals 
are always too small to admit of being measured with precision, 
(Fig. 15.) This plane, assuming a more considerable ex- 
tent, replaces the same edge of the prism by another plane, 
much broader at one extremity than the other, as is shewn by 
the lines of large dots in the same Figure. Sometimes it 
has a very considerable extent, as is represented in Fig. 16. 
It then reduces one of the bases of the triedral prism to the 
form of a very narrow trapezium, while that of the opposite base 
remains very broad. By a still more considerable increase, the 
plane of this base totally disappears ; and the crystal is termi- 
