ON THE CKYSTALLTSATIONS 
the smaller or triangular faces, A, I, H, &c. parts of this 
form are detached, until the remainder is contained under 
faces equal and similar to each other, the form will assume 
the appearance of an isosceles four-sided pyramid, as repre- 
sented Fig. 2. The faces of cleavage are situated like those 
in the figure marked FEG, F'EG. In the hypothesis of 
the regular octahedron, four, or at least two, such faces 
should appear at each of the six solid angles ; the result of 
which, in the first case, would be a hexdhedral trigonal-- 
icositetrahedron, in the second its half, the pentagonal- 
dodecahedron. This is, however, not confirmed by an 
accurate examination, since we find only four directions in 
which cleavage takes place, and this produces an isosceles 
Jbur-sided pyramid^ Fig. 3, the terminal edges AD, AD' 
of which are parallel to the lines AD, AD' in the figure 2^ 
drawn perpendicularly from the apex A to the lateral edges 
of the given form. Supposing the axes of the two forms, 
of that of crystallisation, and that of cleavage equal, the 
horizontal projection or basis of the first CCCC (Fig. 4.) 
will be a square circumscribed to the horizontal projection 
DD'D^D"' of the second, and the sides of the bases CC : 
DD' will be in the ratio of : 1. If, on the contrary, 
the bases are supposed equal, the axes of the same two 
forms will be in the ratio of \- ^l. Let the more obtuse 
pyramid be designated by P, the more acute one will be 
represented by P+l.* 
These forms belong to the Pyramidal System of Mohs, 
and the crystallisation of Gopper-pyrites can therefore, by 
no methodical means of proceeding, be derived from the 
* For the derivation of these pyramids, their belonging to a series of 
pyramids, and for the designation of its members, compare the Supplement 
to Encydopcedia Britannica^ article Mineralogy. 
