triple Sulphuret, of Lead, Antimony, and Copper, &c. 35 
combined merely with those of the primitive crystal ; but it is 
yet more common than in the preceding modification, to meet 
with them combined with the planes of the secondary prism ; 
and they are then situated along the edges of the terminal faces, 
as is shown in Fig. 7. More frequently, however, the prism is 
very short, as is represented in Fig. 8 ; and it usually preserves 
some traces, more or less considerable, of the sides of the pri- 
mitive crystal, as in Fig. 9. 
It sometimes happens, that the decrease producing this modi- 
fication has taken place only in two of the opposite angles, 
situated in the same direction, on the terminal faces ; then, if 
the planes arising from this modification happen to be combined 
with those of the secondary prism, they are situated only on 
two opposite edges of each of the terminal faces of this prism. 
See Fig. 10. I have observed this variety in pretty large crystals; 
and the specimen in which I met with them (and which, like 
all the others, came from Cornwall) contains also some very 
regular aggregations, each consisting of four of these crystals, 
in the form of a cross. These crystals penetrate each other for 
a certain part of their extent, as is shown in Fig. 1 1. 
If the planes produced by this third modification were com- 
bined with those arising from the second modification, the result 
would be, the variety represented in Fig. 12, in which the 
planes of the two last modifications meet together at an angle 
of 170°; but I have not yet met with this variety in so simple a 
state. 
Fourth Modification. This is produced by a decrease along the 
edges of the terminal faces of the primitive crystal ; in conse- 
quence of which, each of those edges is replaced by a plane, 
which is equally inclined upon the adjacent faces, and makes, 
F 2 
