OF COPPER-PYRITES. 
15 
lisation, of any simple form belonging to the species ; aftei? 
having previously received such a position, that one of them 
is removed 180° from the parallel position with the otherj 
in a plane, which is always parallel to a face of crystallisation 
of that species to which the individuals belong. 
The Face of Composition, as well as the Plane of Revo^ 
lution, may assume a great many different positions, only 
they must always be similarly situated in respect to both 
the individuals that are joined together. 
The varieties of regular aggregation, or composition^^ 
hitherto observed in Copper-pyrites are the following : 
I. The plane of revolution parallel to a face of P. 
1. The face of composition parallel to the same face 
of P. Porm of crystallisation : P, fig. 28. 
% The face of composition perpendicular to a ter- 
minal edge of P-j-1. Supposed form of crystal- 
lisation : P+1, fig. £9. 
II. The plane of revolution parallel to a face of P—- 1. 
1. The face of composition perpendicular to a ter- 
minal edge of P. Supposed form of crystallisa- 
tion: P, fig. 30. (PI. III.) 
III. The plane of revolution parallel to a face of 
[p+»]. 
1. The face of composition parallel to the same 
face. The result of this law in the form P^ 
whose alternating faces are enlarged hemi- 
pyramidally, would be fig. 81. This, however, 
does not occur ; the component individuals do 
not end in their face of contact, but parts of 
them appear also on the other side of this face, 
