OF COPPER-PYPvItES. 
we have just described, and which takes place perpendicu- 
larly to one only of the terminal edges of P, is rare* 
and the very few instances I know of, are to be found 
among the varieties from Nassau-Siegen. But it is also 
met with, at the same time, upon all the terminal edges 
of P, and then it is one of the most common regular 
aggregations in the present species. The faces a a' a"^ 
b y h\ fig. 37, of the different crystals, fall into one 
and the same plane; which, of course, cannot be the 
ease with the faces c d (f\ d df d", on account of the 
difference of the pyramid P from the regular octahedron. 
Crystals thus produced have been taken for simple, and 
described accordingly as octahedrons truncated on ail tlieir 
edges and angles. The appearance, indeed, is very sedu-, 
cing, if we do not attend to the stria?,_which, in the different 
individuals, follow different directions, parallel to lines 
drawn from the point E to the respective apices of the 
pyramid P in the three individuals, whose faces meet in this 
point. The remaining parts of the faces of P-[-l make 
sometimes re-entering angles, as in fig. 38, which adds to 
the evidence of such forms being compound. 
The tetrahedral crystals from Cornwall are subject to the' 
same law, only the individuals appear more decidedly hemi- 
pyramidal, so as to present only half the number of the 
faces of P. Generally they assume the form fig, 39, where 
P 
the faces of ^ are deeply streaked by the accession of the 
faces of P+l ; though the mathematical regularity would 
require it to be so as represented fig. 40, where the lines 
AE, A'E ; AC, AC, &c. denote the junction of the differ- 
ent individuals. The striae of the crystals are so deep and 
so numerous, that, instead of the faces AEA', A'EA", 
A"EA falling into one plane, the lines of junction AE, 
A'E, A"E are marked by re-entering angles. 
VOL. IV. B 
