S36 M. Levy o/i the Determinat ion of Secondary Faces in 
with great accuracy the incidences of the planes of most of the 
modifications he has described, and in having given drawings of 
a great many varieties, which, though inexact, are perfectly suffi- 
cient to convey an idea of the form. 
Twelve different modifications are mentioned in this paper. 
!For nine of them, the angles of one plane of each upon two 
different faces are given. Eight of these may be determined 
without knowing any angles. But first it is necessary to cor- 
rect some errors in the drawings. 
From the angles given, it necessarily follows, that the planes 
of the seventh modification should not only be parallel to the in- 
tersection of 1 and 1, as it is Indicated in the figures, (see 
Plates 18. and 19. to the 2d volume of the Transactions of the 
Geol. Soc.), but also to the line of intersection of 2 and 5. 
2d^, That the planes of the sixth modification parallel to the 
lateral edges of the prism, should also be parallel to the inter- 
. section of two corresponding planes of the seventh modification, 
the one belonging to the superior, the other to the inferior sum- 
mit of the crystal ; or otherwise, that the line of intersection of 
the sixth and seventh modification should be perpendicular to 
the lateral edge of the prism. 
All the drawings in which these parallelisms do not exist, are 
consequently incorrect, or the faces marked with the same fi- 
gure do not belong to the same modification. These parallel- 
isms are the essential character of the sixth and seventh modifi- 
cation, in the same manner as it is the essential character of the 
4th, 5th, 6th, to be parallel to the axis of the prism, of the face js? 
to be parallel to the intersection of two adjacent planes of the 
second modification'; and unless it is argued that it is not neces- 
sary to indicate the latter parallelisms in the drawings, I see no 
reason why the former should not be preserved in the represen- 
tation of the crystal. 
Now, nothing is easier than the determination of the modifi- 
cations. For the sake ot simplicity, I shall here, as in the red 
oxide of copper, assume an hypothetical primitive form. It will 
be a square prism, whose lateral faces will correspond to the first 
modification, and the altitude of which will be determined by 
supposing the facek jp to be produced by a decrement by' one 
row on the angles. 
