Troost on the Pyroxene. 
57 
30. Triuniternary P. The Sexoctonal, augmented by 
two faces at the summit. This variety was also discovered 
at Franklin, near Sparta, N. J. and belongs to that variety 
of Pyroxene which was described by W Keating as a new 
mineral, under the name of Jeffersonite. (See Jour. Acad. 
Nat. Sc. Phil. vol. ii. p. 194.) 
31. Epointed P. A four sided prism, the primitive faces 
of the prism having disappeared, terminated by a four sided 
pyramid having the apex truncated — from Compton Hill^ 
N. J. 
Such are the crystaline forms of the Pyroxene which I 
have though proper to exhibit; a great number of them 
forming varieties known by the name of Hemitrope, made 
by the revolution of one half of the crystal upon the other, 
which we suppose to have remained at rest. We know 
such hemitropes formed by crystals which have not yet 
been found as simple crystals. In general, the hemitropes 
are not rare among the Pyroxene, though as yet a small 
number has been described. The most known are, 
Triunitary hemitropal P. * An eight sided prism, ter- 
minated at one of the extremities by a summit with four 
faces, and at the other extremity by a summit also with 
four faces, but forming a re-entering angle, and differing 
from the former summit, which has a projecting angle — as 
the Subtractive hemitropal P. 
Hemitrope Cross P. Formed by a junction of crystals 
in such manner as to form a right angle with each other. 
The Pyroxenes do not offer always distinct determina- 
* Traite, vol. iii. p. 86, fig. 144, 
