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 



