334 . Mr. Sullivan on some Prismatic Forms of 
to be the limiting form of the rbombohedron, and is represented 
by Mohs by the symbol R+, and by Haidinger by oR. 
Zippe also adopts the latter. 
According to Haiiy d! or oP is rare, and Dufrénoy states 
that only some examples are known. According to Zippe, it is 
frequent enough in combination as a secondary form, butseldomer 
as the dominant form. Surmounted by the primitive rhombo- 
hedron (R or P), it is noticed by Dufrénoy as “a very rare ex- 
ample of the prism on the edges, associated with the primitive 
rhombohedron*” from Cumberland. He also mentions another 
in which 4! or 4R! (the équiawe of Haiiy) replaces P or R, but 
does not give the locality. Further on he notices a third ex- 
ample from the Samson mine in the Hartz, in which the hori- 
zontal edges of the prism are truncated by rudimentary planes 
of the pyramid. 
The prism oR or e?, although comparatively rare as a simple 
form, is very frequent in combination ; according to Dufrénoy, 
indeed, it is the only one found complete. A little before, he 
says that it is of a milky whiteness, and almost always opake. 
The base sometimes bears striz parallel to the edges, which are 
indications of cleavage. Examples of ©R surmounted by }R! 
or 6' from the Hartz, Cumberland, and the department of I’ Isére, 
have been described. 
The position of the rhombohedrons surmounting the prisms is 
different in each kind. In oP the surmounting rhombohedral 
faces lie so that the edges of combination with the prismatic 
faces coincide with the lateral edges of the rhombohedron. In 
oR the edges of combination in three alternate faces are hori- 
zontal ; the truncatures at either end of the prism alternating, 
so that each face of truncature is parallel to one at the opposite 
end. The directions of the cleavages correspond perfectly with 
the dispositions of the modifying planes, so that every alternate 
basal edge of the prism oR or e*? may be removed by cleavage 
with the greatest facility, by which a prism surmounted by the 
faces of the rhombohedron may be obtained. 
Although the prismatic faces oR are sometimes dull, they 
always, at least in all the crystals which I recollect to have seen, 
possess more lustre than the faces oP associated with them. 
The former are, indeed, usually very bright in transparent 
crystals. This circumstance is noticed by Dufrénoy, who, in 
speaking of the example of oP or (d') with pyramidal trun- 
eatures of the lateral edges, from Samson mine in the Hartz, 
von F, X. M. Zippe.—Denkschriften der Kaiserl. Akademie der Wissen- 
schaften. Mathematisch-naturwissenschaftliche Classe, Bd. iii. 1st Lief. 
p- 109. 
* Traité de Minéralogie, par A, Dufrénoy. 2 ed. tomeii. p. 297. 
