336 Mr. Sullivan on some Prismatic Forms of 
perfectly hyaline. Except for the difference of form, a mass of 
these crystals, resting on crystalline quartz, resembles, in a strik- 
ing manner, a mass of large crystals of chlorate of potash. Layers 
of growth in the direction of the secondary axes can be observed 
in some of the prisms; in many of these the outer shell, about 
one millimetre thick, is frequently free from indications of 
cleavage, and perfectly transparent. The prismatic faces are dull, 
exactly like the appearance of white wax when sufficiently thin 
to be translucent; they are also uneven. These faces exactly 
resemble those of the prism ooP (d'), in specimens which I have © 
seen from Andreasberg. On this account, I concluded, at first 
sight, that I had the combination oP, oP, which would be not 
merely rare as an example of the pyramidal prism, but still more 
so as a tabular form of it, in which the base would impress its 
character upon the crystal, and of which I have not seen 
any example recorded. I found, however, that the alternate 
edges were modified by rudimentary facets of a rhombohedron, 
which was placed in the same position, as regards the faces of 
the prism, that I have before mentioned as characteristic of oR, 
The basal edges not modified were easily removed by cleavage. 
I found the modifying facets to be those of the rhombohedron 
4 Ror B. 
Asséciated with the crystals just described, were sometimes 
found white opake crystals, like those from Andreasberg, and 
others three or four millimetres thick, upon which were rudi- 
mentary facets of a scalenohedron. I have not been able to get 
any good specimens of these varieties. 
It may be worth while to enumerate, from Zippe’s excellent 
memoir, the tabular prismatic forms which have been hitherto 
observed, with a view of determining the exact position of 
the example just described in the series. They are as follow :— 
1. oR, 2P, oR, oP (a!, e°, e?, d') figured by Levy*. 
2. oR, $R/, oR (a, 61, ce?) white tabular crystals from Wear- 
dale in Durham. 
3. oR, oR, oP (a), e?, d') from Andreasberg. 
4. oR, 2R/, oR, oP (a', e!, e?, d') from Andreasberg. 
5. oR, 1R’, 3 R, oP from Andreasberg. 
6. oR, ©R from Andreasberg, Marienberg, Schneeberg, 
Joachimsthal, and Schemnitz. 
The last-mentioned form from Luganure, which is oR, oR, 
tR! (a, ec, b'), approaches nearest to No. 6, from which it differs, 
so far as can be expressed by a formula, only by the rudimentary 
rhombohedral facets. Ifthe faces }R! became so developed as to 
* Description d’une collection de minéraux formée par H. Heuland, &c. 
Londres, 1837, fig. 87. 
