454. S&. L. Penfield—Tremannite and Metacinnabarite. 
It will be seen that onofrite stands, as it should, intermediate 
between the selenide and sulphide. No. IV of the above list 
was analyzed by Mr. Wm. J. Comstock* and consists of 13°7 
per cent HgSe and 86° per cent HgS. Calculating the den- 
sity of an isomorphous mixture in the above proportions 
taking the density of HgSe=8-2 that of HgS=7°8 we obtain 
7°85 as against 7-98 observed. Onofrite is therefore physically 
intermediate between tiemannite and metacinnabarite, and rep- 
resents an isomorphous mixture of these two isometric mole- 
cules. It is strange that the conclusions arrived at by Prof. 
Brush in the article already referred to, should have been the 
same, although he based his conclusions on earlier and incor- 
rect determinations of the density of tiemannite. 
As to the position which these minerals should occupy in & 
natural grouping, it seems that they would naturally come 
into the sphalerite group with which they have many things in 
common, being simple selenides or sulphides, with mercury 
replaced in part by zinc or cadmium, crystallizing in the isomet- 
ric system, tetrahedral, and with strong tendency to twinning 
parallel to the octahedron. 
In closing I take pleasure in expressing my thanks to Prof. 
Brush and Mr. ©. S. Bement for the material which they 
provided for carrying on this investigation. 
Note as to the occurrence of Tiemannite; by Prof. J. B. CLAY- 
TON. Communicated by letter to Prof. Brush, dated Salt Lake 
City, October 6th, 1884. 
he mine is situated above five miles S.W. of Marysvale, 
Mineralogical Laboratory Sheffield Scientific School, March 28th, 1885. 
* This Journal, III, xxi, 312. 
