
PROFESSOR HEDDLE ON THE RHOMBOHEDRAL CARBONATES. 495 
foliated plates, and, rarely, fibrous in structure; Brucite very rarely; magnetite 
in octohedra; pearly white Dolomite; and Breunnerite.. 
This Breunnerite occurs in pale brown rhombohedral crystals, imbedded 
either in the tale or in the Dolomite. Its specific gravity is 3-095; its cleavage 
angle, 106° 50’. 
The analysis executed on 25 grains afforded— 

Insoluble (talc), ; 2 : é : 5 ‘096 
Carbonate of magnesia, é : : : ‘ 91°394 
* ofiron, . : F ? : P 6:°784 
> of manganese, : ; ; F , “780 
Silica, : ‘ : P : : : ‘60 
Alumina, 4 3 : ; . F i 136 
99°76 
Here the magnesian carbonate is to the conjoined carbonates of iron and 
manganese in the ratio of 16 to 1; and the mineral is within, though barely 
within, the margin of the ferriferous magnesites, to which HaIDENGER assigned 
the name of Breunnerite. 
The theoretical specific gravity of a carbonate compounded in the above 
ratio is 3087, and its theoretical angle, 107° 27’. There was not in the mineral 
even a trace of lime. 
The most frequent order of position of the associated minerals here is—talec, 
Breunnerite, Dolomite; both tale and Breunnerite are, however, sometimes 
inclosed in Dolomite.. 
* 
DOLOMITES. 
The Dolomite which has been mentioned as occurring at North Cross 
Geo is found in numerous ramifying veins; its colour is a pure watery white; 
its lustre is pearly; it is semi-transparent; though not showing itself in free 
crystals, it is in large crystalline masses, and is altogether finer than at any other 
Scottish locality. Its specific gravity is 2°865; its cleavage angle, 106° 17. 
Portions from two veins were examined, with a view to ascertain if the 
replacement of the carbonates was constant,—the first analysis being that of the 
specimen whose gravity and angle were taken. 
1 2 3 1 
On 25 grs. On 25:17 grs. 
Insoluble (talc), 1 
Carbonate of lime, 52:548 55°344 
99 of magnesia, 43°772 41°911 
d of iron, 1972 2:193 1:304 1:36 
5 of manganese, 1:368 6 84 1108 
99-76 100-048 

