496 PROFESSOR HEDDLE ON THE RHOMBOHEDRAL CARBONATES, 
The only difference that could be seen between the specimens analysed was. 
that the first was more distinctly crystallised than the others; and it is a 
fact calling for remark, that in the presence of this free and almost indefinite 
replacement, the Breunnerite, which is imbedded in this highly calcareous 
matrix, and which is a mineral in itself obeying the law of homeomorphous 
replacement, and evidently of contemporaneous origin, should so absolutely in 
crystallising expel every particle of lime, whilst it incorporated iron and man- 
ganese—the former, at least, to a greater extent than had been accomplished 
by that matrix. 
The specific gravity and angle of No. 1 were so near the usual, and the mineral 
altogether so typical, as to call for no remark. 
Dolomite from Scalpa, Harris.—This was analysed on account of its having 
been mentioned by Maccuttocu and others as ‘ the great vein of Dolomite.” 
It forms a vein (? bed), associated with penninite on the one side and 
steatite on the other, running nearly east and west from a spot a few yards to 
the south-east of the Lighthouse pier at Scalpa. The enclosing rock is serpen- 
tine, distinctly bedded in hornblendic gneiss. 
The appearance of this Dolomite is unusual—it is confusedly crystalline, 
somewhat foliaceous in structure, with its cleavage faces so curved as to be f 
incapable of measurement. It is of unusual hardness and toughness, is milk : 
white, perfectly opaque, and with little lustre. Its specific gravity is 2°87. 
Its analysis of 25 grains afforded— 


Insoluble (steatite), . : : : : : 16" 
Carbonate of lime, . ; : : : 3 50°244 
. of magnesia, : ; j F 43-028 
iG ofiron, . ‘ : ; : ‘ 2:504 ey 
5 of manganese, : ; : ; ; 2:896 
Silica, : ; : é : : : 64 
Alumina, : ; : ) : é : 112 
99°644 
The four carbonates are here in nearly the ratios 
CaC MgC FeC MnC 
50 49 2 24 
and the theoretical specific gravity of such a compound is 2°913. 
Dolomite from the old lead and copper mine on the roadside a little to the 
south-east of Newton Stewart, Galloway.—It occurs here associated with chalco- 
pyrite and galena in large pale brown simple and twin crystals ; also rarely in 
fine crystals of a beautiful pink colour;—the brown crystals were analysed. 
Their specific gravity was 2°906 ; their cleavage angle, 106° 10’. 
