THE MINERALOGY OF SCOTLAND. 457 
serpentine of Portsoy are chromite; and very probably there is an admixture of 
that substance with magnetite. 
The substances, the analyses of which follow, cannot however be regarded 
as mere mixtures, 
1, A black iron sand was found somewhat sparingly by Mr. DupGEon and 
myself among the ordinary siliceous sands of the Dale Burn, in Unst, Shetland. 
The locality was at the point where the burn turns abruptly to the northward. 
This sand was of a blue-black colour, decidedly but not strongly magnetic ; 
under the microscope it seemed much split or cleaved; of a hackly fracture, 
and it showed no crystalline forms. 
It was, so far as the small quantity obtained arated, separated from 
siliceous and non-magnetic sand by repeated stirring up in water and decanta- 
tion, and ultimately by the magnet. 
When magnified it appeared to be, with the exception of a small admixture 
of siliceous grains, uniform. 
On an analysis it afforded— 
1 2. 
Ferric Oxide, ; : 57° 285 62 - 464 
Chromium Sesquioxide, 9°4 10+ 25 
Ferrous Oxide, . ; 24:944 27:°199 
Manganous Oxide, , "4 '436 
Lime, ; hae ey Pos 
Silica, . ‘ ‘ : fae 
100 - 349 100 * 349 
2. Gives the proportion, after abstracting the silica and lime, as these are 
undoubtedly present from mere mechanical admixture. 
The question is, can this be an admiature of magnetite with chromite ? 
No one point connected with it favours such a view. Its geographical and 
geological position does not ;—it is found about two miles from the nearest 
chromite, with more than one hill ridge between ; its altitude is greater than 
that of the chromite, and the stream which sweeps it towards the sea, flows 
from a hill of mica schist. 
It was physically purified by the magnet, which readily abstracted it from 
the siliceous admixture. Chromite is, at most, and that only rarely, feebly 
magnetic. 
Its powder had a uniform blue-black colour; the crushed powder of 
chromite is brown. 
Lastly, an admixture of chromite with magnetite would not yield the per- 
centages above given. The results of the analysis point rather to a replacement 
