458 PROFESSOR HEDDLE ON 
of ferric oxide by chromic oxide, though it is not altogether accordant with 
that. 
3. A “black magnetic sand” was noted as occurring on the shores of the 
Loch of Trista, by Dr. Fiemine (£d. Phil. Jour., vol. iv. p. 114). He says 
that it “occurs along with iron sand, imbedded in small grains, in the primitive 
limestone in the neighbourhood. Small crystals of sphene occur along with 
the iron sand imbedded in the limestone.” 
It is found both on the north and south shores of the lake ina granitic sand; 
in this it occurs to the amount of about one hundredth part of the whole. 
The Rey. Davin WEBSTER writes the author that it probably was derived 
from a valley to the north-west, called the Dullans, from whence a stream runs 
into the lake. The high state of the lake prevented the author from obtaining 
more than a trifling quantity of the sand; and he is indebted to Mr. WEBSTER 
for the supply which he examined and analysed. 
Mr. WesstTeER holds the view that the sand is derived from masses of bog 
iron ore which sprinkle the surface in profusion, at the Dullans ; these masses, 
however, when examined by the writer, yielded none of the sand; and even 
eranting that they had done so, it could only have been caught up from the 
surface during the formation of a substance now recognised universally as an 
outcome of organic change. 
Mr. WEBSTER also sent the writer a very similar sand from the sea-shore on 
the east side of Trista Voe. 
The sand from the Loch of Trista presents an appearance under the micro- 
scope which is different from that of any magnetic or “ black iron sand” which 
I have examined. Among the grains there occur a few well-defined, and very 
slightly abraded octahedral crystals, evidently of the regular system ; they are, 
as is the rest of the sand, jet black and lustrous. The great bulk of the sand 
is composed of rounded grains, which have at first sight a vitrified appearance ; 
but this is due to their surfaces being pitted with a multitude of minute conchoidal 
fractures, doubtless from repeated collision in the surf of the lake ; their frac- 
ture therefore is conchoidal, and the lustre is extremely high. Many of the 
grains still retain adherent transparent quartz, whence I assign their matrix to 
the gneissic rock. 
Excepting the octahedral crystals, the appearance of the grains is uniform; 
there is no admixture ; the proportional quantity of crystals is very small. 
The sand had been originally separated from the granitic granules by the 
magnet, and the process was repeated several times to free it from quartz ; it 
did not appear, however, that there was, as is frequently the case with mag- 
netic sands, a more- and a less-strongly magnetic portion. 
Mr. Wesster, from observations on - spot, came to the same conclusion. 
