loo Dr Brewster on a stngulm^ Structure in the Diamond, 
included air on the diamond and the amber, when they were in 
such a soft state as to he susceptible of compression from so 
small a force. That this compressible state of the diamond 
could not arise from the action of heat, is manifest from the na- 
ture and the recent formation of the soil in which it is found : 
that it could not exist in a mass formed by aqueous deposition 
is still more obvious, and hence we are led to the conclusion, 
rendered probable by other analogies, that the diamond origi- 
nates, like amber, from the consolidation of perhaps vegetable 
matter, which gradually acquires a crystalline form by the in- 
fluence of time, and the slow action of corpuscular forces *. 
As the preceding results were obtained from flat diamonds, 
which did not seem to have been regularly crystallised, I was 
anxious to detect the same structure in those which had a re- 
gular crystalline form. With this view I examined several of 
the diamonds in Mr Allan'’s collection, and was fortunate 
enough not only to detect in a perfect octohedral crystal the same 
structure which I had observed in the flat specimens, but also 
an air-bubble of considerable size, which had produced, by its 
expansion, the polarising structui’e already described^ 
Edinburgh, March 
Art. XVIII . — Notice regarding the Submarine llc7nains of a 
Grove of Fir-Trees in Orlmey, Contained in a Letter from 
W. G. Watt, Esq. of Skaill, to Mr Stevenson, Civil En,, 
gineer. 
On the west coast of the Mainland of Orkney, about midway 
between Hoy Mouth and the Westray Frith, lies the Bay of 
Skaill, almost the only one on this part of the coast. It is com- 
pletely exposed to all the violence of the western oct?an, the 
waves of which often roll into it with tremendous force. 
About three years ago, the sand on the shore of this bay vras 
^ See Edia. Phil. Journah vol. i. p. 53. ; and JMM. Eyrie and Maltcbrun's An^ 
nales des Voj/ages-t tom. iv. p. 
