92 
Williamson on Faujasina. 
of contour, will approach much more closely to the calcareous 
sponges than any hitherto described. 
I am well aware that, to many, these dry details will appear 
unnecessarily and tediously minute ; but it must be remem- 
bered that, until we are accurately familiar with all the lead- 
ing types of structure existing in this interesting group of 
organisms, we cannot be in a condition to arrive at final con- 
clusions respecting their nature and zoological position. 
Manchester, May 2lst, 1851. 
Notice of a Diatomaceous Earth found in the Isle of Mull. By 
William Gregory, M.D., F.R.S.E., Professor of Chemistry 
in the University of Edinburgh. Communicated by Pro- 
fessor John E. Quekett. (Read March 23rd, 1853.) 
This earth was discovered, about two years ago, by the Duke 
of Argyll, who gave a short account of its geological position 
to the Royal Society of Edinburgh. It constitutes a bed, 
resembling marl in appearance, lying in a rough piece of 
ground, at Knock, near Aros, between Loch Baa, a fresh-water 
lake, 3 miles long and 1 mile broad, and the sea. The lake 
is about 30 feet, the land about 40 feet, above the sea-level, 
and the lake is surrounded with high mountains on all sides 
except the west, where its waters flow towards the sea, passing 
through the rough district, boggy in parts, above mentioned, 
which is about a mile broad. The marl-bed, as it is called 
on the spot, lies within 50 yards of the lateral granite rock, 
and half-way from the lake to the sea. The surface of the 
land between the lake and the sea is very uneven, covered 
with large stones, gravel, and sand. At one part there is a 
hollow, which in winter used to become a small loch, in 
summer only a stagnant pool, and in draining this the bed of 
marl was discovered. It was filled in summer by a small 
stream unconnected with the lake. The bed rests on the 
gravel, which again rests on the granite of w^hich the whole 
district is formed. As there is no formation of an epoch be- 
tween those of the granite and of the gravel, we cannot, from 
its position, ascertain precisely the geological period at which 
the bed was deposited. The Duke of Argyll regards the 
gravel as belonging to the Diluvium, and the Infusorial de- 
posit as comparatively of very recent origin. But there is 
reason to think, from the character of the species, that the 
deposit may belong to a more remote epoch. Ehrenberg, to 
whom 1 sent a portion of it, writes to me, that he thinks it 
probably connected with the Tertiary, or at all events, with 
