t consists in great part of infusoria with siliceous shields,
and of the siliceous tissue of plants. (1/3. I must take this
opportunity of acknowledging the great kindness with which this
illustrious naturalist has examined many of my specimens. I have
sent (June 1845) a full account of the falling of this dust to the
Geological Society.) In five little packets which I sent him, he
has ascertained no less than sixty-seven different organic forms!
The infusoria, with the exception of two marine species, are all
inhabitants of fresh-water. I have found no less than fifteen
different accounts of dust having fallen on vessels when far out in
the Atlantic. From the direction of the wind whenever it has
fallen, and from its having always fallen during those months when
the harmattan is known to raise clouds of dust high into the
atmosphere, we may feel sure that it all comes from Africa. It is,
however, a very singular fact, that, although Professor Ehrenberg
knows many species of infusoria peculiar to Africa, he finds none
of these in the dust which I sent him. On the other hand, he finds
in it two species which hitherto he knows as living only in South
America. The dust falls in such quantities as to dirty everything
on board, and to hurt people's eyes; vessels even have run on shore
owing to the obscurity of the atmosphere. It has often fallen on
ships when several hundred, and even more than a thousand miles
from the coast of Africa, and at points sixteen hundred miles
distant in a north and south direction. In some dust which was
collected on a vessel three hundred miles from the land, I was much
surprised to find particles of stone above the thousandth of an
inch square, mixed with finer matter. After this fact one need not
be surprised at the diffusion of the far lighter and smaller
sporules of cryptogamic plants.

The geology of this island is the most interesting part of its
natural history. On entering the harbour, a perfectly horizontal
white band in the face of the sea cliff, may be seen running for
some miles along the coast, and at the height of about forty-five
feet above the water. Upon examination, this white stratum is found
to consist of calcareous matter, with numerous shells embedded,
most or all of which now exist on the neighbouring coast. It rests
on ancient volcanic rocks, and has been covered by a stream of
basalt, which must have entered the sea when the white shelly bed
was lying at the bottom. I