i882.] ATMOSPHERIC DUST WITH INFUSORIA. 5 
injured the astronomical instruments. ‘The morning before we 
anchored at Porto Praya, I collected a little packet of this 
brown-coloured fine dust, which appeared to have been filtered 
from the wind by the gauze of the vane at the mast-head. Mr. 
Lyell-has also given me four packets of dust which fell ona 
vessel a few hundred miles northward of these islands. Professor 
Ehrenberg* finds that this dust consists in great part of infusoria 
with siliceous shields, and of the siliceous tissue of plants. 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 every- 
thing 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 Jand, I was much surprised to find particles of stone above 
the thousandth of an inch square, mixed with finer matter. After 
this fatt 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 
* 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 Geolo- 
gieal Society. 
