1832.] ATMOSPHERIC DUST WITH INFUSORIA. 



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 on a 

 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 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 



* 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- 

 gical Society. 



