b ST. JAGO CAPE UE VEKU ItiLAADS. 



was found to have slightly injured the astronomical 

 instruments. The morning before we anchored at 

 Porto Praya, I collected a little jiacket 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. Pro- 

 fessor Ehrenberg* finds that this dust consists in 

 great part of infusoria with siliceous shields, and 

 of the siliceous tissue of plants. In five little pack- 

 ets which I sent him, he has ascertained no less 

 than sixty-seven different organic forms ! The in- 

 fusoria, with the exception of two marine species, 

 are all inhabitants of fi-esh-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, how- 

 ever, a very singular fact, that, although Professor 

 Ehrenberg knows many species of infusoria pecu- 

 liar 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 quanti- 

 ties as to dirty everything on board, and to hurt 

 people's eyes; vessels even have run on shore ow- 

 ing to the obscurity of the atmosphere. It has oft- 

 en fallen on ships when several hundred, and even 

 more than a thousand miles from the coast of Afri- 



* 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 Societj% 



