28 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [JunC 4, 



plants and the ovules of Infusoria. The dust which fell on the 

 three succeeding days resembles in appearance and in its action 

 under the blowpipe that collected by myself off St. Jago, and is so 

 excessively fine, that Lieut. James was obliged to collect it with a 

 sponge moistened with fresh water. As the wind continued nearly 

 in the same direction during the four above-mentioned days, and 

 the distance from the land was only a little increased after the first 

 day, it would appear probable that the coarser dust was raised by a 

 squall with which the breezes on this coast so often begin blowing. 



With respect to the direction of the wind during the falls of dust, 

 in every instance where recorded it has been between N.E. and S.E. ; 

 generally between N.E. and E. In the case however given by the 

 Rev.W. Clarke (^), a hazy wind which had blown for some time 

 from E. and S.E. first fell calm, and was succeeded for a few hours 

 by a S.W. wind, and then returned strongly to the east ; during this 

 whole time dust fell. With respect to the time of year, the falls 

 have always occurred in the months of January, February, March 

 and April ; but in the case of the Princess Louise in 1840, as late as 

 on the 9th of May. In the one year of 1839, it has chanced that dust 

 has been recorded as having fallen in the Atlantic (as may be seen 

 in the references) on the 14th and 15th of January, and on the 2nd, 

 4th, 9th, 10th, 11th, 12th and 13th of February. I may add, that 

 Baron Roussin (^), during his survey of the north-western African 

 coast, found, that whilst the wind keeps parallel to the shore, the 

 haze and dust extend seaward only a short distance ; but when du- 

 ring the above four specified months the harmattan blows from the 

 N.E. and E.N.E., accompanied by tornados, the dust is blown far 

 out, and is raised on high, so that stars and all other objects within 

 30° of the horizon are hidden. 



From the several recorded accounts (^) it appears that the quan- 

 tity of dust which falls on vessels in the open Atlantic is considerable, 

 and that the atmosphere is often rendered quite hazy ; but nearer 

 to the African coast the quantity is still more considerable. Ves- 

 sels have several times run on shore owing to the haziness of the air : 

 and Horsburgh(^) recommends all vessels, for this reason, to avoid 

 the passage between the Cape Verd Archipelago and the main-land. 

 Roussin also, during his survey, was thus much impeded. Lieut. 

 Arlett found the water so discoloured (^), that the track left by his 

 ship was visible for a long time ; and he attributes this in part to the 

 fine sand blown from the deserts, " with which everything on board 

 soon becomes perfectly caked." 



Professor Ehrenberg * has examined the dust collected by Lieut. 

 James and myself ; and he finds that it is in considerable part com- 

 posed of Infusoria, including no less than sixty-seven diff'erent forms. 

 These consist of 32 species of siliceous-shielded Polygastrica ; of 34 

 forms of Phytolitharia, or the siliceous tissues of plants ; and of one 



* These microscopic organized bodies have been described in the * Monats- 

 berichten der Berlin Akad. der Wissens. Mai 1844 ; u. 27 Februar 1845.' In the 

 latter paper a full hst of the names is given : the column marked St. Jago includes 

 those collected by myself. 



