4 ST. J AGO CAPE DE VERD ISLANDS. [CHAP. i. 



of Impalpably fine dust, which was found to have slightly 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 it in 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. It is interest- 

 ing to trace the changes, produced by the heat of the overlying lava, 

 on the friable mass, which in parts has been converted into a crystalline 

 limestone, and in other parts into a compact spotted stone. When 

 the lime has been caught up by the scoriaceous fragments of the lower 



* I must take this opportunity of acknowledging the great kindness with 

 which this illustrious naturalist has examined many of my specimens. 1 

 have sent (June, 1845) a - u ^ account of the falling of this dust to the 

 Geological Society. 



