1832.] ATMOSPHERIC DUST WITH INFUSORIA. 5 



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 mouths when the harmattan is known to raise clouds of dust 

 high into the atmosphere, we may feel sure that it all comes from 

 Africa. Jt 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 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 tho laud, 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 interesting 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. Where the lime has 

 been caught up by the scoriaceous fragments of the lower surface 

 of the stream, it is converted into groups of beautifully radiated 



