1832.] STRANGE DUST. 19 



siliceous shields, and of the siliceous tissue of plants. In 

 five little packets which I sent him, he has ascertained no 

 less than s?xty-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 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 crypto- 

 gamlc plants. 



The geology of this Island Is the most Interesting part of 

 its natural history. On entering the harbour, a perfectly 

 horizontal white ban^ 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 over- 

 lying 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 scorlaccous fragments of the lower surface 



