in the Ocean at the South Pole. 179 



sphere off the Cape Yerd Islands, and also on the high sea of 

 that region, while he was there ; and like^^^se on a ship, which, 

 according to the account in his letter, was 380 sea-miles distant 

 from land. The wind was then blowing fr-om the African coast. 

 I\Ir. Darwin has sent to the author for examination a sample of 

 the dust which fell on the ship on the high sea at that gi-eat 

 distance from land. This dust has been universally regarded 

 hitherto as volcanic ashes. The microscopic analysis has clearly 

 sho^^'n that a considerable portion, perhaps one-sixth of the mass, 

 consists of numerous species of Siliceous Polygastrica and por- 

 tions of sihcated terrestrial plants, as follows : — 



The forms included in this catalogue, mostly known and for 

 the most part European, prove — 



1. That this meteoric shower of dust was of terrestrial origin. 



2. That it was not volcanic ash. 



3. That it was dust which had been lifted up to a great height 

 from a dried-up marshy district by an unusually strong current 

 of air or a whii-lwind. 



4. That the dust did not necessarily and evidently come ft-om 

 Africa, as being the nearest land, although the wind blew from 

 thence when the dust fell ; for this reason, that no exclusively 

 African forms are among it. 



5. That as Himantidium Papilio, a very marked fonn, has hi- 

 therto occmTcd only in Cayenne (see the Mikroskopische Leben 

 in Slid- und Nord-Amerika, plate 2. fig. 2.), and as the Surirella 

 is also probably an American form, only two conclusions present 

 themselves : either that the dust was raised in South America into 



