1845.] DARWIN ON DUST IN THE ATLANTIC. 29 



Polythalamia. The little packet of dust collected by myself would not 

 have filled a quarter of a tea-spoon, yet it contains seventeen forms. 

 Professor Ehrenberg remarks, that as 37 species are common to se- 

 veral of the packets, the dust collected by myself, and on four suc- 

 cessive days by Lieut. James, must certainly have come from the 

 same quarter ; yet mine was brought by a E.N.E. wind, and Lieut. 

 James's by a S.E. and E.S.E. wind. The Infusoria are all old known 

 species, excepting one allied to a Hungarian fossil ; and they are of 

 freshwater origin with the exception of two {Grammatophora oce- 

 anica and Textilaria glohulosd), which are certainly marine. Prof. 

 Ehrenberg could not detect any of the soft parts of the Infusoria, as 

 if they had been quickly dried up, and hence it would appear that 

 they must have been caught up by the wind some time after having 

 been dead. The greater number of the species are of wide or mun- 

 dane distribution ; four species are common to Senegambia and 

 S. America, and two are peculiar to the latter country : moreover it is 

 a very singular fact, that out of the many forms known to Professor 

 Ehrenberg as characteristic of Africa, and more especially of the 

 Sahara and Senegambian regions, none were found in the dust. 

 From these facts one might at first doubt whether the dust came 

 from Africa ; but considering that it has invariably fallen with the 

 wind between N.E. and S.E., that is, directly from the coast of Africa ; 

 that the first commencement of the haze has been seen to come on 

 with these winds ; that coarser particles have first fallen ; that the 

 dust and hazy atmosphere is more common near the African coast 

 than further in the Atlantic; and lastly, that the months during 

 which it falls coincide with those when the harmattan blows from 

 the continent, and when it is known that clouds of dust and sand are 

 raised by it, I think there can be no doubt that the dust which falls 

 in the Atlantic does come from Africa. How to explain the enigma 

 of the absence of characteristic African forms and of the presence 

 of two species from S. America, I will not pretend to conjecture. 

 Finally I may remark, that the circumstance of such quantities of dust 

 being periodically blown, year after year, over so immense an area 

 in the Atlantic Ocean, is interesting, as showing by how apparently 

 inefficient a cause a widely extended deposit may be in process of 

 formation ; and this deposit, it appears from the researches of Prof. 

 Ehrenberg, will in chief part consist of freshwater Polygastrica and 

 of Phytolitharia. 



List of References. 



{}) Nautical Magazine, 1839, p. 364. The dust fell from the 9th to the 13th of 

 February 1839, whilst sailing from (lat. 10° N., long. 29° 59') to (lat. 2° 50' N., 

 long. 26° 30' W.). The wind on the 9th was E.N.E., on the 10th N.E. by E., and 

 on the three succeeding days N.E. 



(2) Geographical Journal, vol. vi. p. 296. " Survey of some of the Canary 

 Islands and part of the coast of Africa, by Lieut. \V. Arlctt, R.N." 



(3) Edinburgh New Phil. Journal, vol. xxxii. p. 134. The account is taken from 

 Berghaus' Almanack of the dust which fell on the Princess Louise on Jan. I4tli 

 and 15th, 1839, between (lat. 24° 20' N., long. 26° 42' W.) and (lat. 23° 05' N., and 

 long. 28° 18' W.) : and again in 1840 from the 0th to the 9th of May, whilst between 

 (lat. 10° 29' N., long. 32° 19' W.) and (lat. 16° 44' N., long. 36° 37' W.). During the 



