1845.] DARWIN ON DUST IN THE ATLANTIC. 27 



Phytolitharia. I have found fifteen distinct statements of dust 

 having fallen ; and several of these refer to a period of more than 

 one day, and some to a considerably longer time. Other less distinct 

 accounts have also appeared. At the end of this paper I will give 

 the particular cases, and will here only refer to the more striking 

 ones and make a few general remarks. 



The phsenomenon has been most frequently observed in the neigh- 

 bourhood of the Cape Verd Archipelago. The most southern point 

 at which dust is recorded to have fallen is noticed by Capt. Hay- 

 ward (^), on whose vessel it fell whilst sailing from lat. 10° N. to 

 2° 56' N. ; the distance from the nearest of the Cape Verd Islands 

 being between 450 and 850 miles. Respecting the northern limit, 

 the water for a great distance on both sides of C. Noon (in lat. 

 28° 45') is discoloured, owing in part, according to Lieut. Arlett ('^), 

 to the quantities of falling dust. Hence the phaenomenon has been 

 observed over a space of at least 1600 miles of latitude. This dust 

 has several times fallen on vessels when between 300 and 600 miles 

 from the coast of Africa: it fell, in May 1840, on the Princess 

 Louise (3) (in lat. 14° 21' N. and long. 35° 24' W.) when 1030 miles 

 from Cape Verd, the nearest point of the continent, and therefore 

 half-way between Cayenne in S. America and the dry country north 

 of the Senegal in Africa. 



On the 16th of January (1833), when the Beagle was ten miles off 

 the N.W. end of St. Jago, some very fine dust was found adhering 

 to the under side of the horizontal wind- vane at the mast-head ; it 

 appeared to have been filtered by the gauze from the air, as the ship 

 lay inclined to the wind. The wind had been for twenty-four hours 

 previously E.N.E., and hence, from the position of the ship, the dust 

 probably came from the coast of Africa. The atmosphere was so 

 hazy that the visible horizon was only one mile distant. During our 

 stay of three weeks at St. Jago (to February 8th) the wind was N.E., 

 as is always the case during this time of the year ; the atmosphere 

 was often hazy, and very fine dust was almost constantly falling, so 

 that the astronomical instruments were roughened and a little in- 

 jured. The dust collected on the Beagle was excessively fine-grained, 

 and of a reddish brown colour ; it does not effervesce with acids ; it 

 easily fuses under the blowpipe into a black or gray bead. 



In 1838, from the 7th to the 10th of March, whilst Lieut. James 

 in H.M.S. Spey was sailing, at the distance of from 330 to 380 

 miles from the continent, between lat. 21° 10' N., long. 22° 14' W., 

 and lat. 17° 43' N., long. 25° 54' W., considerable quantities of dust 

 fell on his vessel, four packets of which, together with a written 

 communication, I owe to the kindness of Mr. Lyell. The dust 

 which fell on the first day (or the 7th) was preceded by a thick 

 haze, and it is coarser than that which fell on the succeeding days : 

 it contains numerous irregular, transparent, variously coloured par- 

 ticles of stone about the yo\joth of an inch square, with some lew 

 a little larger, and nmch fine matter. The fact of particles of this 

 size having been brought at least 330 miles from the land is inter- 

 esting, as bearing on tiic distribution of the sporuKis of cryptogainic 



