III EXPEDITION OF THE " CHALLENGER " 77 



minute productions may perform ; they may also be the purifiers 

 of the vitiated atmosphere, and thus execute in the Antarctic 

 latitudes the office of our trees and grass turf in the temperate 

 regions, and the broad leaves of the palm, &c., in the 

 trop'cs." .... 



With respect to the distribution of the 

 Diatomacece, Dr. Hooker remarks : - 



" There is probably no latitude between that of Spitzbergen 

 and Victoria Land, where some of the species of either country 

 do not exist : Iceland, Britain, the Mediterranean Sea, North and 

 South America, and the South Sea Islands, all possess Antarctic 

 Diatomaccce. The silicious coats of species only known living 

 in the waters of the South Polar Ocean, have, during past 

 ages, contributed to the formation of rocks ; and thus they out- 

 live several successive creations of organized beings. The 

 phonolite stones of the Rhine, and the Tripoli stone, contain 

 species identical with what are now contributing to form a sedi- 

 mentary deposit (and perhaps, at some future period, a bed of 

 rock) extending in one continuous stratum for 400 measured 

 miles. I allude to the shores of the Victoria Barrier, along 

 whose coast the soundings examined were invariably charged 

 with diatomaceous remains, constituting a bank which stretches 

 200 miles north from the base of Victoria Barrier, while the 

 average depth of water above it is 300 fathoms, or 1,800 feet. 

 Again, some of the Antarctic species have been detected floating 

 in the atmosphere which overhangs the wide ocean between 

 Africa and America. The knowledge of this marvellous fact we 

 owe to Mr. Darwin, who, when he was at sea off the Cape de 

 Verd Islands, collected an impalpable powder which fell on 

 Captain Fitzroy's ship. He transmitted this dust to Ehrenberg, 

 who ascertained it to consist of the silicious coats, chiefly of 

 American D iaf.omacece, which were being wafted through the 

 upper region of the air, when some meteorological phenomena 

 checked them in their course and deposited them on the ship 

 and surface of the ocean. 



" The existence of the remains of many species of this order 



