ORIGIN OF CHALK-FLINTS. 101 



l)y the Molluscs, which in their turn found a grave in the 

 stomach of the Cetacean. " We find that the siliceous 

 particles of the Diatomacece, Polycistina, Acanthometrce, 

 and Sponges, exist not only in a state of the utmost purity, 

 but that they occur precisely in that state of minute sub- 

 division which favours the solvent or aggregative process 

 in an eminent degree. We see that they are gathered 

 together by the Salpse, in the first instance, from the ele- 

 ment in which they live, and that they are freed of all, 

 or nearly all, their soft portions, by the action of the di- 

 gestive cavities of these creatures. We find that the Salpse 

 again, in inconceivably vast numbers, afford almost the 

 entire food of the largest orders of Cetaceans ; and I there- 

 fore think we are able to infer, with certainty, that, in 

 the comj^lex stomachs and intestines of the latter, the 

 further process of aggregation of siliceous particles goes 

 on upon a gigantic scale, aided by the presence of the 

 alkalies, and that the aojo-reo'ated masses beino; voided at 

 intervals, slowly subside, without interruption, to the bed 

 of the ocean.'^ 



Darwin records having seen clustered objects in the 

 sea near Keeling Atoll, which he does not name, but 

 which from the figures he has given must have been 

 Diatoms. But all the streaks and bands of colour seen on 

 the ocean are not attributable to plants : some of them 

 are certainly of an animal nature. The following pheno- 

 menon was noticed bv the observer last named on the coast 

 of Chili. The vessel passed through broad bands of red- 

 dish water, which when examined microscopically swarmed 



