4 Dr. G. C. Wallich on the Diatomacca. 



Throughout the entire scries of the two great kingdoms of 

 nature, there is no class of objects so universally and lavishly 

 distributed as the Diatomacesc. In every latitude, on land and 

 by sea, and under every known variation of temperature, where- 

 soever are combined the primary conditions of light and mois- 

 ture, these minute but wondrously beautiful structures are to be 

 found, in inexhaustible profusion. 



Of their immediate purpose and uses we know little, as yet, 

 beyond the bare fact that vast strata of the earth's crust consist 

 more or less entirely of their silicious remains j that these 

 strata have been formed, in bygone ages, as marine, fluviatile, 

 or lacustrine deposits ; and that, in our own day, similar strata 

 are being gradually but incessantly built up, in the dark abysses 

 of the sea-bed, far beyond the depths at which any living struc- 

 tures, with which we are acquainted, could meet with the condi- 

 tions essential to their existence. 



The strata referred to are, by some writers, described as 

 "fossil" (or I should rather say that the Diatomacere discovered 

 in those strata have been so considered), an error at once ob- 

 vious, from the fact of their silicious constituents existing now 

 in precisely the same state as that in which they originally con- 

 stituted the framework of the organisms by which they were 

 eliminated and secreted. 



This is a material point ; and I am desirous of laying stress 

 upon it, inasmuch as I conceive the deposits under notice not to 

 be dependent wholly, or even chiefly, on the subsidence of these 

 silicious frameworks, as the sequel to ordinary death and decay, 

 but on the living structures being subjected, in numberless mul- 

 titudes, to the processes of digestion ; whereby, being divested 

 of the bulk of the particles possessing any buoyant tendency, 

 the mineral remains subside, by their own specific gravity, to the 

 regions wherein they are finally entombed. 



Some faint conception of their numbers may be gathered 

 from what we see in the guanos, which present a considerable 

 percentage of Diatomaceous exuviae, and are thereby enhanced in 

 commercial value. In most descriptions of the sources from 

 whence the silicious element in guano is derived, it is stated 

 that the birds producing this kind of deposit feed directly 

 upon Diatomacese. Professor Carpenter suggests that the birds 

 must have received these minute particles from the " shell-fish 

 to which they serve as ordinary food." 



Professor Quekett, in a paper on " The Examination of Guano 

 by the Microscope " (Trans. Microscop. Soc. vol. ii. p. 29), thus 

 writes: "The silicious animalcules and sponge-spicules, it 

 would seem, become present in the guano from, firstly, being 

 devoured by fishes whilst adhering to sea-weeds or mingled with 



