82 _ GENERAL HISTORY OF THE INFUSORIA. 



speciea wJiicli occur fossil will, ere long, be detected in the recent state. It 

 is at all events certain that a very large proportion of the Diatoms found in 

 the fossil state also occur in the living state, and that every day adds to their 

 number. There is at present no good evidence of the existence of Diatoms 

 earlier than the chalk, if so early. But we must not forget that the shells 

 of Diatoms appear to be altered by long contact with carbonate of lime, so 

 that they may have existed at one time in the chalk. We find them, how- 

 ever, in spite of the action of calcareous matter, in the recent chalk-marls of 

 Meudon and of Caltanisetta, which are rather more recent than the chalk, 

 and probably of about the age of the clay-marl of ^gina. If, as I believe, 

 no Diatoms have become extinct, this may perhaps depend on their minute 

 size and extreme simplicity of structure, which probably render them more 

 indiiferent to climatic changes than more highly organized and larger beings. 

 We have evidence, to a certain extent, that this is the case ; for by Ehren- 

 berg's figures it appears that, in gatherings of recent Diatoms from all parts 

 of the world, in every possible variety of climate, the majority of species are 

 identical with om- own. 



*' Diatoms, therefore, are not materially affected by existing differences of 

 climate, and have probably been as little affected by the geological changes 

 which have occurred, at all events, since the period of the Eocene deposits." 



Geological Importance of Diatome^. — Fossil Accumulations. — Although 

 so exceedingly minute and apparently insignificant in comparison with the 

 animals and plants usually claiming our notice, yet, by their excessive multi- 

 plication and accumulation, they assume even a greater importance, in the 

 physical history of the earth, than the largest trees or animals mth which 

 we are acquainted. This lesson is taught us by hving examples of these 

 microscopic beings constituting appreciable masses, and by innumerable in- 

 stances where only the silicious skeletons remain, in a fossil or semi-fossil 

 condition. 



Ehrenberg thus illustrates their rapidity of production and accumulation. 

 " Silicious Infusoria," he says, "form, in stagnant waters during hot weather, 

 a porous layer of the thickness of the Jiand. Although more than 100,000,000 

 weigh harcfly a grain, one may in the coui'se of half-an-hour collect a pound 

 weight of them ; hence it will no longer seem impossible that they may build 

 up rocks. However, one of the most striking examples of the operation of 

 Diatomese as a physical agency on a large scale, is afforded by Dr. Hooker's 

 observations addressed to the British Association {Ueport, 1847). He saj^s — 

 " The waters, and especially the newly-formed ice of the whole Antarctic 

 Ocean, between the parallels of 60° and 80° south, abound in Diatomaceae, — so 

 numerous as to stain the sea everywhere of a pale ochreous bro^vn, the surface 

 having that colour as far as the eye can reach from the ship. Though pecu- 

 liarly abundant in the ley Sea, these plants are probably uniformly dispersed 

 over the whole ocean, but, being invisible from their minuteness, can only be 

 recognized when washed together in masses, and contrasted with some opake 

 substance. They were invariably found in the stomachs of Salpoe and of other 

 sea animals, in all latitudes between that of the tropic and the highest parallel 

 attained in the Antarctic expedition. Their death and decomposition produce 

 a submarine deposit or bank of vast dimensions, consisting mainly of their 

 silicious shields, intermixed with Infusoria and inorganic matter. Its position 

 is from the 76th to the 78th degree of south latitude, and occupies an area 

 400 miles long by 120 wide. The lead sometimes sank two feet in this pasty 

 deposit, and on examination showed the bottom made up in great measure of 

 the species now living on the surface. This deposit may be considered as 

 resting upon the shores of Victoria Land and of the Barriers, and hence on the 



