620 MICROSCOPIC FORMS OF VEGETABLE LIFE THALLOPHYTES 



retiring tide at the mouths of our larger rivers. They are not, how- 

 ever, confined to the localities I have mentioned they are, in fact, 

 most ubiquitous, and there is hardly a roadside ditch, water- trough, 

 or cistern, which will not reward a search and furnish specimens 

 of the tribe.' Such is their abundance in some rivers and estuaries 

 that their multiplication is affirmed by Professor Ehrenberg to have 

 exercised an important influence in blocking up harbours and 

 diminishing the depth of channels ! Of their extraordinary abundance 

 in certain parts of the ocean the best evidence is afforded by the 

 observations of Sir J. D. Hooker upon the Diatomacece of the southern 

 seas ; for within the Antarctic Circle they are rendered peculiarly 

 conspicuous by becoming enclosed in the newly formed ice, and by 



FIG. 467. Fossil Diatomacese, &c., from Oran : a, a, a, Coscinodiscus; 0, b, b, 

 Actinocyclus ; c, Dictyochya fibula ; d, Lithasteriscug radiatus ; .e, Spongolithis 

 acicularis ; /, /, Grammatophora parallela (side view) ; g, g, GrammatopJiora 

 angulosa (front view). 



being washed up in myriads by the sea on to the ' pack ' and * bergs,' 

 everywhere staining the white ice and snow a pale ochreous brown. 

 A deposit of mud, chiefly consisting of the siliceous valves of Diato- 

 macece, not less than 400 miles long and 120 miles broad, was found 

 at a depth of between 200 and 400 feet on the flanks of Victoria Land 

 in 70 south latitude. Of the thickness of this deposit no conjecture 

 could be formed ; but that it must be continually increasing is evi- 

 dent, the silex of which it is in a great measure composed being 

 indestructible. A fact of peculiar interest in connection with 

 this deposit is its extension over the submarine flanks of Mount 

 Erebus, an active volcano of 12,400 feet elevation, since a commu- 

 nication between the ocean waters and the bowels of a volcano, sucli 



