GENERAL DISTRIBUTION OF DIATOMACE.E. 311 



Of their extraordinary abundance in certain parts of the Ocean, 

 the best evidence is afforded by the observations of Dr. J. D. 

 Hooker upon the Diatomacea? of the southern seas ; for within the 

 Antarctic Circle they are rendered peculiarly conspicuous by be- 

 coming enclosed in the newly-formed ice, and by being washed-up 

 in myriads by the sea on to the 'pack' and 'bergs,' everywhere 

 staining the white ice and snow of a pale ochreous brown. A 

 deposit of mud, chiefly consisting of the siliceous loricse of Diato- 

 mace®, 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 in- 

 creasing is evident, the silex of which it is in a great measure 

 composed being indestructible. A fact of peculiar interest in con- 

 nection with this deposit is its extension over the submarine flanks 

 of Mount Erebus, an active Volcano of 12,400 feet elevation ; since 

 a communication between the ocean-waters and the bowels of a 

 volcano, such as there are other reasons for believing to be occa- 

 sionally formed, would account for the presence of Diatomacese in 

 volcanic ashes and pumice, which was discovered by Prof. Ehren- 

 berg. It is remarked by Dr. Hooker, that the universal presence 

 of this invisible vegetation throughout the South Polar Ocean is 

 a most important feature, since there is a marked deficiency in 

 this region of higher forms of vegetation ; and were it not for them, 

 there would neither be food for aquatic Animals, nor (if it were pos- 

 sible for these to maintain themselves by preying on one another) 

 could the ocean-waters be purified of the carbonic acid which 

 animal respiration and decomposition would be continually impart- 

 ing to it. It is interesting to observe that some species of Marine 

 Diatomaceae are found through every degree of latitude between 

 Spitzbergen and Victoria Land, whilst others seem limited to 

 particular regions. One of the most singular instances of the 

 preservation of Diatomaceous forms is their existence in Guano ; 

 into which they must have passed from the intestinal canals of the 

 Birds of whose accumulated excrement that substance is composed, 

 those birds having received them, it is probable, from Shell-fish, 

 to which these minute organisms serve as ordinary food (§ 2557.) 



236. The indestructible nature of the Lorica? of Diatomacea? 

 has also served to perpetuate their presence in numerous localities 

 from which their living forms have long since disappeared ; for the 

 accumulation of sediment formed by their successive production 

 and death, either on the bed of the Ocean, or on the bottoms of 

 Fresh-water Lakes, gives-rise to deposits which may attain consi- 

 derable thickness, and which, by subsequent changes of level, may 

 come to form part of the dry land. Thus very extensive Siliceous 

 strata, consisting almost entirely of marine Diatomacece, are found 

 to alternate, in the neighbourhood of the Mediterranean, with Cal- 

 careous strata chiefly formed of Foraminifera (Chap, x.) ; the 



