BIATOMACKiE — FOSSILIZED DEPOSITS. 303 



there would neither be food for aquatic animals, nor (if it were 

 possible 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 

 imparting to it. It is interesting to observe, that some species 

 of marine Diatomacese are found through every degree of lati- 

 tude 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 sub- 

 tance is composed, those birds having received them, it is pro- 

 bable, from shell-fish, to which these minute organisms serve as 

 ordinary food (§ 192.) 



. 191. The indestructible nature of the epiderms of Diatomacece 

 has also served to perpetuate their presence in numerous locali- 

 ties, from which their living forms have long since disappeared ; 

 for the accumulation of sediment formed by their successive pro- 

 duction and death, either on the bed of the ocean, or on the bot- 

 toms of fresh-water lakes, gives rise to deposits which may attain 

 considerable thickness, and which, by subsequent changes of 

 level, may come to form part of the dry land. Thus very exten- 

 sive siliceous strata, consisting almost entirely of marine Diato- 

 macece, are found to alternate, in the neighborhood of the Medi- 

 terranean, with calcareous strata, chiefly formed of Foraminifera 

 (Chap. X) ; the whole series being the representative of the Chalk 

 formation of Northern Europe, in which the silex that was pro- 

 bably deposited at first in this form, has undergone conversion 

 into flint, by agencies hereafter to be considered (Chaps. X, XIX). 

 Of the Diatomaceous composition of these strata, we have a 

 characteristic example in Fig. 101, which represents the fossil 

 Diatomacese of Oran, in Algeria. The so-called "infusorial 

 earth" of Richmond, in Virginia, and that of Bermuda, also ma- 

 rine deposits, are very celebrated among Midl-oscopists for the 

 number and beauty of the forms they have yielded ; the former 

 constitutes a stratum of 18 feet in thickness, underlying the whole 

 city, and extending over an area whose limits are not known. 

 Several deposits of more limited extent, and apparently of fresh- 

 water origin, have been found in our own islands ; as for instance 

 at Dolgelly, in ISTorth Wales, at Lough Mourne, in Ireland (Fig. 

 102), and in the island of Mull, in Scotland. Similar deposits in 

 Sweden and E'orway are known under the name of berg-mehl or 

 mountain flour ; and in times of scarcity, the inhabitants of those 

 countries are accustomed to mix these substances with their 

 dough in making bread. This has been supposed merely to have 

 the efiect of giving increased bulk to their loaves, so as to render 

 the really nutritive portion more satisfying. But as the berg- 

 mehl has been found to lose from a quarter to a third of its 

 weight by exposure to a red-heat, there seems a strong proba- 



