302 THE MICROSCOPE AND ITS REVELATIONS. 



intestinal canals of the Birds of whose accumulated excrement that sub- 

 stances is composed; those birds having received them, it is probable, 

 from Shell-fish, to which these minute organisms serve as ordinary food 

 ( 300). 



299. The indestructible nature of the silicified casings of Diatomacece 

 has also served to perpetuate their presence in numerous localities from 

 which their living forms have long since disappeared; for the accumula- 

 tion of sediment formed by their successive production and death, even 

 on the bed of the Ocean, or on the bottoms 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 extensive Siliceous strata, consisting almost entirely of marine 

 Diatomacece, are found to alternate, in the neighborhood of the Mediter- 

 ranean, with Calcareous strata chiefly formed of Foraminifera (Chap, 

 xii.); the whole series being the representative of the Chalk formation 

 of Northern Europe, in which the silex that was probably deposited at 

 first in this form has undergone conversion into flint, by agencies here- 

 after to be considered (Chaps, xii., xxi.). Of the Diatomaceous compo- 

 sition of these strata we have a characteristic example in Fig. 191, which 

 represents the Fossil Diatomacese of Oran in Algeria. The so-called 

 ' infusorial earth ' of Eichmond in Virginia, and that of Bermuda, also 

 Marine deposits, are very celebrated among Microscopists for the number 

 and beauty of the forms they have yielded; the former constitute's 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 lim- 

 ited extent, and apparently of fresh- water origin, have been found in our 

 own islands; as for instance at Dolgelly in North Wales, at South Mournc 

 in Ireland (Fig. 192), and in the island of Mull in Scotland. Similar 

 deposits in Sweden and Norway are known under the name of berg-meld 

 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 effect 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 probability that it contains Organic matter enough to 

 render it nutritious in itself. When thus occurring in strata of a fossil 

 or sub-fossil character, the Diatomaceous deposits are generally distin- 

 guishable as white or cream-colored powders of extreme fineness. 



300. For collecting fresh Diatomacece, those general methods are to 

 be had recourse to which have been already described ( 269). "Their 

 living masses," says Prof. W. Smith, "present themselves as colored 

 fringes attached to larger plants, or forming a covering to stones or rocks 

 in cushion-like tufts or spread over their surface as delicate velvet or 

 depositing themselves as a filmy stratum on the mud, or intermixed with 

 the scum of living or decayed vegetation floating on the surface of the 

 water. Their color is usually a yellowish-brown of a greater or less in- 

 tensity, varying from a light chestnut, in individual specimens, to a shade 

 almost approaching black in the aggregated masses. Their presence may 

 often be detected without the aid of a microscope, by the absence, in many 

 species, of the fibrous tenacity which distinguishes other plants: when 

 removed from their natural position they become distributed through the 

 water, and are held in suspension by it, only subsiding after some little 

 time has elapsed." Notwithstanding every care, the collected specimens 



