622 MICROSCOPIC FORMS OF VEGETABLE LIFE THALLOPHYTES 



accumulation 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 J)iatomacece, are found to alter- 

 nate, in the neighbourhood of the Mediterranean, with calcareous 

 strata chiefly formed of Foraminifera, 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 under- 

 gone conversion into flint, by agencies hereafter to be considered. 

 Of the diatomaceous composition of these strata we have a character- 

 istic example in fig. 467, which represents the fossil Diatomacece of 

 Oran in Algeria. The so-called ' infusorial earth ' of Richmond in 

 Virginia, and also that of Bermuda, both marine deposits, are very 

 celebrated among microscopists for the number and beauty of the 

 forms they have yielded ; the former constitutes a stratum of eighteen 

 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 North Wales, at South 

 Mourne in Ireland (fig. 468), and in the island of Mull in Scotland. 

 Similar deposits in Sweden and Norway are known under the name 

 of Bergmehl, or mountain -flour ; and in times of scarcity the inha- 

 bitants 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 

 Bergmehl 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 distinguishable as 

 white or cream-coloured powders of extreme fineness. 



For collecting fresh Diatomacece those general methods are to be 

 had recourse to which have been already described. * Their living 

 masses,' says Mr. W. Smith, ' present themselves as coloured 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 inter- 

 mixed with the scum of living or decayed vegetation floating on the 

 surface of the water. Their colour is usually a yellowish-brown of a 

 greater or less intensity, 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 are liable to be 

 mixed with much foreign matter ; this may be partly got rid of by 



