Fossil diatoms 111 



organic Eiibstance in the sea, and the life in the sea is therefore very largely 

 dependent upon the inconceivable numbers of plankton-diatoms. In many 

 of them the cell-walls are thin and rather delicate, with only a slight 

 impregnation of silica. 



The valves of diatoms of both the plankton and the benthos are found in 

 quantity in the alimentary tracts of Molluscs, Tunicates and Fishes. They 

 are also found in abundance in Guano, having passed through the digestive 

 tracts of birds which feed on marine animals. 



Some of the freshwater species, notably Asterionella, are sometimes the 

 cause of foulness of drinking water (Whipple & Jackson, '99). This is due 

 to the escape from the dead frustules of the oily products of metabolism. 

 Such foulness can be obviated by storing water in the dark, or by treatment 

 with copper sulphate in the proportion of not more than one part in two 

 million parts of water. 



FOSSIL DIATOMS. Great accumulations of diatoms are now being formed 

 on the floor of both the Arctic and Antarctic Oceans, and beds are also 

 accumulating in many of the larger freshwater lakes. Not only are these 

 minute plants actively engaged at the present time, however, in forming 

 oceanic and lake deposits, but the numerous Diatomaceous Earths bear 

 testimony to their activity in former ages. These earths are mostly of 

 a white or grey colour, sometimes hard, but more often so friable as to 

 crumble readily between the fingers, and they are composed almost entirely 

 of the siliceous valves of diatoms. Thus, large numbers of fossil diatoms are 

 known. They may have had a marine or a freshwater origin, and most 

 of the forms contained in the deposits belong to genera, and many of them 

 to species, now living. The deposits are for the most part relatively recent, 

 being principally associated with rocks belonging to the Tertiary formations. 

 Some of them are of economic importance, being used as polishing powders 

 (' Tripoli '), as non-conducting materials in the manufacture of fire-proof and 

 sound-proof partitions, as absorbents for nitre-glycerin in the manufacture 

 of dynamite (' Kieselguhr '), as constituents of dentifrices, and for other 

 purposes. In deposits of this kind there are usually a number of species, 

 but one or more may be dominant, forming the great bulk of the material. 



Diatomaceous Earths have been found in many parts of the world, notably 

 in Hungary, Bohemia, the United States, Barbados, and Trinidad. In the 

 British Islands the best-known deposits are those at Dolgelly in Wales, and 

 at Toome Bridge in Antrim, Ireland. The famous deposit at Biln, in 

 Bohemia, which averages 14 feet in thickness, was estimated by Ehrenberg 

 to contain some 40,000,000 frustules of diatoms in every cubic inch. It 

 attains in places a thickness of 50 feet and is hard and flinty, the cementing 

 silica having been formed by the solution of some of the diatom valves. The 



