198 SEAWEEDS 



They are not only engaged in forming oceanic 

 deposits at the present day, but the fossil diatom- 

 aceous earths of Tertiary and Quaternary age are 

 witnesses to their activity in past ages. The first 

 occurrence of fossil Diatoms is in the chalk, and here, 

 illustrating the persistence of the type, there are 

 preserved species still existing in the waters. The 

 extensive fossil deposits which are used as polishing 

 powders (tripoli), in the manufacture of dynamite 

 (Kieselguhr, &c.), arid for other purposes, are in- 

 teresting from the fact that though many species 

 occur in them, one is always predominant, and it, or 

 at most a few, form the mass of the particular 

 deposit. All these fossil forms belong to genera and 

 a large number of them to species now living. Some 

 deposits have been laid down in fresh-water, others in 

 salt-water, and the latter contain species that are 

 still marine. The so-called edible earths of China 

 and Japan, of Siberia, Lapland, &c., which are' 

 mixed with meal and so eaten, are of diatomaceous 

 origin. 



The earlier rocks have been very carefully searched, 

 notably the Silurian, for fossil Diatoms, but without 

 any trustworthy record. Castracane has stated that 

 he found several species in the ash of English coal, 

 but though he claims to have used precautions, the 

 record is open to doubt, since an exhaustive and 

 fruitless search has been made in many coals by very 

 careful observers. The fact that Castracane's Diatoms 

 are fresh-water forms now living adds to the doubt, 

 from the possibility of a mistake in manipulation thus 

 suggested, in spite of what we know of the persist- 



