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.), and 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- 
