256 FIRST FORMS OF VEGETATION. 
inconceivable multitudes amid the mud and 
detritus deposited by rivers at their mouths, and 
by the accumulation of their exuvie, year after 
year, occasion a vast deal of labour and cost to 
the dredger. The mud of the Nile and the 
Ganges, which have formed the great deltas of 
Egypt and Bengal, is full of them. Naturalists, 
who have explored the virgin forests of the 
tropics, inform us that the very branches of the 
trees are covered with vast numbers of them. 
They have been discovered in the stomach of 
the oyster, the clam, and the barnacle ; and Dr. 
Hooker says, in the Botany of the Antarctic 
Voyage, that the stomachs of the salpe and 
other molluscous animals, which were washed up 
in immense masses on the ice, invariably con- 
tained several species of diatoms. On the soil 
of our fields they occur in myriads among guano, 
the product of those vermivorous shore-birds 
which inhabit the desolate islands of the South 
Seas; and on the tops of the highest British 
mountains—Ben Lawers, Ben Nevis, and Ben 
Macdhui—I have repeatedly gathered them in 
great quantities from the black mud which is 
generally found under masses of melting snow. 
The ice-bound seas of the north are peopled by 
them. They form the brown staining matter of 
the ‘rotten ice’ so well known to all Northern 
