SEAWEEDS AND PONDWEEDS 49 
in all waters, fresh and salt, but their chief interest lies 
in the fact that the cell-wall is always encrusted with 
silica, the mineral which is found almost pure in flint. 
In the diatoms this takes most beautiful symmetrical 
shapes, and this shell remains when the plant is dead, 
and sinks to the bottom of the water. Some idea of the 
enormous number that exist in the sea 
will be gained when you realise that 
though each cell is microscopic, fossil 
beds have been discovered in various 
parts of the world in which the diatoms 
lie to the depth of several feet. The 
most famous of these beds of “in- 
fusorial earth” is in America, near 
Richmond, the capital of the Southern 
States during the great war of North against South. This 
extends for many miles, and is on the average forty feet 
deep. Now the number of shells in two cubic feet has 
been estimated at twelve billion, so one can hardly con- 
ceive the number in a bed such as this. 
Another example of the group is the tiny alga, which 
gives rise to the appearance known as “ Red Snow.” This 
was first noticed in Savoy by De Saussure, in 1760, who 
failed altogether to account for it; but a much more 
splendid exhibition was seen in Greenland by Captain J. 
Ross, on the so-called Crimson Cliffs. Patient study 
with the microscope revealed the true nature of this 
extraordinary phenomenon. The whole surface of the 
snow is covered with the cells of one of the alge, the 
cells of which are permeated with the brilliant red. One 
asks how it can live in such a position, and the answer 
is that it dissolves the snow for water, and the rest of its 
_ nourishment it obtains from the dust blown by the wind 
E 
DIATOMS. 
