DESMIDS AND DIATOMS. 591 
or less limited duration, among the Diatoms the new cells 
become new individuals ; and though, as living forms, their 
duration is brief, yet incorporating as they do into their 
tissues the almost indestructible element, silica, to a 
greater extent than in any other group of organisms, they 
become as it were petrified, even while still alive, and at 
death leave behind relics, minute indeed, but imperish- 
able, the most perfect of fossils, in which every groove 
and marking of their former selves is accurately and 
beautifully preserved. 
We have, then, only to reflect for a moment upon the 
almost universal distribution of the Diatomacez, to un- 
derstand how, by rapid growth and the formation of 
indestructible remains, they may readily become of great 
importance in a physical and geological point of view. 
They are found alike in fresh, salt, and brackish water ; in 
moist earth and in tidal muds; in hot springs and in river 
ice, from the poles to the equator, coloring vast tracts of 
the surface of the sea, as well as composing the great 
bulk of the ocean’s bed. Even in the lava and cinders 
of volcanoes their presence has been recognized, and 
they form a large portion of the dust-showers and “blood- 
rains” formerly so dreaded, and which cover at times with 
powder the sails of ships at sea. Mr. Roper, an English 
microscopist, tells us, that, excluding coarse sand, one- 
fourth of the finer part of the residuum of the mud of the 
Thames is composed of the silicious remains of the Diato- 
macex, and expresses his belief that their silicious shells 
“have a perceptible influence in the formation of shoals 
and mud-banks.” Dr. Hooker, again, in speaking of the 
results obtained by the Antarctic Expedition, observes that 
sare abound in the newly-formed ice of the Polar Seas, 
ucing by their death a submarine deposit of vast 
