592 DESMIDS AND DIATOMS, 
dimensions, occupying probably an area of 400 miles long 
by 120 wide, resting in part upon a glacier 400 miles in 
length, and in part upon the submarine flanks of Mount 
Feebua, an active voleano 12,000 feet in height! Finally, 
Ehrenberg considers that at Pillau, in PE Ai , “there 
are annually deposited from the water from 7,200 to 14,- 
000 cubic metres of fine microscopic organisms, which, in 
the course of a century, would give a deposit of from 
720,000 to 1,400,000 cubic metres of Infusory-rock or 
Tripoli-stone.” 
So much for the rapidity of growth and the physical 
importance of the Diatoms in our own era; let us now 
glance for a moment at earlier periods, and see whether 
these minute organisms were then too at work, producing 
results at all comparable to those which we witness at the 
present day. 
To begin with the more recent geological pent i 
Every tyro in microscopic inquiry has, among his other 
curiosities, obtained at least one slide with the label 
“Fossil Infusoria.” These are Diatomacez, and the de- 
posits from which they are derived may be found in all 
parts of our country, cropping out on the borders of 
ponds, or underlying layers of peat. It is, however, 
often a matter of doubt, especially in the former case, 
where the forms of the deposit and those still living in 
the water are apparently identical, how far these may 
“Sealy be entitled to the designation of “fossils.” That 
ey are so in many cases, and almost always when un- 
derlying beds of peat, is shown by the entire absence in 
these latter of certain species (especially Witschia and - 
dra), while these species are growing in the waters 
meron eS a profusion. The period 
