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minute and apparently insignificant in comparison with the majestic 
trees that in all time have composed, and the huge animals that 
have tenanted, the forests of our planet, yet by their excessive 
multiplication and accumulation, the Diatomaceze assume even a 
greater importance in the physical history of the earth than any of 
these. 
The truth of this statement is proved by the fact of living 
examples of these tiny atoms forming appreciable masses in an 
inconceivable short space of time, and also by innumerable 
instances where only their siliceous skeletons remain in a fossil, or 
a semi-fossil, condition. Numerous instances are to be met with 
illustrative of the wide and rapid diffusion of the Diatomacez. So 
great is their abundance in many rivers and estuaries, that their 
multiplication is affirmed by Professor Ehrenberg to have exercised 
an important influence in blocking up harbours and diminishing 
the depths of channels. This statement is confirmed by Mr. 
Roper, who says that the shells of diatoms have a perceptible 
influence in the formation of shoals and mud-banks in the bed of 
the Thames. : 
On the coast of Victoria Land, Dr. Hooker found a deposit 
of mud, chiefly consisting of their shells, not less than four hundred 
miles long and one hundred miles broad. Of the thickness of 
this deposit no conjecture could be formed; sometimes the lead 
sank in it to a depth of two feet. 
The siliceous nature of this group of plants has served to 
preserve and to perpetuate their presence in numerous localities 
all over the world, where now, even the conditions essential to 
their existence as living organisms are no longer found. 
In these places they occur as soft white creamy or faint reddy- 
blue coloured rock. Lough Mourne, Dolgethelly, and Toomebridge 
in Ireland ; Holderness and Ulverstone in England ; Peterhead in 
Scotland ; Bilin in Bohemia; and Oran in Algiers, are a few such 
instances. But by far the largest deposits are found in America. 
The river Columbia, in one part of its course, runs between two 
precipices seven hundred or eight hundred feet high, which are 
composed of porcelain clay five hundred feet thick, covered by a 
