DIATOMACE^E. 97 



Sir John Ross and other Arctic explorers speak of a large 

 bank called the Victoria Barrier, 400 miles long, and 120 

 miles wide, composed almost entirely of infusoria. Dur- 

 ing the last week I was engaged in examining a sounding 

 from the bottom of the ocean at the depth of 2000 

 fathoms, on the exact spot where the Atlantic telegraph 

 unfortunately gave way; although the quantity was 

 minute, still I discovered a great number of interesting 

 forms. What is known as Tripoli powder in the arts 

 consists almost entirely of fossil deposits of the siliceous 

 coats of diatoms, which from their hardness form an 

 excellent means of polishing metals ; these fossil deposits 

 are very numerous and in great quantity in different parts 

 of the world. The town of Richmond, in the United 

 States, is built upon a stratum of these bodies twenty feet 

 in thickness ; in California and America generally, in 

 Bohemia, throughout Europe and Africa, and even in our 

 own country, we find similar deposits, varying of course 



in the different species present I have been 



enabled to examine some of the curious raised fossil beach 

 near Copiapo in Chili, which is gradually forming into 

 stone. Though this beach is one mile from the present 

 shore, and 180 feet above the level of the sea, yet I have 

 found in it diatoms of the same species as those that 

 occur on the shore at the present day ; the diatoms are 

 also found in a fossil state in peat, coal, bog iron-ore, 

 flint, and the chalk formation. Thus, in a geological view, 

 though individually invisible, yet numerically they per- 

 form a most important part in the crust of the earth 



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