154 THE WORLD'S LUMBER ROOM. 



extreme minuteness, they have formed whole beds of 

 silicious rock, such as that known as " Barbadoes earth," 

 which is almost entirely composed of their shields. 



Among the most important consumers of silica, how- 

 ever, are the microscopic plants called Diatomaceae (Fig. 5), 

 which swarm on the surface of the ocean, and are found, 

 more or less, in all waters, salt or fresh, and in all latitudes. 



In warm weather they sometimes form a mouldy cover- 

 ing, half an inch thick, on stagnant water; sometimes 

 they collect in a yellow-brown layer at the bottom of 

 ponds, or on water-plants, stones, mosses ; or they may be 

 found filling the towing-net in the Antarctic regions, and 

 staining the floating ice with an ochreous tinge. They 

 are one-celled plants, consisting usually, as their name 

 implies, of two symmetrical portions, or valves, which are 

 coated with pure silica. The variety in their shapes is 

 endless and wonderful, and almost all are delicately marked 

 and sculptured with bands of dots and lines. 



Yet in spite of the exquisite beauty of their glass cases, 

 they are so minute that 41,000,000,000 could be accommo- 

 dated in one cubic inch of space, and 186,000,000 would 

 weigh but a grain and look to the naked eye like a mere 

 pinch of dust. They multiply rapidly, however, and one may 

 become 8,000,000 in forty-eight hours, while in four days its 

 progeny will fill two cubic feet of space. 



It is these diatoms which are the main cause of the 

 silting up of harbours. At Wismar, on the Baltic, they 

 accumulate at the rate of 17,496 cubic feet annually, 

 enough to form a solid cube measuring twenty-six feet each 

 way. As they die, their cases fall through the water, in a 



