AND OTHER WATER WAVES 145 



than this at a depth of 600 feet. The line of 600 



feet, 



depth 



100 fathoms, is a very important 

 in the seas, for it is approximately that to which 

 the edges of the continents are submerged. From 

 this line the sea bottom descends abruptly to the 

 vast depths of the great plains where the soundings 

 are reckoned in thousands of fathoms. 



At such depths the agitation caused by wind- 

 waves must be absolutely insensible, but within 

 depths of 100 fathoms, i.e., on the continental plat- 

 form, continual slight oscillations of 10-20 seconds 

 period, and even as little as i-inch amplitude, must 

 exert an effect in hindering the deposition of the 

 finest kinds of mud. Mud, or dust, when so finely 

 divided that it settles through air or through water 

 with indefinite slowness, only comes to rest where 

 the fluid is free from agitation, unless some agency 

 e.g., chemical or electrical precipitates the par- 

 ticles. Every student has noticed that it is not 

 the exposed papers which he is handling frequently 

 on his desk which become dusty, but those put 

 away e.g., on the top of a high bookshelf. 



We may say with confidence, as a theoretical 

 inference, that the agitation of wind-formed waves 

 affects the bottom of the sea as far as the edge 

 of the continental platform to such an extent as 

 (in co-operation with tidal and other currents) to 



