

Ch. XXII.] 



DEPOSITS AND THEIR WIDE DIFFUSION. 



573 





immense extent over which they may be the means of dif- 

 fusing homogeneous mixtures. Even off coasts where there 

 a re no large rivers, they may still have the power of spread- 

 ing far and wide over the bottom of the ocean, not only sand 

 and pebbles, but the finest mud. Thus for several thousand 

 m iles along the western coast of South America, comprising 

 the larger parts of Peru and Chili, there is a perpetual rolling 

 of shingle along the shore, part of which, as Mr. Darwin has 

 shown, "are incessantly reduced to the finest mud by the 

 waves,' and swept into the depths of the Pacific by the tides 

 and currents. The same author, however, has remarked that, 

 notwithstanding the great force of the waves on that shore, 

 all rocks 60 feet under water are covered by sea weed, show- 

 ing that the bed of the sea is not denuded at that depth, the 

 eflfects of the winds being comparatively superficial. 



In regard to the distribution of sediment by currents it 

 may be observed, that the rate of subsidence of the finer mud 

 carried down by every great river into the ocean, or of that 

 caused by the rolling of the waves upon a shore, must be ex- 

 tremely slow ; for the more minute the separate particles of 

 mud, the slower will they sink to the bottom, and the sooner 

 will 'they acquire what is called their terminal velocity. It 

 is well known that a solid body, descending through a resist- 



medium 



medium more and mor 



becomes 



exam] 

 >h air 



of 



to counteract the further increase of velocity. Foi 

 a leaden ball, one inch diameter, falling throu: 

 density as at the earth's surface, will never acquire greater 

 velocity than 260 feet per second, and, in water, its greatest 

 velocity will be 8 feet 6 inches per second. If the diameter 

 of the ball were T ^ T of an inch, the terminal velocities m air 

 would be 26 feet, and in water -86 of a foot per second. 



Now every chemist is familiar with the fact, that minute 

 particles descend with extreme slowness through water the 

 extent of their surface being very great in proportion to their 

 weio-ht, and the resistance of the fluid depending on the 

 amount of surface. A precipitate of sulphate of baryta, for 

 examnle. will sometimes require more than five or six hours 



