THE SUBSIDENCE OF PARTICLES IN LIQUIDS. 169 



Iiave experinieuted ii\Hm. I believe that the phenomena have an importance not heretofore given 

 to them either by geologists, physical geographers, or engineers. 



The formation of bars at the mouths and in the channels of rivers, and the distribution of 

 silt on the floor of the ocean and of lakes, have usually been discussed and considered from the 

 hy<lraulic side only, the direction and velocity of the cuirent have been considered iis the only 

 factors of any considerable importance, but 1 believe that the clicmical composition of the water 

 plays an essential and controlling part in the effects jiroduced. 



The phenomena attending the formation of bars at the months of rivers which empty into 

 fresh-water lakes, and the depositions which take place in the channels of rivers where the water 

 remains fresh all the year through, are very unlike those attending the formation of bars in salt 

 water at the mouths of muddy rivers, or the silting of the channel just within the mouths of such 

 rivers, where the water becomes brackish before reaching the sea. 



In fresh-water rivers and at their mouths in fresh-water lakes, the more obvious changes in the 

 bars take place only at the time of floods, and the movement and dejjosition of the material are in 

 strict accordance with hydraulic laws. The "hydiaulic value" of particles of known size and 

 specific gravity has been experimentally determined with great care and accuiacy. The deposi- 

 tion of the finer material takes place in the still waters, and at low water aiid is comparatively 

 slight in quantity in any one year. 



But when a muddy river enters salt water, chemical laws inteifere with the purely mechanical 

 ones, another set of phenomena are introduced, and the growth of the bar is different. Then the 

 rate of deposition is affected by the salt more than by the current, and velocities which would 

 be much more than sufficient to carry the finer suspended matter indefinitely if the water were 

 fresh, entirely fail where the water is brackish or salt. Practically it is the degree of saltness 

 which controls the deposition. 



In the phenomena exhibited at the mouth of the Mississippi River we see these principles 

 manifested on a stui)endous scale. 



At time of flood, when the whole water is fresh to the bottom of the river and to its very 

 mouth, then no considerable deposition occurs in the channel within the mouth ; the mud is carried 

 outside and largely deposited on the outer slope of the bar, notwithstanding the agitation by the 

 waves at that point. At the same time the channel within the mouth is scoured out and the 

 inside of the bar is more or less abraded, or, as various engineers have expressed it, the bar is 

 "pushed out into the Gulf" by each high water. 



At low water the salt water from the Gulf runs back into the river, first as a stratum on the 

 bottom, with a layer of the lighter fresh and muddy water over it ; then later the river becomes 

 brackish to its surface, this condition extending to the head of the passes, or further, according 

 to the season and the amount of water. Then a large deposition always takes place on the inner 

 slojje of the bar, and in the channel within the mouth, and up to the head of the passes. I was 

 told on the;, spot that this takes place every year, and that since the jetties have been built, the 

 required depth for navigation is only then maintained between the jetties by some dredging. As 

 the channel is closely watched and soundings made every week from the head of the passes to the 

 Gulf, the phenomena are easily studied. And let it be borne in mind that the time when this 

 deposit takes place within the mouth is when the proportion of mud to water is at the least, and 

 is very much less than at high water. 



When the floods of the next year come, and the waters again freshen to the bottom of the 

 river, this deposit is picked up again by this fresh water and is carried out to the salt water, as 

 already described. It is precisely analogous to the iiickiug up of the material iu the experimental 

 8. Mis. 110 22 



