170 MEMOIRS OF THE NATIONAL ACADEMY OF SCIENCES. 



dasks by successive freshenings. This picking up froiii withiu the river aud the iuner slope of the 

 bar is by water already mucli heavier loaded with clay thau was that from which the low-water 

 deposition took place. Abrasion withiu the channel olteu takes place rapidly where some change 

 of current occurs, aud deep places are locally excavated to such depths that the jetties have to be 

 protected from undermining by temporary or permanent wing-dams. The high water of the year 

 of my visit had excavated one place to the depth of 120 feet wheie the water had been very shoal 

 before the building of the jetties. A short wing-dam at the time of my visit was directing the 

 low- water deposit into this hole, and its depth had already been reduced 40 feet. 



Some of the specimens exhibited to the Academj- are of mud dredged from the inner slope of 

 the bar just within the jetties, when the turbid fresh water formed a layer 14 feet deep over the 

 clearer salt water then on the bottom of the river chauuel. The material is a very unctions, 

 tenacious clay, which becomes very tough on drying. It is as smooth as soap in the hands, but 

 gritty with \ ery line sand between the teeth. The microscope shows the sand to be \ery minute aud 

 the finer grains very active with "Brownian movements." When agitated in fresh water a large 

 quantity of this mud is suspended, and the liquid remains opaquely muddy for a comparatively 

 long time, but it tlocculates and settles quickly when mixed with sea-water. 1 have made many 

 experiments with the several muds obtained at and near the jetties, aud with river water obtained 

 above the passes, where the water was entirely fresh, and all show essentially the same phenomena 

 when in waters of the same freshness. 



My belief is that the deposition in the channel within the mouth at low water is directly aud 

 chiefly due to chemical causes; that the scouring out at time of high water is also due to chemical 

 causes, the velocity of the current being secondary ; that a given volume of salt water, having a 

 given velocity, will not suspend and transport an amount of clay which the same volume of fresh 

 water, having the same velocity, would suspeud aud transport indefiiutely ; that fresh water with 

 a given velocity will pick up from the bottom and scour out deposits which salt water of the same 

 velocity will not, uotwithstanding its greater specific gravity. 



This is not only in accordance with my experiments, but it seems to me to be abundantly illus- 

 trated in the delta phenomena of the Mississippi. The local abrasions between the jetties at floods 

 show what has occurred at successive steps all the way to the head of the delta. When the Gulf 

 extended much farther inland, then the water was shallow both inside aud outside the mouth, as 

 it is uow, but as the delta grew aud the bar was "pushed out iuto the Gulf" by successive floods, 

 the deep liver chaunel followed it up from behind, the mud being picked uj) and carried out as the 

 water freshened, just as it now is done between the jetties. From above New Orleans to near the 

 head of the passes, so far as the water continues fresh at all times of the year, the channel is deep, 

 usually more thau 70 and often 100 or 120 feet deep; but; all this distance the water must have been 

 shallow when the mouth of the river was at the successive points. The shoaling of the river 

 from where the water becomes salt, the upward slope from the head of the passes to the crest of the 

 l)ar, up which slope the river must ruu at flood aud down which the heavier salt water runs when 

 not crowded out by floods, the deposition on the outer slope of the bar at high water and on the 

 iuner slo])e at low water, the deposit within the mouths when enough salt water gains adniissiou 

 there, and the scouring out of this again as the river freshens are all in strict accordance with 

 this theory. My experiments explain i)henomena the causes of which have heretofore been so 

 much in disi)ute between the engineers who have discussed the improvement of the mouth for 

 navigation. 



Some of these pheuomeua were observed iu connection with this very question so long ago as 

 1838 by W. H. Sidell, who experimented upon the action of acids, various salts, sea-water, &c., 



