ANNIVERSARY ADDRESS OF THE PRESIDENT. xHu 



is true that the gradual elevation of that plain by new accessions of 

 matter, would tend to eiface every inequality derived from this source ; 

 but we might certainly have expected to find more broken ground 

 between the opposite bluffs, had local upthrows of alluvial strata been 

 of repeated occurrence*." 



Of the depth of the alluvial deposits spread over this vast region 

 we as yet know little. Mr. Lyell was informed by several engineers 

 that borings 600 feet deep were made near Lake Pontchartrain, 

 north of New Orleans, in which the bottom of the alluvial matter is 

 said not to have been reached. It is possible that the upper part of 

 the plain may have been formed by the gradual raising of the river's 

 bed by accumulating detrital matter, as that of the Po and other 

 rivers has been ; but it is probable, I think, that the greater part has 

 been formed by the discharge of the alluvial matter into the sea, at the 

 mouth of the river, together with deposits on each side when the stream, 

 swollen by floods, periodically overflowed its banks : that at one time 

 this basin of the Mississippi was an arm of the sea, penetrating into the 

 land, which has been gradually filled up, the mouth of the river ad- 

 vancing as the accumulation went on, to its present position. If the 

 alluvial matter be of such vast thickness, as the sinkings at Lake 

 Pontchartrain seem to indicate, I do not see how it could have been 

 accumulated in any other way, unless indeed we suppose an uninter- 

 rupted slow subsidence of the valley for a vast period of time. This 

 supposed bay or deep inlet may have been of considerable depth 

 throughout its whole length, or it may have had a bottom gradually 

 shallowing northward ; and the nature of the bottom, whether 

 deep or shelving, would of course determine the amount of thick- 

 ness of the accumulating matter, and the rate of advancement of 

 the mouth of the river southward. As a basis for calculating the 

 time that may have elapsed since the alluvial plain began to be 

 formed, Mr. Lyell assumes that the newly deposited soil has a depth 

 over the whole area of the delta, comprising 13,600 square miles, 

 equal to that which has been penetrated vertically north of New 

 Orleans, or 600 feet ; that is, similar to the average depth which 

 has been ascertained to prevail in the waters of the Gulf of Mexico, 

 between the southern point of Florida and the Balize, or mouths of 

 the Mississippi ; but for the sake of facility of calculation, he assumes 

 it to be one-tenth of an English mile, or 528 feet. 



From experiments made by Dr. Riddell of New Orleans, Mr. 

 Forshey, and Dr. Carpenter, on the mean annual solid contents sus- 

 pended in the water of the Mississippi, and from observations on its 

 mean width, depth and velocity, and thence the mean annual dis- 

 charge of water, the number of cubic feet of solid matter annually 

 brought down by the river has been estimated ; that is, the finer 

 matter only, that which is suspended, and not taking into account 

 the coarser materials, which, throughout the delta and over a great 

 part of the plain above, from its very slight inclination, would pro- 

 bably amount to very little. The estimated annual quantity being 

 spread over the computed area of the delta, that is, 13,600 square 

 * Ibid, page 216. 



