272 ]Mr. A. Tylor on Changes of the Sea-Level effected by 



the surface of the more ancient formation upon which the Mis- 

 sissippi has formed this great alluvial deposit, the bottom of 

 which is now more than 500 feet below the present sea-level. 



Mr. Charles Ellet, Jun., in a Report to the American Secre- 

 tary at War, January 29, 1851, communicates the information 

 from which the diagrams figs. 1 and 2 are constructed. See p. 259. 



The theory of Mr. C. Ellet is, that the velocity of the stratum 

 of fresh water (fig. 1) is communicated entirely to the underlying 

 stratum, composed of salt water, partially to the next stratum 3, 

 but not at all to stratum 4, which is stationary : stratum 5 is also 

 marine, but it flows in an opposite direction to the rest, and 

 restores the salt water which is carried away by the friction of 

 the upper stratum, No. 1, against the surface of No. 2. 



It is supposed that the rapid increase of deposit at the bar, 

 fig. 1, arises from stratum No. 5 carrying mud to that point, 

 where its velocity is partially neutralized by impinging against 

 stratum No. 1. 



From the following particulars of the deltas of the Ganges and 

 Po, it would appear that they are similarly situated to the Missis- 

 sippi. " An Artesian well at Fort William near Calcutta, in the 

 year 1835, displayed at a depth of 50 feet a deposit of peat with 

 a red-coloured wood similar to that now living. At 120 feet 

 clay and sand with pebbles were met with. At the depth of 350 

 feet a freshwater tortoise and part of the humerus of a ruminant 

 were found. At 380 feet, clay with lacustrine shells was incum- 

 bent upon what appeared to be another dirt-bed or stratum of 

 decayed wood. At 400 feet they reached sand and shingle*.^' 



In the delta of the Po, a well bored 400 feet failed to pene- 

 trate the modern alluvial deposit ; very near the bottom it pierced 

 beds of peat, similar to those now forming. The coarser par- 

 ticles of mud which have already passed the mouths of rivers 

 may contribute to the marine or fluvio-marine deposits forming 

 outside deltas ; but this can only be to a limited extent, as the 

 great bulk of the mud is far too fine to settle near the coast. 

 Little material could be obtained from cliffs along the sea coasts, 

 but we have information of marine currents specially bringing 

 sand and mud from other parts of the sea-bottom to the neigh- 

 bourhood of deltas. (See Mr. EUet's observations.) 



For these reasons, if the further examination of the deltas of 

 the Mississippi and other rivers should lead to the discovery of 

 some recent marine or fluvio-marine strata, it may turn out that 

 such deposits have been more rapidly accumulated than the 

 purely fluviatile beds with which they may be associated. In 

 estimating the age of deltas, allowance, however, ought to be 

 made for such contingences, and also for their organic contents. 

 * Lyell, loc. cit. p. 248 ; and Principles, p. 267-270. 



