A. Tylor — Formation of Deltas. 493 



or above 1,000,000 years for a denudation of 100 feet under present 

 conditions, or 138,393 years on the hj'^pothesis of a Pluvial Period. 



If the whole area of the Mississippi basin was 600 feet higher on 

 the average above the sea than it is now, and glacial and pluvial 

 conditions exerted in greater force (indicated by the coarseness 7*3 

 times of the deposits), the rate of annual denudation would also be 

 much increased and the time shortened in proportion. 



It certainly appears by the diagram, made from soundings in the 

 Gulf of Mexico and Eiver Mississippi, that modern marine strata, 600 

 feet or more in thickness, have been deposited on the bottom of the Gulf 

 of Mexico, extending over some thousand square miles. I say modern, 

 for it must be identical in age with the Mississippi Delta deposit. 



According to Humphreys and Abbot, about one-eighth of a cubic 

 mile of detritus is annually transported to the Gulf of Mexico by the 

 Mississippi. They have calculated the rate of progress seaward of 

 the river, and extension of the Delta, as if the Delta had been re- 

 cently prolonged all along its outer edge, instead of at a mere point. 

 Sir C. Lyell has also drawn attention to the- narrowness of the 

 present deposit near the passes. It is clear that only a very narrow 

 tongue of land has been pushed out into the gulf by the river. The 

 river would have to change its course from time to time in order to 

 push out similar tongues of land round the Delta, which is a sector 

 of a circle ; and it would take evidently above 30 times as long to 

 add 50 miles all round the Delta as it has taken to make the single 

 strip of land, which contains the present river channel. 



There is also great probability that when the river changed its 

 course, the previous accumulation might be removed by the sea and 

 redeposited near the new channel. 



Messrs. Humphreys and Abbot only consider the superficial deposit 

 as belonging to the river, and speak of this exclusively as alluvium, 

 and as totally distinct from the beds of clay and thick sands which 

 occur below the alluvium, and which alluvium proper they describe 

 in different parts of the basin as from 20 feet to 60 feet thick at the 

 maximum. There seems as much distinction in mineral character 

 between the alluvium of the present river and the ancient deposits 

 below it, as there is between the peat and alluvium of England and 

 the gravels below them.^ This indicates, in the opinion of the 

 author, as great a change in the rainfall and climate in America as 

 in England, at a period not long antecedent to the true historical 

 period, in all probability in the proportion of 20 inches to 300 inches, 

 or even a still higher ratio. 



Sir C. Lyell explains the occurrence of decayed wood and of 

 freshwater and estuarine shells as far below the surface of Deltas 

 as 380 feet, in the following manner (page 247, vol. ii. second visit, 

 U.S.): — 'These appearances may readily be accounted for, by 

 assuming there was a gradual subsidence of the ground for ages, 

 which was as constantly raised by the accession of fluviatile sediment, 

 so as to prevent any intrusion of the sea. Occasionally there were 



1 M. Belgrand has, since the date on which this paper was written, 1868, confirmed 

 this view in his great work on the Seine. 



