MISSISSIPPI DELTA. 325 



for the formation of the whole ; and if the alluvial matter 

 of the plain above be 264 feet deep, or half that of the delta, 

 it must have required 33,500 more years for its accumulation, 

 even if its area be estimated as only equal to that of the 

 delta, whereas it is, in fact, larger/' Moreover, as Sir 

 Charles has himself pointed out, a very large proportion of 

 the mud brought down by the river is not deposited in the 

 delta, but is carried out into the Gulf. In the " Antiquity of 

 Man/'* he refers to the above-given calculation, and admits 

 that the discharge of water seems to have been much under- 

 % rated by the earlier 'experimenters. Messrs. Humphreys and 

 Abbot, who have recently surveyed the delta, " also remark 

 that the river pushes along its bottom into the gulf a certain 

 quantity of sand and gravel/' which "would, they suppose, 

 augment the volume of solid matter by about one-tenth." 

 This, of course, would greatly diminish the time required ; 

 but taking into consideration the quantity of mud which is 

 carried out to sea, and which was not allowed for in the 

 previous calculation, Sir Charles Lyell still regards 100,000 

 years as a moderate estimate ; and he considers, that " the 

 alluvium of the Somme containing flint implements and the 

 remains of the mammoth and hyaena," is no less ancient. 



Sir C. Lyell has alsof attempted to form an estimate of 

 the duration of the glacial epoch, assuming that the different 

 movements of elevation and depression proceeded at an average 

 rate of 2J feet in a century. As the simplest "series of 

 changes in physical geography which can possibly account 

 for the phenomena of the glacial period," he gives the fol- 

 lowing : 



"First, a continental period, towards the close of which 

 the forest of Cromer flourished : when the land was at least 

 500 feet above its present level, perhaps much higher, and 



* Appendix to Third Ed. of the Antiquity of Man, p. 16. 

 t Antiquity of Man, pp. 282, 286. 



