248 COMPUTATION OF AGE [CHAP. XXXIV. 



was as constantly raised by the accession of fluviatile 

 sediment, so as to prevent any incursion of the sea. 

 Occasionally there were pauses in the downward 

 movement, when trees grew on the soil, and vege 

 table matter of some thickness had time to ac 

 cumulate. 



Recent observations, by Morlot and others, have 

 demonstrated that, since the time of the Romans, 

 there has been a general subsidence of the coast at 

 the head of the Adriatic, to the amount of five feet, 

 which has not prevented the delta of the Po and 

 other rivers from advancing on the sea, although it 

 must have checked their progress. Of the much 

 greater movements of elevation and depression which 

 have taken place in the delta of the Indus, especially 

 those wrought in the year 1819, I have elsewhere 

 given an account.* It would, therefore, be perfectly 

 consistent with analogy to find, in the neighbour- 

 hood^of New Orleans, ancient swamp formations, with 

 the roots and stumps of erect trees, unmixed with 

 marine remains, far below the level of the sea, as is 

 the fact, if I can rely on the information given me 

 in 1846. t 



Finding it impossible to calculate the age of the 

 delta, from the observed rate of the advance of the 

 land on the Gulf in each century, I endeavoured 

 to approximate, by a different method, to a minimum 

 of the time required for bringing down from the 

 upper country that large quantity of earthy matter 

 which is now deposited within the area of the delta. 



* Principles, Seventh Edition, p. 437. t See ant e, p. 137. 



