CHAP. XXXIV.] AGE OF DELTA OF MISSISSIPPI. 187 



readily be accounted for, by assuming that 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 in 

 cursion of the sea. Occasionally there were pauses in the down 

 ward movement, when trees grew on the soil, and vegetable mat 

 ter of some thickness had time to accumulate. 



Recent observations, by Morlat 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 ele 

 vation and depression which have taken place in the delta of the 

 Indus, especially those wrought in the year 1819, I have else 

 where given an account.^ It would, therefore, be perfectly con 

 sistent with analogy to find, in the neighborhood 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-f 



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 endeavored 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 depos 

 ited within the area of the delta. Dr. Riddell communicated to 

 me, at New Orleans, the result of a series of experiments which 

 he had made, to ascertain the proportion of sediment contained in 

 the waters of the Mississippi. He concluded that the mean an 

 nual amount of solid matter was to the water as r .^ 5 in weight, 

 or about ^ in volume. $ Since that period, he has made 

 another series of experiments, and his tables show that the quan- 



* Principles, Seventh Edition, p. 437. t See ante, p. 109. 



J The calculations here given, were communicated to the British Asso 

 ciation in a Lecture which I delivered at Southampton, in September, 1846. 

 See &quot;Athenaeum Journal,&quot; Sept. 26, 1846, and &quot;Report of British Asso 

 ciation,&quot; 1846, p. 117. 



