PI. nr. OR POISONED BY VEGETABLE GROWTH? 179 



metres.' This is cat the rate of four miles in a 

 hundred years. Let us suppose the Adriatic to be filled 

 up. In this case, would the alluvial plain be a dead 

 level, or would it slope? I think it w^ould slope from 

 the head of the Adriatic to the Mediterranean. If so, 

 as the level of the sea would prevent a fall^ it must rise 

 from the Mediterranean to the present mouth of thePo. 

 This rise, at eight inches a mile, would be considerable. 

 Nor would it stop there. It would be continued up the 

 course of the river to where its unalluvial bed is above 

 this gradient. If this is true, those who have under- 

 taken to embank the Po and the Adige are longi lahoris 

 damnati. 



Indeed, in principle^ it is impossible, under any cir- 

 cumstances, that strata deposited by ponding (which 

 gives the longitudinal slope), or by moving water from 

 materials held in suspension (which gives the lateral 

 slope in alluvial plains) should be absolutely horizontal. 

 They must die out : the first in the contrary, the second 

 in the same direction as that in which the water moves ; 

 although to almost all 'practical intents they generally 

 may be reckoned horizontal, that is, reckoning time 

 and space huraanly. Geologically, the effect of this 

 slight difference between principle and practice, in such 

 a basin as the Mississippi, may expose millions of men 

 to the constant chance of death to themselves and de- 

 struction to their property from inundation, with the 

 certainty that their actual estates are in process of be- 

 coming subterranean, and themselves and their works 



N 2 



