Part III.] OR POISONED BY VEGETABLE GROWTH? 
203 
prevent a fall , it must rise from the Mediter¬ 
ranean to the present mouth of the Po. This 
rise, at three inches a mile, would be considerable. 
Nor would it stop there. It would be continued 
up the course of the river to where its natural 
bed is above this gradient. If this is true, 
those who have undertaken to embank the Po 
and the Adige are longi laboris damnati. 
Indeed, in principle , it is impossible, under 
any circumstances, that strata deposited by 
moving water from materials held in suspension 
should be absolutely horizontal. They must die 
out in the direction in which the water moves; 
although to almost all practical intents they gene¬ 
rally may be reckoned horizontal, that is, reckon¬ 
ing time and space humanly . 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 destruction to 
their property from inundation, with the cer¬ 
tainty that their actual estates are in process of 
becoming subterranean, and themselves and 
their works fossil. 
I only talk of general tendencies. In such 
river basins as have been mentioned, these ope¬ 
rations are on so vast a scale, they are spread 
