SEDIMENT OF THE PO. 283 
northern and north-eastern declivities of the Apennines——have 
annually sent down into the lakes, the plains, and the Adriatic, 
not less than 375,000,000 cubic yards of earth and disintegrated 
rock. We have, then, an aggregate of 750,000,000,000 cubic 
yards of such material, which, allowing to the mountain surface 
in question an area of 50,000,000,000 square yards, would 
cover the whole to the depth of fifteen yards.* There are very 
large portions of this area, where, as we know from ancient 
remains—roads, bridges, and the like—from other direct. testi- 
mony, and from geological considerations, very little degrada- 
tion has taken place within twenty centuries, and hence the 
quantity to be assigned to localities where the destructive causes 
have been most active is increased in proportion. 
If this vast mass of pulverized rock and earth were restored 
to the localities from which it was derived, it certainly would 
not obliterate valleys and gorges hollowed out by great geo- 
logical causes, but it would reduce the length and diminish the 
depth of ravines of later formation, modify the inclination of 
their walls, reclothe with earth many bare mountain ridges, 
essentially change the line of junction between plain and 
mountain, and carry back a long reach of the Adriatic coast 
many miles to the west.t 
It is, indeed, not to be supposed that all the degradation of 
* The total superficies of the basin of the Po, down to Ponte Lagoscuro 
[Ferrara]—a point where it has received all its affluents—is 6,938,200 hec- 
tares, that is, 4,105,600 in mountain lands, 2,832,600 in plain lands.— 
Dumont, Travaux Publics, ete., p. 272. 
These latter two quantities are equal respectively to 10,145,348, and 
6,999,638 acres, or 15,852 and 10,937 square miles. 
+ I do not use these quantities as factors the value of which is precisely 
ascertained ; nor, for the purposes of the present argument, is quantitative 
exactness important. JI employ numerical statements simply as a means of 
aiding the imagination to form a general and certainly not extravagant idea of 
the extent of geographical revolutions which man has done much to accelerate, 
if not, strictly speaking, to produce. 
There is an old proverb, Dolus latet in generakibus, and Arthur Young is not 
the only public economist who has warned his readers against the deceitfulness 
of round numbers. I think, on the contrary, that vastly more error has been 
produced by the affectation of precision in cases where precision is impossible. 
