SEDIMENT OF THE PO. 275 



the depth of fifteen yards.* There are very large portions of thia 

 area, where, as we know from ancient remains — roads, bridges, 

 and the hke — from other direct testimony, and from geological 

 considerations, very Httle degradation has taken place within 

 twenty centuries, and hence the quantity to be assigned to locali- 

 ties 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 geological 

 causes, but it would reduce the length and diminish the depth of 

 ravines of later formation, modify the inchnation of their walls, 

 reclothe with earth many bare mountain ridges, essentially change 

 the hne of junction between plain and mountain, and carry back 

 a long reach of the Adriatic coast many miles to the west.f 



* 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 hectares, 

 that is, 4,105,600 in mountain lands, 3,832,600 in plain lands. — DuiiONT, 

 Travaux Publics, etc., 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. 



The valley of the Po is estimated to contain 587,946 hectares of rock and 

 glacier, 1,131,288 of forest often very thinly wooded, 789,999 bare of vegeta- 

 tion, and 209,107 capable of being wooded. GiornaU di AgricoUura, April, 

 1874. 



f I do not use these quantities as factors the value of which is precisely ascer- 

 tained ; nor, for the purposes of the present argument, is quantitative exact- 

 ness important. I 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 generalibus, and Arthur Young is not 

 the only public economist who has warned his readers against the deceitf uhiess 

 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. 

 In all the great operations of terrestrial nature, the elements are so numerous 

 and so difficult of exact appreciation, that, until the means of scientific obser- 

 vation and measurement are much more perfected than they now are, we must 

 content ourselves with general approximations. I say terrestrial nature, be- 

 cause in cosmical movements we have fewer elements to deal with, and may 

 therefore arrive at much more rigorous proportional accuracy in determina- 

 tion of time and place than we can in fixing and predicting the quantities and 

 the epochs of variable natural phenomena on the earth's surface. 



Travellers are often misled by local habits in the use of what may be called 



