SEDIMENT OF THE PO. 269 



removal of the deposit I have described between the two dams m 

 a single freshet, shows that, in spite of considerable obstruction 

 from roughness of bed, large quantities of sand may be taken up 

 and carried off by streams of no great rapidity of inchnation ; for 

 the whole descent of the bed of the river between the two dams 

 — a distance of four miles — is but sixty feet, or fifteen feet to 

 the mile.* 



The facts which I have adduced may aid us in forming an idea 

 of the origin and mode of transportation of the prodigious de- 

 posits at the mouth of great rivers Kke the Mississippi, the Nile, 

 the Ganges and the Hoang-Ho, the delta of which last river, 

 composed entirely of river sediment, has a superficial extent of 

 not less than 96,500 square miles. But we shall obtain a clearer 

 conception of the character of this important geographical pro- 

 cess by measuring, more in detail, the mass of earth and rock 

 which a well-known river and its tributaries have washed from 

 the mountains and transported to the plains or the sea, within 

 the historic period. 



The Po and its Deposits. 



The current of the River Po, for a considerable distance after 

 its volume of water is otherwise sufficient for continuous naviga- 

 tion, is too rapid for that purpose until near Cremona, where its 

 velocity becomes too much reduced to transport great quantities 

 of mineral matter, except in a state of minute division. Its 

 southern affluents bring down from the Apennines a large quan- 

 tity of fine earth from various geological formations, while its 

 Alpine tributaries west of the Ticino are charged chiefly with 

 rock ground down to sand or gravel. The bed of the river has 

 been somewhat elevated by the deposits in its channel, though 

 not by any means above the level of the adjacent plains, as has 

 been so often represented. The dikes, which confine the current 

 at high water, at the same time augment its velocity and compel 



* In a sheet-irou siphon, 1,000 feet long, -with a diameter of four inches, 

 having the entrance 18 feet, the orifice of discharge 40 feet, below the summit 

 of the curve, employed in draining a mine in California, the force of the cur- 

 rent was such as to carry through the tube great quantities of sand and coarse 

 gravel, some of the grains of which were as large as an English walnut.— 

 RA.YMOND, Mining Statistics, 1870, p. 603. 



