SEDIMENT OF THE PO. 277 
The facts which I have adduced may aid us in forming an 
idea of the origin and mode of transportation of the prodigious 
deposits at the mouth of great rivers like 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 tts Deposits. 
The current of the River Po, for a considerable distance after 
its volume of water is otherwise sufficient for continuous navi- 
gation, is too rapid for that purpose unti] near Cremona, where 
its velocity becomes too much reduced to transport great quan- 
tities of mineral matter, except in a state of minute division. 
Its southern affluents bring down from the Apennines a large 
quantity 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 it to carry most of its sediment to the Adriatic. It 
has, therefore, raised neither its own channel nor its alluvial 
shores, as it would have done if it had remained unconfined. 
But, as the surface of the water in floods is above the general 
level of the plains through which it flows, the Po can, at that 
period, receive no contributions of earth from the washing of 
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.—RAy- 
MOND, Mining Statistics, 1870, p. 602. 
