COAST OF THE NETHERLANDS. 541 
southern end of the valley, which detained the waters of the 
wet season until they could be gradually drawn off into the 
Paglia. They consequently deposited most of their sediment 
in the Val di Chiana and carried down comparatively little 
earth to the Tiber. The lateral streams contributing the largest 
quantities of sedimentary matter to the Val di Chiana original- 
ly flowed into that valley near its northern end ; and the change 
of their channels and outlets in a southern direction, so as to 
raise that part of the valley by their deposits and thereby 
reverse its drainage, was one of the principal steps in the pro- 
cess of improvement. 
We have seen that the north end of the Val di Chiana near 
the Arno had been raised by spontaneous deposit of sediment 
to such a height as to interpose a sufficient obstacle to all flow 
in that direction. If, then, the Roman dam had not been 
erected, or the works of the Tuscan Government undertaken, 
the whole of the earth, which has been arrested by those works 
and employed to raise the bed and reverse the declivity of the 
valley, would have been carried down to the Tiber and thence 
into the sea. The deposit thus created would, of course, have 
contributed to increase the advance of the shore at the mouth 
of that river, which has long been going on at the rate of three 
metres and nine-tenths (twelve feet and nine inches) per 
annum.* It is evident that a quantity of earth, sufficient to 
effect the immense changes I have described in a wide valley 
more than thirty miles long, if deposited at the outlet of the 
Tiber, would have very considerably modified the outline of 
the coast, and have exerted no unimportant influence on the 
flow of that river, by raising its point of discharge and length- 
ening its channel. 
The Coast of the Netherlands. 
It has been shown in a former section that the dikes of the 
*See the careful estimates of Rozet, Moyens de forcer les Torrents, etc., 
pp. 42, 44. 
