430 
RIVER MOUTHS. 
and reverse tlie declivity of the valley, would have been car¬ 
ried 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 consid¬ 
erably 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 lengthening its channel. 
The sediment washed into the marshes of the Maremme is 
not less than 12,000,000 cubic yards per annum. The escape 
of this quantity into the sea, wdiich is now almost wholly pre¬ 
vented, would be sufficient to advance the coast line fourteen 
yards per year, for a distance of forty miles, computing the 
mean depth of the sea near the shore at twelve yards. It is 
true that in this case, as well as in that of other rivers, the 
sedimentary matter would not be distributed equally along 
the shore, and much of it would be carried out into deep 
water, or perhaps transported by the currents to distant coasts. 
The immediate effects of the deposit, therefore, would not be 
so palpable as they appear in this numerical form, but they 
would be equally certain, and would infallibly manifest them¬ 
selves, first, perhaps, at some remote point, and afterward at 
or near the outlets of the rivers which produced them. 
Obstruction of River Mouths. 
The mouths of a large proportion of the streams known to 
ancient internal navigation are already blocked up by sand¬ 
bars or fluviatile deposits, and the maritime approaches to 
river harbors frequented by the ships of Phenicia and Car¬ 
thage and Greece and Pome are shoaled to a considerable 
* See the careful estimates of Roset, Moyens de forcer les Torrents , etc., 
pp. 42, 44. 
