2982, SEDIMENT OF THE PO. 
The present rate of deposit at the mouth of the Po has con- 
tinued since the year 1600, the previous advance of the coast, 
after the year 1200, having been only one-third as rapid. The 
great increase of erosion and transport is ascribed by Lombar- 
dini chiefly to the destruction of the forests in the basin of that 
river and the valleys of its tributaries, since the beginning of 
the seventeenth century.* We have no data to show the rate 
of deposit in any given century before the year 1200, and it 
doubtless varied according to the progress of population and 
the consequent extension of clearing and cultivation. The 
transporting power of torrents is greatest soon after their for- 
mation, because at that time their points of delivery are lower, 
and, of course, their general slope and velocity more rapid, 
than after years of erosion above, and deposit below, have 
depressed the beds of their mountain valleys, and elevated the 
channels of their lower course. Their eroding action also is 
most powerful at the same period, both because their mechan- 
ical force is then greatest, and because the loose earth and 
stones of freshly cleared forest-ground are most easily removed. 
-Many of the Alpine valleys west of the Ticino—that of the 
Dora Baltea, for instance—were nearly stripped of their forests 
in the days of the Roman Empire, others in the Middle Ages, 
and, of course, there must have been, at different periods before 
the year 1200, epochs when the erosion and transportation of 
solid matter from the Alps and the Apennines were at least as 
great as since the year 1600. 
Upon the whole, we shall not greatly err if we assume that, 
for a period of not less than two thousand years, the walls of 
the basin of the Po—the Italian slope of the Alps, and the 
as, according to Lombardini, that river delivers into the Adriatic. Castellani 
supposes the computation of Mengotti to fall much below the truth, and there 
can be no doubt that a vastly larger quantity of earth and gravel is washed 
down from the Alps and the Apennines than is carried to the sea.—-CASTEL- 
LANI, Dell’ Immediata Influenza delle Selve sul corso delle Acque, i., pp. 42, 43. 
I have contented myself with assuming less than one-half of Mengotti’s esti- 
mate. 
* BAUMGARTEN, An. des Ponts et Chaussées, 1847, ler sémestre, p, 175. 
