274 SEDIMEE^T OF THE PO. 



The present rate of deposit at tlie moutli of the Po has contin 

 ued since the year 1600, the previous advance of the coast, aftei 

 the year 1200, having been only one-third as rapid. The great 

 increase of erosion and transport is ascribed by Lombardini 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 ac- 

 cording to the progress of population and the consequent exten- 

 sion of clearing and cultivation. The transporting power of tor- 

 rents is greatest soon after their formation, 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 val- 

 leys, and elevated the channels of their lower course. Their 

 eroding action also is most powerful at the same period, both be- 

 cause their mechanical 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 northern and 

 northeastern declivities of the Apennines — have annually sent 

 down into the lakes, the plains, and the Adriatic, not less than 

 375,000,000 cubic yards of earth and disintegrated rock. We 

 have, then, an aggregate of 750,000,000,000 cubic yards of such 

 material, which, allowing to the mountain surface in question an 

 area of 50,000,000,000 square yards, would cover the whole to 



down from the Alps and the Apennines than is carried to the sea. — Castel- 

 liANi, DelV 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 estt 

 mate. 



* Baumgakten, An, des Pontes et Ghaussees, 1847, ler semestre, p. 175. 



