STATEMENT BY MR MARSH. 235 



filled, or nearly so, even this material could no longer be deposited in 

 . it. The fact of the complete removal of the deposit I have described 

 between the two dams in a single freshet, shows that, in spite of con- 

 siderable obstruction from roughness of bed, large quantities of sand 

 may be taken up and carried off by streams of no great rapidity of 

 inclination ; for the whole descent of the bed of the river between 

 the two dams — a distance of four miles — is but sixty feet, or fifteen 

 feet to the mile. 



" 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 geogTaphical process 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 whole process of the disintegrating efiects of the Po Mr Marsh 

 then details with clearness, and concludes : — '•' 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 north-eastern 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 the depth of fifteen yards. There are very large portions 

 of this area, where, as we know from ancient remains — roads, 

 bridges, and the like — from other direct testimony, and from geo- 

 logical considerations, very little degradation has taken place within 

 twenty centuries, and hence the quantity to be assigned to localities 

 where the destructive causes have been most active is increased in 

 proportion. 



" If this vast mass of pulverized rock and earth were restored to 

 the localities from which it was derived, it certainly would not 

 obliterate valleys and gorges hollowed out by great geological causes, 

 but it would reduce the length and diminish the depth of ravines of 

 later formation, modify the inclination of their walls, reclothe with 

 earth many bare mountain ridges, essentially change the line of 



