RESULTS OF OPERATIONS. 521 



The sediment washed into the marshes of the Maremma is not 

 less than 12,000,000 cubic yards per annum. The escape of this 

 quantity into the sea, which is now ahnost wholly prevented, 

 would be sufficient to advance the coast-hne 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 de- 

 posit in the sea, therefore, would not be so palpable as they appear 

 in this numerical form ; but they would be equally certain, and 

 would infallibly manifest themselves, first, perhaps, at some 

 remote point, and afterwards more energetically at or near the 

 outlets of the rivers which produced them. The elevation of the 

 bottom of the sea would diminish the inclination of the beds of 

 the rivers discharging themselves into it on that coast, and of 

 course their tendency to overflow their banks, and to extend still 

 further the domain of the marshes which border them, would be 

 increased in proportion. 



It has been already stated that, in order to prevent the over- 

 flow of the valley of the Tiber by freely draining the Yal di 

 Chiana into it, the Papal authorities, long before the commence- 

 ment of the Tuscan works, constructed strong barriers near the 

 southern end of the valley, which detained the waters of the wet 

 season until they could be gradually drawn off into the Pagha. 

 They consequently deposited most of their sediment in the Yal 

 di Chiana, and carried down comparatively httle earth to the 

 Tiber. The lateral streams, contributing the largest quantities of 

 sedimentary matter to the Yal di Chiana, originally flowed into 

 that valley near its northern end ; and the change of their chan- 

 nels 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 process of improvement. 



"We have seen that the north end of the Yal di Chiana, near 

 the Amo, had been raised by spontaneous deposit of sediment to 

 such a height as to interpose a sufficient obstacle to aU flow in 

 that direction. If, then, the Roman dam had not been erected, 

 or the works of the Tuscan Government undertaken, the whole 



