COLLATERAL EFFECTS OF WORKS IN VAL DI CHIAN A. 429 
capital town of the district, is now much frequented for its sea 
baths and its general salubrity, at a season when formerly it 
was justly shunned as the abode of disease and death.* 
It is now a hundred years since the commencement of the 
improvements in the Yal di Chiana, and those of the Ma- 
remma have been in more or less continued operation for 
above a generation. They have, as we have seen, produced 
important geographical changes in the surface of the earth 
and in the flow of considerable rivers, and their effects have 
been not less conspicuous in preventing other changes, of a 
deleterious character, which would infallibly have taken place 
if they had not been arrested by the improvements in ques¬ 
tion. It has been already stated that, in order to prevent the 
overflow of the valley of the Tiber by freely draining the Yal 
di Chiana into it, the Papal authorities, long before the com¬ 
mencement 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 Paglia. They consequently deposited most of tlieir sedi¬ 
ment in the Yal di Chiana and carried down comparatively 
little 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 channels and outlets in a southern direc¬ 
tion, 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 Arno had been raised by spontaneous deposit of 
sediment to such a height as to interpose a sufficient ob¬ 
stacle to all flow in that direction. If, then, the Roman 
dam had not been erected, or the works of the Tuscan 
Government undertaken, the whole of the earth, which has 
been arrested by those works and employed to raise the bed 
* Gioegini, Sur les causes de VInsalubrite de Vair dans le voisinage des 
marais, etc., lue d VAcademic des Sciences d Paris, le 12 Juillet, 1825. Re¬ 
printed in Salvagnoli, Rapporto, etc., appendice, p. 5, et seqq. 
