TIIE YAL DI CHIAN A. 
417 
and the prevention of the mixture of salt water with fresh in 
the coast marshes and shallow bays., which is a very active 
cause of the development of malarious influences.* 
Improvements in the Val di Chiana. 
For twenty miles or more after the remotest headwaters of 
the Arno have united to form a considerable stream, this river 
flows southeastward to the vicinity of Arezzo. It here sweeps 
round to the northwest, and follows that course to near its 
junction with the Sieve, a few miles above Florence, from 
which point its general direction is westward to the sea. From 
the bend at Arezzo, a depression called the Yal di Chiana runs 
southeastward until it strikes into the valley of the Paglia, a 
tributary of the Tiber, and thus connects the basin of the latter 
river with that of the Arno. In the Middle Ages, and down to 
the eighteenth century, the Yal di Chiana was often over¬ 
flowed and devastated by the torrents which poured down 
from the highlands, transporting great quantities of slime with 
their currents, stagnating upon its surface, and gradually con¬ 
verting it into a marshy and unhealthy district, which was at 
last very greatly reduced in population and productiveness. 
It had, in fact, become so desolate that even the swallow had 
deserted it.f 
* The fact, that the mixing of salt and fresh water in coast marshes and 
lagoons is deleterious to the sanitary condition of the vicinity, seems almost 
universally admitted, though the precise reason why a mixture of both 
should be more injurious than either alone, is not altogether clear. It has 
been suggested that the admission of salt water to the lagoons and rivers 
kills many fresh water plants and animals, while the fresh water is equally 
fatal to many marine organisms, and that the decomposition of the remains 
originates poisonous miasmata. Other theories however have been pro¬ 
posed. The whole subject is fully and ably discussed by Dr. Salvagnoli 
Marchetti in the appendix to his valuable Rapport o sul Bonificamento dells 
Maremme Toscano. See also the ALcmovic Econonnco-Statistiche suite ALa- 
remme Toscane , of the same author. 
f This curious fact is thus stated in the preface to Fossombroni 
{Memorie sopra la Val di Chiana , edition of 1835, p. xiii), Irom whicn 
also I borrow most of the data hereafter given with respect to that valley: 
“ It is perhaps not universally known, that the swallows, which come from 
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