The rivers of Tuscany have wrought great changes in the plains through which they flow, and along the sea-coast. Their labour has been facilitated by the nature of the soil which they traverse. The least rain converts the barren hill-slopes into a semi-fluid paste, which is carried by the rivers down to the sea. The mouth of the Arno has thus been pushed forward to the extent of seven miles in the course of a few centuries. In former times the Serchio and the Arno united before they flowed into the sea, but the Pisans diverted the former river to the {243} north, in order to rid themselves of its unwelcome deposits. Pisa, in the time of Strabo, stood at a distance of only twenty Olympian stadia from the Tyrrhenian Sea, and when the _cascina_ of San Rossore was built, towards the close of the eleventh century, its walls were close to the beach, which is now at a distance of three miles. Extensive plains intersected by dunes, or _tomboli_, and partly covered with forests of pines, have been added to the land in the course of centuries. These sandy wastes have become the home of large herds of horses and half-wild cattle, and the camel has been acclimatised there, it is said, since the Crusades. These changes in the coast-line may not, however, be due exclusively to the agency of the rivers, for there exists evidence of an upheaval of the land. The building stone known at Leghorn as _panchina_ is clearly of marine origin, and the shells which enter into its composition are still met with in the Tyrrhenian Sea. [Illustration: Fig. 84.—MONTE ARGENTARO. From the French Chart. Scale 1 : 168,000.] [Illustration: Fig. 85.—VAL DI CHIANA. From the Austrian Staff Map. Scale 1 : 218,000.] Amongst the changes effected by human agency in the basin of the Arno those referring to the Val di Chiana are, perhaps, the most important. This depression connects the basins of the Arno and Tiber, and may possibly have served as an outlet to the former river before it had opened itself a way through the {244} gorge below Florence. Formerly the water-parting between the two rivers was close to the Arno. A small portion of its drainage was carried to the Tuscan river, but by far the greater portion of the vale was occupied by stagnant pools, extending to the south as far as the latitude of Montepulciano, a distance of twenty miles. The whole of this region was a breeding-place of fever. Dante and other Italian writers speak of it as an accursed place. The inhabitants made vain attempts at drainage. The illustrious Galileo, when consulted on the subject, {245} declared that nothing could be done to mend this evil; and though Torricelli conceived that it would be possible to drain the valley, he took no steps to put his theories into practice. [Illustration: Fig. 86.—THE LAKE OF BIENTINA. From the Austrian Staff Map. Scale 1 : 328,000.]