Book 7, on the water which, with various qualities of angles, strikes and hollows out beneath the banks of rivers. What are those waters that reign in the rivers after their inundations. What are those waters that fail in the subsiding of the rivers' floods. All the walls ruined by the waters fall toward the oncoming of those waters. The ruins of the bank arise from their bulges which impede the straight course of the inundations, or from the course of the water which does not follow the straightness of the water of that inundation. The water in straight and narrow channels will have a higher surface in the middle and at the sides than in any other part of its width. At the sides it is higher because, striking against the bank, it leaps upward and reflects backward and strikes the bottom and makes itself low; then it reflects from the bottom and leaps upward and meets in the middle with the oncoming of the opposite water which, striking with it, leaps upward, and returns backward to repeat the blow against its bank, and so it follows successively. The water that runs through straight channels does not pass except from the bank to the middle of the channel, and the other half of its width does the same, and so it makes a long course before the water struck at one bank passes to the opposite bank; but if it is swift, it will not pass the middle. Many are the mountains whose stones, of which they are composed, do not reach the roots of those mountains; but, following the same order of the strata, they are converted into tufa, and under the tufa is clay. And this is seen at the sides of all the rivers that issue from great cuts of the mountains into the great plains, which have cut the mountains down to their roots. The water that moves strongly has its eddies that hollow out the bottoms; and this happens because the course of the eddy follows the obliquity of the surface, whence it directs itself with its motion to the striking of the bottom, always drilling with its front, and thus it goes wearing away the bottom.

Solid stone of the Mugnone hollowed out by the water into the form of vases. It seems the work of a hand, so exact is it.