Very great rivers run beneath the earth. The rains wear away the roots of mountains more than their summits, for two reasons. The first is that the impact of the rain is more powerful in falling, from one and the same height, upon the roots of the mountains than upon the summits, by the seventh of this [book], which says: a heavy body becomes the swifter the more it descends through the air, and the swifter it becomes the heavier it grows. Therefore, there being more space from the roots of the mountains to the cloud than from those clouds to the summit of the mountain, the rain is, as has been said, heavier and more powerful upon those roots of the mountains than upon the summit of that same mountain; and so, little by little, it wears away the less the less it falls. The second cause is that a greater quantity of water is that which descends from the middle of the mountain to its roots than from the summit of that mountain to the said middle. And so we have concluded our aim. The valleys widen at every degree of time. The valleys deepen little, because the land that the rain gives back to the valley is almost as much as that which the river carries away, and in some places more and elsewhere less.

Rivers leave more soil where peoples are near than where there is no human kind, because in such places the mountains and hills are worked, and the rains carry away the loosened soils more easily than the hard soils covered with turf. The water of rivers has its origin not from the sea but from the clouds. The rubbing of stones one against another in the courses made by rivers wears away the angles of the stones.

Stones are composed by the courses of rivers. Stones are formed in layers, or in grades, according to the deposit of the sediments carried by the course of the rivers. Stones are not found where there was no sea or lake. Gravels are created by the course of rivers and in the end are consumed. The gravels are the smaller the nearer the river that generates them approaches the sea. A stone is the harder the farther it is from its base. The veins in stones are fractures made by the layers when they dried and were deprived of moisture, for then such cracks were filled by fine matter. Waters carrying fine and imperceptible sediment generate stone on the side where they slacken.

All the Mediterranean seas and the gulfs of those seas are made by rivers that pour into the sea. Mountains are made by the courses of rivers. Mountains are unmade by the rains and the rivers. Those mountains are more eternal and more permanent in their heights which are covered with snow throughout the whole winter. The bases of mountains continually narrow. Mountains continually become more pointed. Rivers continually lower their beds, except where they are dammed back, for there they do the contrary. Lakes among the mountains are generated by the ruins of those mountains, which close off the valleys. The destruction of lakes arises from the courses of their rivers, which consume the bank or the mountain that generated that lake. The Mediterranean seas continually diminish. The generation of the Mediterranean seas arises from ruins of mountains undermined by the courses of rivers. The valleys continually grow, both in breadth and in length.