The key to the interpretation of the origin of the scenery of the Campagna is supplied to us by the lines of drainage. On the uplift of the region into land, the streams that descended from the steep front of the mountains would make their way seaward along the lowest levels which they could reach. The channels thus chosen by them would be maintained for the future, save where some landslip or volcanic eruption drove them to seek new courses for their waters. Failing such exceptional causes of diversion, the original lines of drainage would gradually be carved deeper into the framework of the land by the erosion of the water running in them. Thus the streams and the valleys which they have cut out for themselves are the most ancient features of the topography. Between them lie ridges and plateaux, which have gradually become prominent owing to the excavation of the intervening hollows. These eminences, though not subject to such marked and rapid demolition as the channels where running water is allowed free play, nevertheless undergo an appreciable decay. Attacked by the alternate expansion and contraction, due to the heat of clear Italian noons, quickly followed by the chill of starry Italian nights, the faces of the crags and cliffs are slowly disintegrated. Heavy rain washes off the loose crust and exposes a fresh surface to renewed attack. Alternate saturation by rain and drying in sunshine, the effects of frost and the abrading influence of wind, all contribute their share to the process of carving. By this universal process of denudation, while the topographical features have been made continually more pronounced, a considerable thickness of rock has no doubt been removed from the general surface of the whole ground since its elevation into land.