346 THE ROMAN CAMPAGNA 



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 abrad- 

 ing 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. 



While it is thus easy to realise, as one traverses 

 the Campagna, how its main topographical character- 

 istics have been evolved, there is a special fascination 

 in pursuing the investigation of particular features and 

 trying to trace their origin in detail. Take, for in- 

 stance, the story of the valleys. I can hardly imagine 

 a more delightful task for a lover of geology than 

 to work out the history of the Tiber an investigation 

 which, from the point of view we are here considering, 

 still remains to be accomplished. Even from any of 

 the hills of Rome it is not difficult to follow some 



