SUBAERIAL EROSION 345 



debris and the greater accumulation of the material 

 around the submarine vents. Such inequalities would 

 not only remain but grow more pronounced when 

 the sea-bottom became a land-surface. Every sub- 

 sequent outbreak of eruptive energy would aggravate 

 them. But ultimately more potent still, because 

 incessant in its operation, would be the influence of 

 the various subaerial agencies by which the land is 

 continually abraded. It is to these agencies that we 

 must mainly ascribe the present topography of the 

 district. By a ceaseless process of sculpture, the volcanic 

 platform has been ultimately carved into hillock and 

 ridge, crag and cliff, valley and ravine. The tools 

 which Nature has employed in this task have been 

 the air, with its wide range of temperature and mois- 

 ture, frost, rain and running water in all its manifold 

 forms, from the tiniest rill to the broad current of 

 the Tiber. 



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 



