ORIGIN OF TOPOGRAPHY 349 



in the scenery of the Campagna. In this investiga- 

 tion we would mark how these differences of contour 

 have mainly arisen from variations in the character of 

 the volcanic tuff. Where the rock has possessed little 

 coherence, and has consequently yielded more easily 

 to the weather, it has crumbled away for the most 

 part into gentle slopes which have been more or less 

 shielded from further decay by a mantle of vegetation. 

 But where the loose ashes and scoriae have been 

 accompanied with much fine dust, and have consoli- 

 dated into a hard, compact stone, this material has 

 survived to form the more rugged features of the 

 scenery. Every stage in the progress of the sculpture 

 may be instructively seen at the edge of the volcanic 

 plain, where it winds round the projecting spurs and 

 into the retreating hollows v and bays of the great 

 Apennine wall. Little imagination is needed to pic- 

 ture this plain as a sea-floor over which the waves 

 rolled along the base of the mountains. But when 

 this sea-floor was raised into land, the torrents from 

 the uplands began to dig out of it winding channels, 

 which were gradually deepened into gullies that unite 

 into wider ravines as they descend. Between these 

 defiles portions of the old plain have been left, along 

 the sides of which the successive sheets of tuff run 

 as bold ribs or as green slopes, according to their 

 relative durability. Here and there one of these out- 

 liers, girdled round with vertical walls of rock, rises 

 as an almost inaccessible platform above the ravines 

 around, and has served as the site of some pre- 

 historic citadel or of some medieval fortalice. All 

 over the Campagna, indeed, from the very earliest 



