124 IRRiaATION CANALS. ch. v. 



such as Suceda and Garoxylon prevail, it is reasonable to 

 conjecture the presence of nitre, gypsum, or other salts in 

 the superficial layer ; but such plants as Artemisia Herha 

 alba, Genista monosperma, and a local form of Helian- 

 themum virgatum will sometimes take almost exclusive 

 possession of the surface, though this in some places is 

 mainly composed of siliceous sand, in others of disinte- 

 grated calcareous tufa, and in others of decomposed vol- 

 canic rock, nowhere seen by us in situ, but derived from 

 scattered blocks of various sizes. In the plain near the 

 city siliceous sand predominates, and, as a consequence, the 

 vegetation is more meagre than elsewhere. 



We hereabouts first saw the only works of public 

 utility which we encountered during our journey. What 

 first struck the eye were long lines of irregular earthen 

 mounds traversing the plain in a north and south direction, 

 and we soon ascertained that these were watercourses 

 rudely arched over. The streams from the mountains 

 south of the city are distributed through irrig-ation canals 

 over a large part of the plain, and thus render it fit for 

 cultivation. Early experience must have taught the people 

 that by protecting these canals from evaporation, they 

 could be made available to a much greater extent ; and it 

 is probable that the construction of these -covered water- 

 ways, some of which were in a ruinous condition, goes back 

 to a remote period. In point of fact, the whole drainage 

 of three considerable valleys, whose torrents we afterwards 

 crossed, appears to be intercepted by this irrigation process, 

 and absorbed by the vegetation of the plain. It is pro- 

 bable that by the skilful extension of the same system 

 wide tracts, now barren, might be made productive. 



