METHODS OF IRRIGATION 39 



(November to March) that maize, which needs only the 

 Slimmer rain, can be cultivated without irrigation. 

 But when we follow the Valle de Lerma southward 

 from Salta the maize harvest becomes more and more 

 uncertain, and it is no longer sown in dry soil when we 

 get to about twenty miles from Salta, in the latitude of 

 the confluence of the Arias and the Juramento, How- 

 ever, the summer rains, which are good for maize, 

 are very injurious to the vine ; they spoil the grapes. 

 Thus the southern limit of the cultivation of maize in 

 dry soil almost coincides with the northern limit of 

 the vine. At that point we have the real beginning of 

 the typical scenery of the valles. 



The need of irrigation is due to the scarcity of rain, 

 but it is accentuated by a number of causes which 

 tend to increase the aridity. The valles are the scene 

 of scorching day-winds, the zonda, like the Fohn of the 

 Swiss Alps, which, there being no snow, dry up the 

 water of the springs and of the irrigation trenches, 

 or use the deposits left by the waters to form dunes, 

 which they push southward, sometimes like veritable 

 glaciers of sand. Moreover, the soil of the valles is 

 generally composed of coarse and permeable alluvial 

 deposits, which absorb the rain-storms immediately. 

 There is at the foot of both sides of the hills which 

 enclose each valle an immense and far-lying bed of 

 imperfectly rounded shingle. This double zone of 

 detritus is strangely desolate, for the vegetation on 

 it is restricted to isolated bushes of jarilla and tola. 

 From the sheepfolds on the mountains to the oases 

 in the valleys one hardly meets a single house. The 

 bed of the valley is not so desolate. A broad ribbon 

 of sand marks the dry bed of a torrent, and on the 

 clays of its banks, if the sheet of water underground 

 is not too deep, one finds, in spite of the goats and 

 asses and charcoal-burners, little forests of algarrohas, 

 which the foundries use for fuel. 



The modern alluvial beds, gravel and sand, represent 



