THE CONTOUR OF THE PLAIN 165 



These various shades of the Pampean climate are 

 of essential importance in the history of colonization 

 and the spread of cultivation. The belt of summer 

 rain is the belt of maize-growing, whereas the cultiva- 

 tion of wheat requires spring rain and a comparatively 

 dry summer. 



While the isohyetic curves, which represent the 

 precipitation for the whole year, are orientated from 

 north-west to south-east, the curves of rainfall during 

 the cold season, from April to September (dry season 

 in the north), cut diagonally across the preceding, and 

 are oriented directly north and south. Bahia Blanca 

 receives in winter as much rain as Rosario, and General 

 Acha (in the district of the central Pampa) as much 

 as Cordoba. Unless one attends to this, one cannot 

 explain the extension of wheat -growing, in the south- 

 west, as far as the 400 millimetre curve, and even beyond 

 it on the Atlantic coast. 



The relief of the Pampean plain is known fairly 

 accurately, thanks to the observations made along 

 the railways. The ground rises slowly toward the 

 west. The loo-metre curve describes a deep gulf 

 some 300 miles west-south-west of Buenos Aires. The 

 belt comprised between 100 and 150 metres above sea- 

 level is more than sixty miles broad in the latitude of 

 Santa Fe, and 130 miles in the latitude of Buenos Aires. 

 Beyond the 150-metre curve the land rises rapidly 

 toward the west and north-west, and reaches 400 metres 

 in the Cordoba district and 500 in the Villa Mercedes 

 district. It is at the altitude of 150 metres, and the 

 break in the inclination which this marks, that the Rio 

 Quinto is lost, near Amarga, south of General Lavalle. 



and a secondary maximum appears in the spring. The predominance 

 of the spring rains, which is a characteristic of southern Brazil, is 

 conspicuous on the middle Uruguay. On the lower part of that river 

 the rain-system approaches that of Buenos Aires, with maxima in 

 spring and autumn, a principal minimum in winter, and a secondary 

 minimum in summer. 



