MAN AND THE SOIL 21 



controlling forces of the landscape are distributed. 

 In the north the moist winds come from the east ; the 

 rains lessen as they pass westward. The clays, capped 

 with black soil, of Buenos Aires are seolian deposits, 

 brought by the wind from the desolate steppes which 

 close the Pampa to the west, fixed and transformed 

 by the vegetation of a moister region. In the south, 

 on the contrary, the rains come from the Pacific, 

 and the fluvio-glacial alluvial beds of the Patagonian 

 tableland are evidence of copious reserves of moisture 

 in the Andes ; but the arid climate in which the waters 

 have left them has made its mark upon their surface. 



This diversity of the physical environment is only 

 fully brought out by colonization. It is colonization, 

 the efforts and attempts of human industry to adjust 

 agricultural or pastoral practices to the natural condi- 

 tions, which enable us to assign the limits of the natural 

 regions. In this differentiation it is essential to notice 

 the historical element. 



The introduction of new crops gives a geographical 

 meaning, which had hitherto escaped observation, 

 to climatological limits such, for instance, as the line 

 of 400 millimetres of rainfall which is the western 

 frontier of the region of cereals. These limits of crops 

 remain uncertain for a time, then experience and 

 tradition gradually fix them. They always keep a 

 certain elasticity, however, advancing or receding 

 according as the market for the particular produce 

 is favourable or unfavourable. 



Improvement in the methods of exploiting the soil 

 the adoption of better agricultural machinery, dry 

 farming, etc. usually leads to the extension of the 

 sphere of a particular type of colonization, as it enables 

 this type to overcome some natural obstacle which 

 restricted its expansion. Sometimes, however, it 

 brings to light a new obstacle and creates a new 

 geographical limit. 



To this category belongs the northern limit of the 



