MINERAL DEPOSITS OF FERTILIZERS 23 



sole natural repository of nitrate of soda ? If we first consider 

 the topographical and climatic conditions of the northern 

 provinces of Chili, we find them to be quite abnormal. 

 Excepting for small fractions of the southern part of the 

 nitrate deposits, the district which yields nitrate of soda 

 lies in the tropic of Capricorn. Rain only occurs at infre- 

 quent intervals, there being often no rain for a whole year ; 

 when rain falls it merely wets the surface of the ground, 

 although there are rare occasions when local floods may 

 prevail. Had there been much rain, the nitrates would have 

 been washed away long ago. The dryness of this particular 

 coastal region is due to the cold south polar wind, 

 which is colder than the local atmosphere and blows from 

 the coast towards the nitrate region according to the rough 

 sketch in Fig. i. It is noticeable that the places where 



Plains Mountains 



FIG. i. Section of Chili nitrate region. 



The wind blowing over the sea does not bring moisture on to the land 

 because it is a cold wind from the antarctic regions. Passing inland the 

 wind becomes heated and drier, but after crossing the dry plains it rises 

 up the steep slopes of the Andes, where it expands, cools, and deposits 

 moisture. The drainage from the mountains percolates through the soil 

 and dries up, forming the deposit of caliche. 



the nitrates are deposited are cut off from the sea by a 

 low range of coastal hills ; also that they lie at the lowest 

 point of an extensive plain, bounded on the inland side by 

 the high mountains of the Andes. With such a section 

 before one, there can be little doubt that the nitrates result 

 from slow drainage from mountain ranges, which has accumu- 

 lated in the low levels from which there is no exit to the 

 sea. The nitrates have doubtless originated by a slow 

 process of oxidation of organic matter in higher regions of 

 the mountains. As the soil contains a fair amount of sodium, 



