THE SOIL 5 



trolling humidity. When a warm wind laden with 

 moisture from the seapasses over the land, it becomes 

 chilled in rising over high hills or mountains, since, 

 as we have seen, these are cooler than the lower levels. 

 Cool air cannot contain as much moisture as warm 

 air, hence part of it is squeezed out and falls as rain. 

 As a rule the windward side of a mountain range is 

 wet, while the leeward side is dry. We thus find 

 that the east coast of Mexico and Central America, 

 which is exposed to the sweep of the northeast trade 

 wind, has a heavy rainfall. In passing over the high 

 mountains of the interior these warm, moist winds 

 become cooled and lose their moisture, so that as they 

 descend on the western side and again become 

 warmed they are very dry. What are practically 

 desert conditions prevail over large areas of the west- 

 ern side of these countries. The same thing occurs 

 in South America and in the more mountainous of the 

 West India Islands. Porto Rico and Jamaica have 

 a very heavy rainfall on the north side of the moun- 

 tains, but on the southern side it is so dry that irriga- 

 tion is necessary for the profitable production of most 

 crops. If our hypothetical chain of mountains ex- 

 tended across the Mississippi Valley, the south side 

 would not only be warmer, but it would have much 

 heavier rainfall, since more of the moisture from the 

 Gulf would be deposited; while the northern side 

 would not only be cold, but very dry. 



The Soil 



Formation of Soils. — The earth or finely divided 

 substratum in which the ordinary land plants grow 



