8 THE RUBBER TREE BOOK 



of dust are always being added to the surface of the soil. Even 

 such comparatively modern buildings as those constructed 

 in our own country by the Romans a few centuries ago have 

 been buried under twenty or thirty feet of wind-swept dust 

 and soil. 



The effects of what is called " weathering " on the ancient 

 rocks has for long centuries been adding, and is still constantly 

 adding, to the soil. The changes of temperature between day 

 and night, summer and winter, have all an influence in the dis- 

 integration of the rocks. This will be readily understood if 

 one considers for a moment the composition of such a common 

 rock as granite, for example. Granite is principally composed 

 of felspar, quartz and mica, and under the influence of changes 

 of temperature these do not expand and contract equally. As 

 a result of this cracks and small fissures are constantly occurring 

 and water gets in. Water is the great solvent of Nature and 

 it immediately attacks any mineral salts present, honeycombs 

 the rocks and sets free particles of mineral and salts to be added 

 to the soil. Outside of the Tropics the water acts in a second 

 way. Present in these cracks or fissures in time of frost, it 

 becomes ice, and in expanding increases the size of the fracture. 

 It is conceivable that soils now in tropical areas owe their 

 existence in part to the disintegrating action of frost upon 

 rocks during times when, owing to changes in the position of 

 the earth's axis, areas that are now tropical were temperate or 

 even colder, just as others now arctic, such as Greenland, were 

 once temperate and even tropical. The weathering effects of 

 oxygen, various acids and carbonic acid gas in the water (the 

 acids formed largely by bacterial action) have also played 

 an important part in adding to the soil. In rocks and stones 

 there are many forms of mineral which, when moisture is present, 

 readily combine with oxygen or carbonic acid gas and dissolve 

 or break up. Iron, for example, is a very common mineral, 

 and when moisture is present and oxygen attacks it, it oxidizes, 

 turns into rust, and is easily powdered away. So with carbonic 

 acid gas; when combined with water it dissolves many forms 

 of mineral readily and renders them more available for the 

 purposes of vegetation. 



Although the variety of mineral substances which compose 

 the crust of the earth is immense, some of them are present 



