CONDITIONS IN BELT OF WEATHERING. 431 



occupy much of the space, this leaves little space for the gases. On the 

 other hand, in so far as water does not occupy the openings in the rocks, 

 this space is taken up by the gases which may exist anywhere in the belt 

 of weathering. Varying amounts of water solution's and gaseous solutions 

 furnish favorable conditions for the gases to pass into the water solutions, 

 in which form they are most effective in rock alteration. 



Further, in the belt of weathering there is a great variety of plant and 

 animal life, and consequently the complex action of organic bodies upon the 

 inorganic compounds. These organic forms vary in magnitude from the 

 smallest bacteria to large trees. Moreover, the organic bodies act both 

 while alive and after death. Therefore reactions of many kinds result from 

 them, involving gases, liquids, and solids. 



Furthermore, in the belt of weathering there are great variations in 

 temperature and other physical conditions. The temperature varies from 

 far below 0° C. in polar regions to 50° C. or even 60° or 70° C. in the sun 

 in some of the arid regions of the Tropics. For much of the globe, as a 

 result of varying temperature, the surface water is alternately in the form 

 of liquid and of ice. Also the conditions are those of variable pressure. 



Finally, there are all variations in topographic conditions. Some areas 

 show very slight relief, while others have the most abrupt slopes. Some 

 areas are near sea level, others are 3,000 or more meters above sea level. 



The work in the belt of weathering is accomplished by mechanical and 

 chemical agencies. The mechanical work is largely that of disintegration; 

 the chemical work is largely that of decomposition and solution. Disinte- 

 gration may entirely separate the constituent minerals from one another, or 

 even siibdivide the mineral particles. Decomposition is complete when all 

 of the original minerals have changed to other minerals. Solution is com- 

 plete only when all the original minerals are dissolved. The ultimate result 

 of the action of the mechanical and chemical agencies is to produce soils. 



MECHANICAL WORK. 



Mechanical work subdivides the rocks or disintegrates them. 

 Mechanical work is accomplished by the following: (1) Water, (2) ice, 

 (3) wind, (4) change in temperature, (5) change from water to ice, (6) 

 plants, and (7) animals. 



