RAIN 73 



screen removed, the ruggedness of the underlying rocks would be 

 seen to be much more decided than now appears. So long as 

 the land has these irregularities, it is said to possess relief ; and 

 when it is all planed down to a flat or gently sloping surface but 

 slightly raised above the sea, it is said to have reached the base- 

 level of erosion, or to be base-levelled. 



The atmospheric agencies may be conveniently divided into 

 (i) rain, (2) frost, (3) changes of temperature, (4) wind. 



1 . Rain 



Perfectly pure water, containing neither gases nor solids in solu- 

 tion, would have very little disintegrating effect upon most rocks, 

 but pure water does not occur in nature. The raindrops, gener- 

 ated by the condensation of the watery vapour of the atmosphere, 

 absorb certain gases, which add very materially to the dissolving 

 powers of the water. Of these gases the most important are 

 oxygen (O) and carbon dioxide (CO*), and all rain-water con- 

 tains them. 



As rain-water percolates through the soil, it acquires additional 

 destructive powers by dissolving the acids formed by the decompo- 

 sition of vegetable matter, which are grouped together under the 

 name of humous acids. These acids have the power of decompos- 

 ing or dissolving many minerals which resist the action of ordinary 

 rain, and thus it is that many rocks which, when exposed to the 

 action of the atmosphere only, waste very slowly, disintegrate with 

 comparative rapidity underground. One of the first and simplest 

 effects of atmospheric moisture consists in the oxidation and hydra- 

 tion of the minerals exposed to it. Hydration, or the taking up 

 of water into chemical union, is an important agency of decay. 

 It is accompanied by an increase of bulk, and hence in the lower 

 layers of a rock-mass greatly augments the pressure. In the 

 District of Columbia "granite rocks have been shown to have 

 become disintegrated for a depth of many feet, with loss of but 

 some 13.46 per cent of their chemical constituents. . . . Natural 

 joint blocks brought up from shafts were, on casual inspection, 



