78 THE ATMOSPHERE — RAIN 



these movements. On vertical cliffs and steep hillsides it is 

 quickly removed, and in such places it is thin or quite lacking, 

 while in the valleys it often accumulates to great depths. Even 

 on gentle slopes and almost level stretches the rains slowly wash it 

 downward, and eventually into the streams which carry it to the 

 sea. The soil is thus not stationary, but under the influence of 

 the rains and streams is slowly but steadily travelling seaward. 

 Disregarding the alluvial deposits made by rivers, and soils accu- 

 mulated by the action of ice or wind, the soil of any district is 

 thus a residual product, and its quantity represents the surplus of 

 chemical disintegration over mechanical removal. 



The mechanical action of rain is greatly increased by extreme 

 violence and volume of precipitation; a single "cloud-burst" will 

 do far more damage than the same quantity of rain falling in 

 gentle showers. Those who know only the temperate regions can 

 form but imperfect conceptions of the violence of tropical rains. 

 On the southern foot-hills of the Himalayas, for example, the rain- 

 fall is exceedingly great (in some localities as much as 500 inches 

 per annum), and almost all of it is precipitated in six months of the 

 year ; especially remarkable is the quantity which often falls in a 

 single day. " The channel of every torrent and stream is swollen 

 at this season, and much sandstone and other rocks are reduced 

 to sand and gravel by the flooded streams. So great is the super- 

 ficial waste, that what would otherwise be a rich and luxuriantly 

 wooded region is converted into a wild and barren moorland " 

 (Lyell). 



The action of rain is thus by no means uniform, the results de- 

 pending upon so many and such varying factors, that we may find 

 marked differences in closely adjoining regions, and even in one 

 and the same mass of rock. One of the most remarkable monu- 

 ments of rain-erosion is exhibited by the curious districts in the 

 far western states known as the "bad laizds" which cover many 

 thousands of square miles in the Dakotas, Nebraska, Wyoming, 

 Utah, etc. The bad-land rocks are mostly rather soft sandstones 

 and clays, with prevailingly calcareous cements, and formed in 

 nearly horizontal beds or layers. The rainfall is light, though 



