xiii Action of Water on the Land 239 



rising 5000 feet above the plain, is believed by Mr. Im 

 Thurn, who first succeeded in reaching its summit, to be 

 simply a rain-wrought earth pillar on a gigantic scale ; the 

 soft sandstone, when freshly exposed, being rapidly washed 

 away by the torrents of one of the rainiest regions of the 

 world, while the harder conglomerate resists erosion and 

 protects the rock beneath. 



313. Underground Water. Of the rain which falls 

 upon the surface of the Earth in a region like Great Britain 

 it is estimated that one -third is returned to the air by 

 evaporation, one-third flows off over the surface, and one- 

 third sinks into the ground. Where the rocks are imper- 

 meable by water, such as shales and stiff clays, more flows 

 off over the surface, but where they are permeable, like 

 sandstone, gravel, or many limestones, a greater proportion 

 soaks through. The movement of water underground is 

 slow or rapid, according to the facility with which the rocks 

 allow it to work its way through them. In time some water 

 undoubtedly filters downward, until, under the influence of 

 great pressure and high temperature, it combines chemically 

 with the rock substance ( 293), but the greater part of it 

 returns to the surface at a level lower than that it started 

 from. Each variety of rock can absorb by capillarity ( 39) 

 a certain definite proportion of water, which remains in it as 

 in a sponge, until enough accumulates to overcome friction, 

 when it percolates through. The rate of percolation is 

 often greatly increased by the presence of cracks or joints. 

 Soft porous rocks becoming saturated may give rise to 

 landslips, especially in cases where they, rest on beds of 

 stiff clay that become lubricated and slippery when wet. 

 As the percolating water dissolves out narrow crevices 

 between the grains of rock, the pressure of the strata above 

 forces them together again, thus producing a slow general 

 settling down of the land-surface. 



314. Wells and Springs. When a thick layer of per- 

 meable rock rests on an impermeable bed, water accumulates 

 until the pressure of the liquid suffices to force a way 

 between the rocks and so reach the surface on the slope of a 

 hill or the side of a valley. This outflow of underground 



