THE NON-LIVING WORLD 163 



bordered by high land to its mouth as in the Tyn 

 and the Tweed. The excavation by a river of its own 

 valley may leave here and there, high up in sheltered 

 positions, accumulations of drifted materials, marking 

 the level at which the river flowed at successive earlier 

 periods. 



Every child who has the opportunity of examining an 

 undulating sandy surface after violent rains, may see 

 clearly both the eroding and the depositing action of 

 water. It needs but a much prolonged action of that 

 kind to cause profound modifications of the earth's 

 surface. There is no mountain which is not almost 

 incessantly being in this way rendered more steep and 

 precipitous, and thus the whole land of the globe 

 constantly tends to be washed down by rivers, and 

 spread out beneath the surface of the sea. But the 

 finer debris of the land carried down incessantly into the 

 sea by rivers, is, when the action of the river-water 

 ceases, caught up by the great marine currents and 

 swept to places more or less widely distant and out of 

 the reach of tidal action. 



The lowering of the earth's surface by the wear and 

 tear of water is more or less counterbalanced by a slow, 

 or rapid, upheaval of other parts of its surface through 

 volcanic action. The number of burning mountains 

 active volcanoes in the world may be estimated at 

 about 300, and some of these give forth vast quantities 

 of lava.* For example, in the Island of Ha wai a burning 

 deluge of lava broke forth in 1840 from the crater of 

 Kilauea ; it spread from one to four miles wide and 

 reached the sea in three days, at a distance of thirty 

 miles, and for fourteen days it plunged, in a vast fiery 



* See ante, p. 147. 



