xni Action of Water on the Land 243 



flow through them. In all limestone regions rivers 

 disappear beneath the surface, and although most of 

 them, like the Guadiana in Spain and the Poik in the 

 Adelsberg caves, reappear on land, several vanish alto- 

 gether and ultimately well up through the salt water of the 

 sea, sometimes from depths of 100 fathoms or more. 



318. Surface Water. During a shower, and for some 

 time after it has ceased, little runnels of water flow down 

 the steeper slopes of the land, uniting where opposed slopes 

 meet to form streams, which ultimately- converge in rivers 

 and flow on to lakes or to the sea. If the land were com- 

 posed of impermeable rock the whole of the rain-water not 

 lost by evaporation would run off over the surface, and 

 rivers would flow only during and immediately after the 

 fall of rain ; this is in fact the case in many mountainous 

 regions where the smooth rock walls are too steep to allow 

 soil to form upon them. On gentler slopes the rain first 

 soaks into the soil, and the streamlets swell gradually and 

 are kept flowing long after the rain stops by the subsequent 

 oozing of moisture. About one-half of the water in large 

 rivers enters them from springs which have pursued an 

 underground course from higher levels, and being inde- 

 pendent of local fluctuations of rainfall these give perma- 

 nence to the flow. When the melting of snow takes place at 

 one period of the year, or when heavy rains occur at definite 

 seasons, the springs are replenished as a store to be drawn 

 on gradually, and the increased supply of surface water pro- 

 duces a regular periodical rise in the level of the river. The 

 Ganges always rises and overflows its banks in summer, 

 when the melting snow of the Himalayas and the rains of 

 the south-west monsoon fill its higher tributaries. Similarly 

 the Nile ( 375), after the monsoon rainfall of Abyssinia, 

 overflows its channel in the rainless land of Lower Egypt 

 every autumn, covering a narrow strip on each side with 

 soft and fertile mud. The Amazon ( 361), on the other 

 hand, is almost always high, as the rainy seasons of its 

 southern and northern tributaries occur at opposite times 

 of the year with the shifting of the trade winds ( 178), 

 but its floods are greatest in June. Dr. John Murray 



