332 THE BEAtJTiES OF NATURE chap. 



time if permitted, after a long cycle of years, 

 to its first course. 



In evidence of the vast quantity of sediment 

 which, rivers deposit, I may mention that the 

 river-deposits at Calcutta are more than 400 

 feet in thickness. 



In addition to temporary " spates," due to 

 heavy rain, most rivers are fuller at one time 

 of year than another, our rivers, for instance, 

 in winter, those of Switzerland, from the 

 melting of the snow, in summer. The Nile 

 commences to rise towards the beginning of 

 July ; from August to October it floods all the 

 low lands, and early in November it sinks 

 again. At its greatest height the volume of 

 water sometimes reaches twenty times that 

 when it is lowest, and yet perhaps not a 

 drop of rain may have fallen. Though we 

 now know that this annual variation is due 

 to the melting of the snow and the fall of 

 rain on the high lands of Central Africa, still 

 when we consider that the phenomenon has 

 been repeated annually for thousands of years 

 it is impossible not to regard it with wonder. 

 In fact Egypt itself may be said to be the 

 bed of the Nile in flood time. 



