LAKES. 97 



courses and highways of civilisation. These great rivers 

 are navigable for large ships, even for many hundreds of 

 miles, and it is most fortunate that this navigation is so 

 seldom impeded by falls; for these, although among the 

 grandest and most beautiful objects of nature, yet put a 

 stop to the navigation of a river which would otherwise be 

 passable for hundreds of miles further. Bivers which run 

 through districts subject to periodical rains, or those whose 

 sources lead from mountain slopes where vast quantities of 

 snow are deposited in winter which melts on the approach 

 of summer, are at such seasons of the year suddenly aug- 

 mented, and generally overflow the banks and inundate the 

 country for miles. This is in many cases a very great mis- 

 fortune, as it spoils the land, reducing it to a swamp, but 

 not in every case ; the overflowing of the Nile brings down 

 with it a great quantity of fertilising vegetable matter, and 

 serves to manure the whole district, for the c soil being 

 extremely porous and the heat great, the moisture is soon 

 got rid of, and hence it was a custom of the Egyptians to 

 celebrate the rising of the Nile by feasts and ceremonies. 



Lakes are formed in the same way as rivers, but, being sur- 

 rounded by high land, have no outlet for the waters so that 

 they may flow back into the sea. If a lake were about to 

 be formed it would be in this way the rain descending on 

 the surrounding high lands would run in rills to the 

 lowest part, and there collect, forming a pool which would 

 gradually increase in size as the water flowed into it, 

 this increase continuing until the surface became so large 

 that the evaporation from it exactly equalled the supply, 

 it would then extend no further, but remain a permanent 

 lake or inland sea, giving off vapour to refresh the surround- 

 ing country, and again receiving it back by streams and 

 rivers, as does the ocean. 



There is scarcely a spot known on the surface of the 

 earth, which does not show strong evidence of having, at 

 some former period, been the bed or bottom of some sea or 

 lake. Some of these evidences may be seen in the strata or 

 layers of earth forming the sides of the cliffs which abut on 

 the sea-shore, and which could not have been produced by 



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