158 THE BOOK OF NATURE STUDY 



shorten their tortuous course, or else join the upper waters of 

 rivers in distinct basins so as to facilitate intercommunication 

 between them. And finally, the harbours which facilitate transport 

 to countries outside, and depend very closely on the natural 

 facilities given where old river valleys have subsided and been 

 " drowned " by the sea, so that deep water penetrates far into the 

 land and has not yet been filled up by the deposits brought down 

 by the river. 



