THE COURSE OF A GREAT RIVER. 465 



able object lesson is conveyed to mankind, illustrating 

 the truth of the old adage that "Perseverance will 

 effect wonders," for with what seem to all appearance 

 very small and inadequate resources, water alone has 

 gradually excavated these wonderful channels of enorm- 

 ous length and magnitude which we call rivers. 



Their courses, which are engineered with marvellous 

 skill and cunning, have like their channels been automati- 

 cally determined simply by the force of gravitation; 

 wherever the lowest level was, there the water flowed, 

 winding and turning as the contour of the ground 

 required; ultimately by dint of constant labour it cut 

 its way through every opposing obstacle, often literally 

 for thousands of miles, until at last it effected its junc- 

 ture with the ocean. 



The course of every great river may be divided into 

 three parts: (i) The Mountain Track; (2) The Valley 

 Track ; (3) The Plains Track ; * to which for the main 

 streams we may add a fourth, namely that highly im- 

 portant portion, from a commercial point of view, known 

 as the estuary or tidal section, where the salt water 

 mingles with the fresh. The mountain track will generally 

 be represented, in extensive river systems, by torrents 

 descending from the hills, whose turbulent course has 

 been briefly noticed in the preceding section on great 

 mountain regions. The valley track will be represented 

 by the river, already perhaps grown into a stream of 

 considerable magnitude, as it issues from the hills, and 

 winds its way still tumbling and tossing through the 

 highlands of the interior of the continent, where though 

 its course is necessarily less turbulent than upon its 

 first portion, it is still generally greatly obstructed by 



* Encyd. Brit., 9th edit, Vol. x., p. 275 (Article "Geology"). 

 VOL. II. 30 



