THE BIRTH OF A GREAT RIVER. 469 



pear to be the most noteworthy features of a few of 

 the principal river systems, it is we think desirable 

 to briefly consider the probable process of formation 

 of a great river channel, in its geological sense. 



A moment's reflection will be sufficient to convince 

 any observant mind how vast must have been the 

 changes which time has produced upon every river 

 channel; as for instance, where the water has worn 

 deep gorges for long distances through masses of 

 solid rock. Let us endeavour to conceive what must 

 have been the probable condition of such a river, in 

 the earlier stages of its existence. Originally, there 

 can be little doubt that as its waters descended from 

 the highlands they must have encountered numerous 

 obstructions, -which barred their progress to the sea; 

 that must have entailed the formation of more or 

 less gigantic lakes at each of these points, where the 

 waters were pent up until they filled the whole catch- 

 ment basin to the rear of them, and finally overflowed 

 the crest of the barrier: the stream would then have 

 been free to descend on the opposite side, gradually 

 eating its way through the obstruction, as it flowed. 

 Most of us can doubtless recall numerous instances 

 where we have ourselves seen such lakes, which still 

 exist in the courses of streams; and where, if we 

 view the matter aright, we can daily see them at 

 their unceasing work of cutting through the obstruct- 

 ing barrier. 



Visitors to the Upper Engadine in Switzerland will 

 see a good example of this in the chain of lakes oc- 

 cupying the bed of that valley, which empty their 

 surplus waters, proceeding from the melting snows, 

 from one lake into the other, by the connecting torrents, 



