234 THE HISTORY OF THE NIAGARA. RIVER. 



of geology. Let us cousider for a moment tbe tendency of stream his- 

 tories and tbe tendency of lake histories. Wherever streams fall over 

 rocky ledges in rapids or in cataracts, t'^eir power of erosion is greatly 

 increased by tbe rapid descent, and they deepen their channels. If 

 tbis process continnes long enough, tbe result must be that each stream 

 will degrade its channel through the bard ledges until the descent is no 

 more rapid there than in other parts of its course. It follows that a 

 stream with cascades and water falls and numerous rapids is laboring 

 at an unfinisbed task. It is either a young stream, or else nature has 

 recently put obstructions in its patb. 



Again, consider what occurs where a lake interrupts the course of a 

 stream. Tbe lower part of tbe stream, the outflowing part, by deepen- 

 ing its channel continually tends to drain the lake. Tbe upper course, 

 the inflowing stream, briugs mud and sand with it and deposits them 

 in tbe still water of tbe lake, thus tending to fill its basin. Thus, by a 

 double process, tbe streams are laboring to extinguish the lakes that 

 lie in tbeir way, and given sufiScient time, they will accomplish tbis. 



Now, if you will study a large maj) ot Nortb America, you will find 

 that the region of the Great Lakes is likewise a region of small lakes. 

 A multitude of lakes, lakelets, ponds, and swamps where ponds once 

 were, characterize tbe surface from tbe (ireat Lakes northward to tbe 

 Arctic Ocean, and for a distance southward into tbe United States. In 

 tbe same region waterfalls abound, and many streams consist of mere 

 alternations of rapids and pools. Further south, in tbe region beyond 

 the Ohio River, lakes and cataracts are rare. The majority of tbe 

 streams flow from source to mouth with regulated course, their waters 

 descending at first somewhat steeply, and gradually becoming more 

 nearly level as they proceed. At the south the whole drainage system 

 is mature ; at the north it is immature. At the south it is old ; at the 

 north, young. 



Tbe explanation of this lies in a great geologic event of somewhat 

 recent date — tbe event known as tbe age of ice. Previous to the ice 

 age our streams may have been as tame and orderly as those of the 

 Southern States, and we have no evidence that there were lakes in this 

 region. During the ice age the region of tbe Great Lakes was some- 

 what in the condition of Greenland, It was covered by an immense 

 sheet of ice and the ice was in motion. In general it moved from north 

 to south. It carried with it whatever lay loose upon the surface. It 

 did more than this, for just as the soft water of a stream, by d ragging- 

 sand and pebbles over tbe bottom, wears its channel deeper, so the 

 plastic ice, holding grains of sand and even large stones in its under 

 surface, dragged these across the underlying rock, and in this waj' not 

 only scoured and scratched it, but even wore it away. 



In yet other ways the moving ice mass was analogous to a river. Its 

 motion was perpetual, and its form changed little, but that which 

 moved was continually renewed. As a river is supplied by rain, so the 



