12 AQUEOUS AGENCIES. 



on the contrary, sedimentary deposit. The high lands, therefore, espe- 

 cially mountain-chains, are the great theatres of erosion. Pure water, 

 however, erodes very slowly, the main agents of erosion being the gravel 

 and sand carried along by the current. The general effect of erosion is 

 leveling. If unopposed, the final effect would be to cut down all lands 

 to the level of the sea, at an average rate of about one foot in five thou- 

 sand years. But the immediate local effect is to increase the inequalities 

 of land-surface, deepening the furrows, gullies, and gorges, and increas- 

 ing the intervening ridges and peaks. The effect, therefore, is like 

 that of a graver's tool, constantly cutting at every elevation, but making 

 trenches at every stroke. 



Thus land-surfaces everywhere, especially iu mountain-regions, are 

 cut away by a process of sculpturing, and the debris carried to the low- 

 lands and to the sea. The smaller lines and more delicate touches are 

 due to rain, the deeper trenches or heavier chiselings to rivers proper. 

 The effects of the former are more general and far greater in the aggre- 

 gate, but the effects of the. latter are far more conspicuous. It is only 

 under certain conditions that rain-sculpture becomes conspicuous. 

 These conditions seem to be a bare soil and absence of frost. Beautiful 

 examples are found in the arid regions of southern Utah. 



We now proceed to discuss the more conspicuous effects of water 

 concentrated in river-channels. 



Examples of Great Erosiox xow goixg on : Waterfalls. 



The erosive power of water is most easily studied in ravines, gorges, 

 canons, and especially in great waterfalls. One of the most interesting 

 of these is Niagara. 



Niagara : General Description. — The plateau on which stands Lake 

 Erie (P N, Fig. 6) is elevated about three hundred feet above that of 



£ N L.ERIE. 



Fig. 6.— Ideal Longitudinal Section through Niagara River from Lake Erie to Lake Ontario. 



Lake Ontario, and is terminated abruptly by an escarpment about three 

 hundred feet high (P). From this point a narrow gorge with nearly 

 perpendicular sides, and two hundred to three hundred feet deep, runs 

 backward through the higher or Erie plateau as far as the falls (X). 

 The Niagara Biver runs out of Lake Erie and upon the Erie plateau as 

 far as the falls, then pitches a hundred and sixty-seven feet perpendicu- 

 larly, and then runs in the gorge for seven miles to Queenstown (Q), 

 where it emerges on the Ontario plateau. Long observation has proved 



