20 DYNAMICAL GEOLOGY. 



divided for convenience into two parts, which, however, 

 graduate completely into each other viz., rain-erosion 

 and stream-erosion : the one is universal, but small and in- 

 conspicuous in any one place ; the other is confined to 

 water-channels, but works with concentrated and con- 

 spicuous effects. The one may be compared to a univer- 

 sal sand-papering, the other to the action of the graver's 

 tool, cutting ever deeper along the same lines. Of the 

 two, the general rain-erosion, though less conspicuous, is 

 probably far the greater in aggregate amount. They co- 

 operate in cutting away the land, and, if unopposed, would 

 finally destroy it. Pure water, however, has comparatively 

 little effect. Its graving-tools are the sand, gravel, peb- 

 bles, and rock-fragments, which it carries along in its 



course. 



Conspicuous Examples of Stream-Erosion. The 



effects of erosion are most conspicuously seen in water- 

 falls, ravines, gorges, and canons ; but also, in less degree, 

 on every hillside, and in every furrow and gully. 



Waterfalls ; Niagara. The Niagara Falls and gorge 

 are an instructive"^xample of stream-erosion, because the 

 effects are easily observed from year to year. 



General Configuration of the Country. Lake Erie 

 is situated on a nearly level plateau, several hundred feet 

 above a similar plateau, on which is situated Lake On- 

 tario. The plateaus are separated by an almost perpen- 

 dicular cliff, running east and west, near Lake Ontario. 

 The Niagara River runs out of Lake Erie, and on the 

 Erie plateau, fifteen to eighteen miles, then drops, by a 

 perpendicular fall, into a narrow gorge, with nearly per- 

 pendicular sides, and runs in the gorge for seven miles, 

 and then emerges on the Ontario plateau just before 

 emptying into that lake. Fig. 5 is an ideal section through 

 the middle of the river, and showing these facts. The 

 light lines show the cliffs on the. other side of the gorge. 



Recession of the Falls. Ever since their discovery, 



