650 DYNAMICAL GEOLOGY. 



recesses ; and thus the opportunities for tearing away masses by sim- 

 ple impact are great, especially when a stream has rocky walls, and 

 the flood season has given the waters their maximum volume and 

 velocity. The Colorado region affords just the conditious for such 

 action, especially since the rocks are, for the most part, horizontal 

 and not very hard. The blows also find much material already half 

 decomposed, which they easily carry away. Great devastations are 

 often made by this means when a flooded stream descends between 

 earthy, forest-covered banks. By the same means, bends may be made 

 in a stream, or cut away, or a new channel may be opened through 

 alluvial flats, to the sea. But in this, and frequently other parts of 

 its work, impact is aided by abrasion. 



This erosion by impact is exemplified in the gold washings of the 

 Sierra Nevada and other regions, where a stream, brought from a high 

 level, is directed in a large jet against the compacted auriferous gravel. 

 The hard gravel-bed melts away with wonderful rapidity before the 

 blows. 



(2.) Abrasion. — The transported sand and gravel which is carried 

 by a current along the bottom or sides of a stream acts like the em- 

 ery' of an emery wheel, yet under only slight pressure. The particles, 

 and especially the pebbles or stones, that are thrown by violent tor- 

 rents against the surfaces within reach, whether those of other stones 

 and pebbles or of solid rock, work more effectively; but less constantly. 

 Together they tend to clear a rapid stream of the stones in its bed and 

 enlarge as well as deepen its channel. At the same time the trans- 

 ported particles are wearing upon one another, tending to reduce the 

 material to that fine impalpable state in which even slow-moving 

 waters will transport them. This mutual abrasion is largely the cause 

 of the rarity of pebbles along the lower part of some rivers. 



Abrasion is also carried forward, in the case of a waterfall, by means 

 of the spray sent off from the water at its final stroke ; and, when 

 there is an air chamber behind the fall, through the forcible move- 

 ment of the air caused by the fall, an effect that was first observed by 

 James Hall at Niagara. 



4. Effect of the Earth's rotation on River-erosion. — The earth's 

 eastward rotation, together with the increase in rate from the pole to 

 the equator, tends to throw the waters of streams in the northern hem- 

 isphere against the right bank (the right, looking down stream), and, 

 in the southern, against the left. Although the ratio between the im- 

 pact on the two banks differs little from equality, the difference is suffi- 

 cient to cause an undermining of earthy deposits and make the bank 

 struck the high and steep one, and the other low. The expression for 

 this ratio at the depth z, is gz-\- b vw sin I: g z, in which b is the 



