LESSONS IN GEOLOGY. 345 



work of erosion seems to be done by sand and gravel carried or 

 pushed along the bed of the river. The rate of erosion will de- 

 pend on the strength of the current, the amount and kind of sed- 

 iment carried, and the nature of the rock through which the 

 channel was located. In the Missouri river, in many places, 

 there are from 40 to 50 feet of sand and mud between the bed 

 rock and the water. When the river is low, or at a medium stage, 

 this bed of mud and sand is quiet, and there is no erosion of the 

 rock-bed; but when the river is in flood, the whole mass is in 

 motion, sliding along like a rasp over the bottom of the rocky 

 channel. Limestone rocks erode more easily than some others, 

 and they are more easily soluble. Sometimes flint nodules in 

 limestone rocks hasten erosive action. The limestone eroding, 

 more rapidly, leaves the nodule as a projection; a piece of 

 floating ice or a log strikes the nodule, knocking it from its bed, 

 leaving a hollow. Sand and gravel collect in the hollow and, kept 

 in motion by the water, soon forma pot-hole, which is an interest- 

 ing and effective mode of erosion in rocks, where in any way little 

 cavities are formed. 



Another special form of erosion, is by water-falls, which are 

 formed in various ways. The falling water splashing against the 

 foot of the cliff gradually undermines the upper portions, which 

 at length break down, and thus the falls recede and a gorge is 

 formed. If the lower portions are easily eroded, the action is 

 more rapid. At the Niagara Falls, the upper stratum is hard lime- 

 stone, the lower shale -which are favorable conditions for rapid 

 work. It is estimated that at the present time the falls are reced- 

 ing at the rate of about 2.4 feet per year. The Grand Canon of 

 the Colorado river about 300 miles long, and in some places 

 6,000 feet deep, is the work of the erosive forces of the river. 

 Sometimes a fissure determines the course of a river, and helps to 

 form its channel; but such cases are not as common as was for- 

 merly supposed. Ice in streams often adds considerably to their 

 erosive power. Land ice, or glaciers, are efficient geological 

 agents, which will be discussed in another place. 



A stream of water carrying sediment begins to drop it when- 



