26 DYXAMICAL GEOLOGY. 



Fig. 156 (page 250), as will be more fully explained here- 

 after. 



Time. The time during which the whole of this enor- 

 mous work was done is but a small portion of the geolog- 

 ical history. It commenced in Middle Tertiary (page 

 383), continued to the present time, and is still going on. 



Pot holes. If we examine the bare rocky beds of 

 swift streams in mountain regions,, we often find deep 

 holes with vertical walls like small rock wells. In their 

 bottoms we are sure to find gravel and a good many 

 rounded pebbles. These are called pot holes. They are 

 formed thus : Swift streams form whirling eddies, in 

 which sand, gravel, and rock fragments carried by the 

 stream are whirled about in the same spot until they hol- 

 low out these holes, while the fragments themselves are 

 rounded into pebbles in the process. These become signs 

 of old river beds where rivers no longer exist. 



2. Transportation and Distribution of Sediments. 



River agency, it will be, remembered, is taken up under 

 three heads. We have already taken up one Erosion. 

 The other two are best taken up together, as Transporta- 

 tion and Distribution of Sediments. 



Transporting 1 Power of Water. Every one is fa- 

 miliar with the fact that running water carries along 

 materials of different degrees of fineness, but the rate at 

 which the carrying or lifting power increases with the 

 velocity is almost incredible to those who have not inves- 

 tigated the subject. It is found that the size or weight 

 of the separate particles or fragments movable by running 

 water increases at the enormous rate of the sixth power of 

 the velocity of the current. Thus, if the velocity of a 

 current be doubled, it can carry a stone sixty-four times 

 as great as before ; if it be increased ten times, it can 

 carry a stone 1,000,000 times as great as before. We can 



