372 ALTERATION OF THE RIVER LEVEL. 



may have been effected by an annual volume of water, differ- 

 ing little from that of the present river. 



A given quantity of water will, however, produce very dif- 

 ferent effects, according to the rapidity with which it flows. 

 " We learn from observation that a velocity of three inches 

 per second at the bottom will just begin to work upon fine 

 clay fit for pottery, and however firm and compact it may be, 

 it will tear it up. Yet no beds are more stable than clay 

 when the velocities do not exceed this ; for the water even 

 takes away the impalpable particles of the superficial clay, 

 leaving the particles of sand sticking by their lower half in 

 the rest of the clay, which they now protect, making a very 

 permanent bottom, if the stream does not bring down gravel 

 or coarse sand, which will rub off this very thin crust, and 

 allow another layer to be worn off. A velocity of six inches 

 will lift fine sand, eight inches will lift sand as coarse as 

 linseed, twelve inches will sweep along fine gravel, twenty- 

 four inches will roll along rounded pebbles an inch diameter, 

 and it requires three feet per second at the bottom to sweep 

 along shivery angular stones of the size of an egg."* 



If, therefore, we are justified in assuming a colder climate 

 than that now existing,, we should much increase the erosive 

 action of the river, not only because the rains would fall on a 

 frozen surface, but because the rainfall of the winter months 

 would accumulate on the high grounds in the form of ice and 

 snow, and would every spring produce floods much greater 

 than any which now occur.-)- 



Moreover, as Mr. Evans has well pointed out, in ancient 

 times, and before the river valleys were excavated to their 

 present depths, the chalk might have been saturated with 



* Cyc. Brit., article "Rivers," t See Murchison's Geology of 

 p. 274. Russia and the Ural Mountains 



p. 572. 



