THE LEVELLING POWER OF RAIN. 373 



headlong down the steep side of a hill indicates an upward 

 action exerted by the force of gravity. 



Even small rivers, when greatly swollen by rain, exhibit 

 great energy in removing heavy masses. Thus Lyell 

 mentions that in August, 1827, the College, a small river 

 which flows down a slight declivity from the eastern water- 

 shed of the Cheviot Hills, carried down several thousand 

 tons' weight of gravel and sand to the plain of the Till. This 

 little river also carried away a bridge then in process of 

 building, " some of the arch stones of which, weighing from 

 half to three-quarters of a ton each, were propelled two miles 

 down the rivulet" " On the same occasion the current tore 

 away from the abutment of a mill-dam a large block of green- 

 stone porphyry, weighing nearly two tons, and transported it 

 to a distance of nearly a quarter of a mile. Instances are 

 related as occurring repeatedly, in which from 1000 to 3000 

 tons of gravel are in like manner removed by this streamlet 

 to still greater distances in one day." 



It may appear, however, to the reader that we have in 

 such instances as these the illustration of destructive agencies 

 which are of their very nature limited within very narrow 

 areas. The torrent, or even the river, may wear out its bed 

 or widen it, but nevertheless can hardly be regarded as 

 modifying the aspect of the region through which it flows. 

 Even in this respect, however, the destructive action of water 

 is not nearly so limited as it might appear to be. Taking a 

 few centuries or a few thousand years, no doubt, we can 

 attribute to the action of rivers, whether in ordinary flow or 

 in flood, little power of modifying the region which they 

 drain. But taking that wider survey (in time) of fluviatile 

 work which modern science requires, dealing with this form 

 of aqueous energy as we deal with the earth's vulcanian 

 energies, we perceive that the effects of river action in the 

 course of long periods of time are not limited to the course 

 which at any given time a river may pursue. In carrying 

 down material along its course to the sea, a river is not 

 merely wearing down its own bed, but is so changing it that 



