92 ON EROSIONS OP THE EARTH'S SURFACE. 



Terraces. During the rise of the land, the water would remain only over its 

 depressed portions, and it might be that the lakes or ponds thus formed, if ranged 

 along some extended depression of surface, would constitute a chain of lakes. 

 The water poured into them from the neighboring hills, would produce an ocean- 

 ward current, and this, passing through the barriers of the lakes, would begin to 

 wear them away. This would be the first step towards a river. 



2. The above processes continuing to go on, the lakes would become narrower 

 and the barriers be more deeply eroded, so that what we call a river would be the 

 result. The matter, however, which was worn away at the barriers would in part 

 be deposited in the deeper and more quiet water between them ; and in part be 

 carried forward to the ocean. Hence the process would be one both of erosion and 

 of filling up. 



3. That, upon the whole, the process of excavation exceeds that of filling up, 

 will be evident from the following facts : — 



1. The increase in the deltas of rivers. 



The Merrimack sends forward, annually, about 839,171 tons of sediment to 

 increase its delta at Newbui^port. 



The Ganges pours into the ocean, each }^ear, 355,361,464 tons of mud. 



The Mississippi carries forward 28,188,383,892 cubic feet, or one cubic mile in 

 five years and eighty-one days. Its whole delta contains 2720 cubic miles : and, 

 therefore, at the rate above indicated, 14,204 years would have been requisite to 

 form it. 



But such examples need not be multiplied, for every river tells the same story. 

 The amount of sediment at its debouchure is ever increasing, and, therefore, its 

 bed must be continually widening and deepening. 



2. Some portions of the banks of most rivers are composed of loose materials, 

 which form precipitous walls, and thus make it almost certain that the depression 

 now occupied by the river, was once occupied by the same sort of materials as the 

 banks, which the waters have carried away. 



3. Wherever rivers run through rocky gorges, especially if cataracts exist, we 

 find distinct evidence, in the worn appearance of the rocky banks, and sometimes 

 by pot-holes, that the stream once ran at a higher level than at present, as at the 

 Great Falls, on the Potomac, near "Washington : at the Falls on Genesee river, at 

 Portage, and on the Mohawk, at Trenton, New York : on the Connecticut river, 

 at Bellows Falls, New Hampshire. If the surface of the rock, however, has been 

 exposed for a very long time, the atmosphere and frost are very apt to cause it to 

 scale off so as to obliterate traces of river action. 



4. The modes in which rivers excavate their beds has been already^ given, 

 essentially, in describing the effects of water and ice upon the rocks. They are 

 briefly as follows : — 



1. By solution of the agents of chemical change. 



2. By direct solution of the constituents of rocks. 



3. By urging forward loose materials, such as sand, gravel, and boulders, over 

 the surface. When a gyratory motion is produced in the water, the eroding mate- 

 rials produce pot-holes. 



