10 AQUEOUS AGENCIES. 



ing from their mountain-home upon the plains, form great rivers, which 

 deposit their freight partly in their course and partly in the sea. Such 

 is a condensed history of rain-water on its way to the ocean whence it 

 came. Our object is to study this history in more detail. 



Erosion of Rain and Rivers. 



The whole amount of water falling on any land-surface may be 

 divided into three parts : 1. That which rushes immediately off the 

 surface, and causes the floods of the rivers, especially the smaller 

 streams ; 2. That which sinks into the earth, and, after doing its 

 chemical work of soil-making, reappears as springs, and forms the 

 regular supply of streams and rivers ; and, 3. That which reaches the 

 sea wholly by subterranean channels. Of these, the first two are the 

 grand erosive agents, and these only concern us at present. Of these, 

 the former predominate in proportion as the land-surface is bare ; the 

 latter in proportion as it is covered with vegetation. 



Hydrographical Basin. — An hydrographical basin of a river, lake, 

 or gulf, is the whole area of land the rainfall of which drains into that 

 river, lake, or gulf. Thus the hydrographical basin of the Mississippi 

 River is the whole area drained by that river It is bounded on the 

 east and west by the Alleghany and Eocky Mountains, and on the 

 north by a low ridge running from Lake Superior westward. The 

 whole area of continents, with the exception of rainless deserts, may be 

 regarded as made up of hydrographical basins. The ridge which sepa- 

 rates contiguous basins is called a water-shed. It is evident that every 

 portion of the land, with the exception of the rainless tracts already 

 mentioned, is subject to the erosive agency of water, and is being worn 

 away and carried into the sea. There have been various attempts to 

 estimate the rate of this general erosion. 



Rate of Erosion of Continents. — This is usually estimated as follows : 

 Some great river, such as the Mississippi, is taken as the subject of 

 experiment. By accurate measurement during every portion of the 

 year, the average amount of water discharged into the sea per second, 

 per hour, per day, per year, is determined. This is a matter of no 

 small difficulty, as it involves the previous determination of the average 

 cross-section of the river and the average velocity of the current. The 

 average cross- section X average velocity = the average discharge per 

 second : from which may be easily obtained the annual discharge. Next, 

 by experiment during every month of the year, the average quantity of 

 mud contained in a given quantity of water is also determined. By an 

 easy calculation this gives us the annual discharge of mud, or the whole 

 quantity of insoluble matter removed from the hydrographical basin in 

 one year. This amount, divided by the area of the river-basin, will give 

 the average thickness of the layer of insoluble matter removed from the 



