18 DYNAMICAL GEOLOGY. 



( Rivers, erosion, transportation, deposit. 

 f Mechanical.. 3 Ocean " " 



AQUEOUS J | j^ 



01 ' Chemical.. . j s P rin S s - chemical deposits in, 

 Lakes, " " 



SECTION I. RIVERS. 



Atmospheric or meteoric water falls on land as rain. A 

 portion sinks into the earth, and, after a longer or shorter 

 subterranean course and doing its appropriate work of 

 rock-disintegration and soil-making, comes up again to 

 the surface as springs. Another portion runs off the 

 surface, cutting and carrying away the soil everywhere. 

 Quickly, however/ it gathers into rills and cuts furrows, 

 these rills uniting into streamlets and cutting gullies. 

 The streamlets, uniting with each other, and with water 

 issuing from springs, form mountain-torrents, and cut out 

 great ravines, gorges, and canons. Finally, the torrents, 

 emerging on the plains from their mountain home, form 

 great rivers, which deposit their freight of gathered earth 

 and rock-fragments in their courses, and finally in the sea 

 or lake into which they empty. Such is a condensed his- 

 tory of the course and work of water from the time it falls 

 as rain until it reaches the ocean from which it came. All 

 of this we include under river-agency. It may be defined 

 as the work of rain and rivers, or the work of circulating 

 meteoric water. All that follows on this subject will be 

 but an expansion of the condensed statement given above, 

 and much of it may be observed by any one who does 

 not commit the mistake of thinking things insignificant 

 because they are common. 



1. Erosion of Rain and Rivers. 



The rain which falls on land-surface may be divided 

 into three parts : One part runs immediately from the 

 surface, producing universal rain-erosion and the muddy 



