FRESH-WATER STREAMS. 635 



in geological changes, through the chemical action of its elements. 

 But these effects properly come under the head of Chemical Geology. 

 The operation loosens the grains of the surface, and thus gives a 

 chance to the winds to do work of erosion, which would otherwise be 

 impossible. The crumbling of the softer decomposable layers, in a 

 series of horizontal beds, may result, and thus the firmer strata be 

 undermined, so that gravitation has a chance to carry forward the 

 work of destruction, and thus make vertical and overhanging bluffs. 

 In the decompositions and recompositions in which the air takes part, 

 the presence of moisture is generally essential ; and the subject is 

 therefore considered under the head of Water. 



IV. WATER. 



Subdivisions of the subject. 



1. Fresh waters ; including especially Rivers and the smaller 

 Lakes. 



2. The Ocean ; including the larger Lakes, whether salt or fresh- 

 water, — the general facts being similar, excepting such as depend on 

 the tides and the kind and density of the water. 



3. Frozen waters, or Glaciers and Icebergs. 



4. Water as a Chemical agent. 



1. FRESH WATERS. 



The Superficial waters and the Subterranean may be separately 

 considered. 



A. SUPERFICIAL WATERS: RIVERS. 



1. General Observations on Rivers. 



1. Water of Rivers. — The fresh waters of the land come from the 

 vapors of the atmosphere ; and these are largely furnished by the 

 ocean. They rise into the upper regions of the atmosphere and, be- 

 coming condensed into drops, descend about the hills and plains, and 

 so begin their geological work, — gravity being the moving power. 



The amount of water in a river depends on (1) the extent of the 

 region it drains ; (2) the amount of rain, mist, or snow of the region; 

 (3) its climate, — heat and a dry atmosphere increasing the loss by 

 evaporation ; (4) its geological nature, — absorbent and cavernous 

 rocks carrying off much of the water; (5) its physical features, — a 

 flat, open, unwooded country favoring evaporation. 



The annual discharge of the Mississippi River averages nineteen 

 and a half trillions (19,500,000,000,000) of cubic feet, varying from 



