636 DYNAMICAL GEOLOGY. 



limiting plane to grade from and toward in all its work. It is thus 

 enabled to make flat plains or terraces of great width along river val- 

 leys and seashores, and give horizontality to its deposits. 



The subject of Water is here presented under the heads, — 



1. Fresh Water; including especially Rivers and Lakes. 



2. The Ocean. 



3.' Frozen Water, or Glaciers and Icebergs. 

 4. Water as a Chemical Agent. 



I. FRESH WATER. 



A. SUPERFICIAL WATEES : 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, in 

 streamlets that combine into rivers and river-systems. 



The area which a river with its tributaries — or a river system 

 (p. 22) — covers is called its drainage-area ; and the amount of water 

 in a river depends largely (1) on the extent of its drainage-area. 

 The extent of that of the Mississippi is about 1,317,500 square miles ; 

 of the St. Lawrence, 298,000 ; of the Mackenzie River, 442,000 ; of 

 the Amazon, 2,264,000 (a third part of South America) ; and of the 

 La Plata, 886,000. North America has one river over 500,000 square 

 miles in drainage area; South America, 2, and Asia, 6 ; and this large 

 number in Asia is due to the size of the Continent, and the great 

 distance of the eastern and northern mountains ranges from the coast. 



The drainage area of the Danube is 23-4,000 square miles; Volga. 397,000; Rhine, 

 65,000; Elbe, 42,000; of the Nile, 520,000; of the Obi, in Asia, 925,000 ; Lena, 594,000 ; 

 Amur, 583,000; the Yenesei, 785,000; Yang-tze-kiang, in China, 548,000; Hoang-ho, 

 537,000 (the words Hang and ho signify river); Ganges, 432,000; Indus, 312,000. 



But the amount of water in a river obviously depends also on 

 (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, and metamorphic or crystalline rocks almost none ; 



(5) its physical features, — a flat, open, unwooded country favoring 

 evaporation. 



The amount of precipitated moisture is vastly the greatest in the 

 tropics, where evaporation is abundant, and where, if there are high 

 mountains, the contrasts of temperature at hand are extreme. It is 



