172 DYNAMICAL GEOLOGY. 



2. Abundant precipitation. — The annual fall of rain (and snow) over the 

 Mississippi drainage area is, for the eastern, or Appalachian part, 40 to 50 

 inches ; for the much larger west-central part, west of the Mississippi River, 

 20 to 25 inches ; for the western part, among the summits of the Rocky 

 Mountains, 25 to 30 inches. In the vast Amazon drainage area the annual 

 precipitation exceeds 50 inches both on the west and north, and is every- 

 where over 25 inches. 



3. Upward tvaste, or that by evaporation, small. — Under a hot and dry 

 climate, and in the absence of forests, the waste is great. The western 

 tributaries of the Mississippi lose a large part of the waters received in the 

 mountains while descending the dry, bare eastern slopes. Where the Nile 

 takes its rise, the annual precipitation is over 50 inches, but it is not more 

 than 10 through the lower two thirds of its course. An extravagant example 

 of this waste is shown on the map of western Maui, on page 179, where there 

 are great channels in the mountains and mere threads over the surface to the 

 west where it seldom rains. 



4. Downward waste, or that by gravity and soil absorption over the drain- 

 age area, small. — Kot only loose sands, but also many sandstones are very 

 absorbent ; and limestones, although nearly impervious to moisture, are often 

 cavernous, and sometimes swallow up rivers. In western New South Wales 

 the rivers take only 2^ per cent of the precipitation, owing chiefly, it is stated, 

 to the porosity of the sandstone of the region. Most lavas are porous and 

 somewhat cavernous, but may lose these qualities by infiltration of earth 

 from decomposition. Further, most stratified uncrystalline rocks are loose 

 in bedding, and take off much water along the open spaces between the 

 layers. Granite and other crystalline rocks make the tightest basins ; for 

 they absorb little. 



Frozen or icy ground is like impervious rock ; almost all the water that 

 falls over it goes to the rivers. Moreover, in cold weather evaporation 

 carries off but little. Hence come the sudden rise and height of many 

 spring floods in cold-temperate latitudes. 



In very dry and warm climates, where the precipitation is reduced to a 

 few inches a year, rivers fail altogether, or flow only during the short rainy 

 season. Between drying up under the hot sun and soaking away in the 

 sandy soil, they are soon gone, and the lakes along their courses, or receiving 

 their waters, may share their fate. 



Other sources of loss in surface waters are (1) the demands of vegetable 

 and animal life ; and (2) the chemical combinations attending the decay of 

 rocks in which hydrous minerals, as the hydrous iron oxide and clays, are 

 made. 



Of the water precipitated, the rivers may get 45 to 50 per cent over regions 

 of crystalline rocks, as is true of the Connecticut River. In other parts of 

 temperate latitudes the amount is usually a third to two fifths of what falls. 

 But in warm latitudes it may be under one tenth. The mean annual dis- 

 charge of the Mississippi River is about 25 per cent of the precipitation; it 



