CHAPTER III. 



THE WORK OF RUNNING WATER. 



Familiar phenomena, both of land and sea, reveal the constant 

 activity and importance of water as a geologic agent. Even when 

 there is no precipitation the moisture in the air influences its activity 

 in certain ways. Just as iron ''rusts" more readily in moist air than 

 in dry, so changes in other mineral substances are influenced by atmos- 

 pheric humidity. Where precipitation takes place the results are 

 more obvious. The passing shower works changes in the surface of 

 the land, striking in proportion to the rate and amount of precipita- 

 tion. The rains feed the streams, and every stream is modifying its 

 bed, and with increasing rapidity as its current is swollen. Even the 

 moisture which is precipitated as snow works its appropriate results. 

 Before it melts it protects the surface against other agents of change; 

 but if it accumulates in sufficient quantity in appropriate situations, it 

 may give rise to avalanches and glaciers, which, like running water, 

 degrade the surface over which they pass. 



A part of the water which falls as rain, and a part of that which 

 results from the melting of snow and ice, sinks into the soil and into 

 the rock below, becoming ground water. It is this ground water which 

 especially justifies the name hydrosphere, often applied to the waters 

 of the earth, for it literally forms a spherical layer in the outer por- 

 tion of the solid part of the earth. During the stay of the water be- 

 neath the surface it effects changes in the rocks through which it 

 passes, dissolving mineral matter here and depositing it there, sub- 

 stituting one substance for another in this place, and effecting new 

 chemical combinations in that. Slow as these processes are, they have 

 worked wondrous changes in the course of the earth's history. 



When the waters are gathered together in ponds, lakes, and oceans, 

 they are still active, and the results of their activity are seen along 

 the shores, where winds and waves produce their chiefest effects. Even 

 the ocean currents, far from land, and the processes of the deep sea, 

 are not without their effect on the course of geological history. 



The work of the surface waters, ground (underground) waters, stand^ 



ing waters, and ice will be considered in order. 



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