ACTION OF RUNNING WATER. 199 



will convince us that the amount is almost beyond estimate. All 

 the rivers on the face of the earth are constantly pouring their 

 waters into the sea, and yet its level is not affected in the slight- 

 est degree, hence we infer that the quantity of mositure evapo- 

 rated from the surface is exactly equal to the sum of all the rivers 

 of the world. If the evaporation and restoration of the waters 

 were all the effect which- is produced by the agencies just de- 

 scribed, little change would be accomplished upon the face of the 

 country over which the waters might flow in their passage to the 

 sea. But in the more elevated tracts of country, -the atmosphere 

 acts powerfully upon the soil, a':d by the influence of heat and 

 cold, by dampness and dryness, and of frost and rain, loosens the 

 most coherent masses and disintegrates the solid rocks. The 

 mountain streams flow down more or less charged with earthy 

 matter, worn from the soil and rocks over which they flow, Jn 

 their passages toward the sea, sometimes over an immense tract 

 of country, they often unite and pour their waters along with al- 

 most irresistible fury. The solvent power of the water assists 

 very materially in degrading the rocky channels through which 

 it flows, and acts powerfully on the alkaline and calcareous ele- 

 ments of the soil, and especially when it holds carbonic acid in 

 solution, which is almost always the case. When the earthy mat- 

 ter and pebbles are thus intermingled with running water, a new 

 mechanical power is gained, by the attrition as they are borne 

 along, thus sapping and gradually undermining high banks and 

 rocks, until at length the overhanging mass is precipitated into 

 the current and swept away by its waters. In this manner, islands 

 are cut off from the main lands, and shoals, and rich earthy de- 

 posits called deltas ure formed at the mouths of rivers. There is 

 nothing so very remarkable in the power of currents to transport 

 even heavy masses of stone, for we must remember that the spe- 

 cific gravity of water is much greater them air, and a stone im- 

 mersed in a stream will loose about half its weight, and many of 

 the lighter particles of the soil will almost float. 



Sir George Staunton estimated that the quantity of sediment 

 borne down by the Yellow River in China, in a single day, was 

 equal to forty-eight millions of cubic feet, and late observations 



