648 DYNAMICAL GEOLOGY. 



Ho, of China, is noted for its devastations* In 1850 it emptied into 

 the Yellow Sea ; now this mouth is dry, and it has a new channel 

 opening to the Gulf of Pechele, nearly 300 miles north of its former 

 outlet, and it departed from the old channel more than this distance 

 from the coast. This is the last of many changes, back and forth, 

 recorded by the Chinese during the past 3,000 years. The changes 

 begin in floods caused by the amount of precipitation in the distant 

 Kuenlun Mountains. Such overflows are destructive to lands and 

 crops that support millions, and often also to human life. 



The windings of a stream, in large alluvial flats, are most numerous 

 where the current is exceedingly slow ; for slight obstacles change the 

 course, throwing the current from one side to the other. Between the 

 mouth of the Ohio and the Gulf of Mexico (Head of the Passes), the 

 length of the Mississippi is 1,080 miles ; and the actual distance in a 

 straight line about 500 miles. 



Pot-holes are incident to the process of erosion, when the waters 

 flow in rapids over a bed of hard rocks. The rushing waters make 

 the large loose masses to revolve or rock, and this wears the surface 

 beneath, and gradually deepens it; and then the rapid whirl begins 

 which carries around stones and pebbles, and keeps up the wear. The 

 " Basin," in the Franconia Notch (White Mountains), is a pot-hole in 

 granite, fifteen feet deep and twenty and twenty-five feet in its two 

 diameters. There are many pot-holes at Bellows Falls, on the Con- 

 necticut ; others on Whi.te River, in the Green Mountains, and else- 

 where. One of those on the White River is fifteen feet deep and 

 eighteen in diameter ; another, twelve feet deep and twenty-six in 

 diameter. 



Nearly all the work of erosion, as well as of transportation, is carried 

 forward in times of floods. Streams that are sluggish and impotent 

 in the dry season, or even burrow out of sight, become torrents of tre- 

 mendous power during rains. The rivers of some dry countries, like 

 Australia, spread out in immense floods in the rainy season, and are 

 strings of pools in the dry. 



Occasionally storms pour down the rain in vast volumes, suddenly 

 making torrents in the mountain valleys of thirty to sixty feet in 

 depth, which tear away forests and remove whole hill sides to the 

 lower plains. 



The bursting of lakes, is one of the consequences of heavy rains, 

 though often due to undermining by subterranean waters, or to tun- 

 neling of the banks by burrowing animals (p. 608). The floods so 

 caused have the character of those arising from a sudden precipitation 

 of rain in the mountains, but sometimes with vaster results, the water 

 plowing profoundly into the slopes before it, and spreading the gravel 



