ORIGIX AND I'ORMATION OF SOILS. 1 '25 



tinu.'il transf>-r of water froin lowrr to liiglier levels. The 

 elevates! hinds are cooler than the valleys. In their re- 

 gion occurs a continual condensation of vapor from the 

 atmosphere, which is as continually supplied from the 

 heated vrvlleys. In the mountains, thus beo:in, as rills, the 

 streams of water, which, gathering volume in their descent, 

 unite below to vast rivers that flow unceasingly into the 

 ocean. 



These streams score their channels into the firmest rocks. 

 Each grain of loosened material, as carried downward by 

 the current, cuts the rock along which it is di'agged so 

 long as it is in motion. 



Tho sides of the channel being undermined and loosen- 

 ed by exposure to the frosts, fall into the strejtm. In time 

 of floods, and always, when the j^ath of the river has a 

 rapid descent, the mere momentum of the water acts pow- 

 erfully upon any inequalities: of surface that oppose its 

 course, tearing away the rocky walls of its channel The 

 blocks and grains of stono, thus set in motion, grind each 

 other to smaller fragments, and when the turbid waters 

 clear themselves in a lake or estuary, there results a bed 

 of gravel, sand, or soil. Two hundred and sixty years 

 ago, the bed of the Sicilian river Simeto was obstructed by 

 the flow across it of a stream of lava from Etna. Since 

 that time the i iver, with but slight descent, has cut a chan- 

 nel through this hard basalt from fifty to several hundred 

 feet wide, and in some parts forty to fifty deep. 



But the action of water in ]»ulverizing rock is not com- 

 pleted when it reaches the sea. The oceans are in i)erpet- 

 xial agitation from tides, wind-waves, and currents like 

 the Gulf-stream, and work continual changes on their 

 shores. 



Glaciers. — What liappens from the rapid flow of water 

 down the sides of mountain slopes below the frost-line is 

 also true of the streams of ice which more slowly descend 

 from the frozen summits. The glaciers a])pear like motion- 



