ORIGIN AND FORMATION OF SOILS. 125 
tinual transfer of water from lower to higher levels. The 
elevated lands are cooler than the valleys. In their re- 
gion occurs a continual condensation of vapor from the 
atmosphere, which is as continually supplied from tie 
heated valleys. In the mountains, thus begin, 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 dragged so 
long as it is in motion. 
The sides of the channel being undermined and loosen- 
ed by exposure to the frosts, fall into the stream. In time 
of floods, and always, when the path 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 stone, 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 river, 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 pulverizing rock is not com- 
pleted when it reaches the sea. The oceans are in perpet- 
ual agitation from tides, wind-waves, and currents hke 
the Gulfstream, and work continual changes on their, 
shores. 
Glaciers.— What happens 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 appear like motion- 
