THE SOIL 81 
Felspar, chemically, is a double silicate of alumina and 
potash or lime. Silicate of alumina is insoluble in water, 
but the silicates of potash and lime are soluble. Felspar, 
therefore, is the weak point in granite, and in the presence 
of water it gradually breaks down, owing to the loss of 
the other constituents which the water dissolves out. 
Just as the strength of every chain is the strength of its 
weakest link, so the durability of granite is measured by 
the durability of its felspar. The disintegration of the 
felspar causes the whole rock to crumble, the insoluble 
silicate of alumina being washed away as clay, and the 
quartz and mica as sand. The mica undergoes a similar 
series of changes as the felspar, but so much more slowly 
that it plays very little part in the disintegration of the 
rock. Water charged with carbonic acid gas also attacks 
limestone and chalk, dissolving out the carbonate of lime. 
It is for this reason that limestone hills are often honey- 
combed with caves and caverns, whose collapse has 
formed some of the most beautiful gorges in England— 
e.g., the Cheddar Gorge in the Mendip Hills of Somerset- 
shire. 
(6) Physically—Moving water carries with it solid 
particles. If the current is swift, heavy stones are rolled 
along which grind against one another and the rocks of 
the river-bed. The destruction of sea-cliffs by the waves 
is partly due to the water undermining the rock chemi- 
cally, and partly to their bombardment by sand and stones, 
especially during storms. 
2. Frost causes the water which has soaked into the 
rocks to freeze. When the water turns into ice it expands, 
and its expansion cracks and breaks the rocks in all 
directions. . 
3. Glaciers grind out the beds in which they move. 
This is not done by the ice itself, but by the angular stones 
and rock-fragments embedded in it. 
4. Changes of Temperature.—In the daytime rocks are 
heated by the sun, and expand ; at night they cool and 
contract. The effect of these movements, if great, is to 
shatter the rocks into fragments. The sands of the 
Sahara Desert have been formed in this way. 
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