THE SOIL 81 



Felspar, chemically, is a double silicate of alumina and 

 potash or lime. SUicate of alumina is insoluble in water, 

 but the silicates of potash and hme 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 hmestone hUls 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. 



