12 ELEMENTARY GEOLOGY 
of streams, and other causes, wear the minerals into 
fragments by mechanical action. The fragments thus 
obtamed may be gathered into beds or layers, form- 
ing new kinds of rocks in the sea. 
Again, the action of the water and air causes some 
of the minerals to decay, and the rocks to crumble. 
These soil fragments may be washed away by rivers 
or waves, and gathered into beds of rock fragments. 
At the same time, from the decay of minerals, soluble 
salts are formed, and under favorable conditions these 
may be borne away by the water and deposited in 
layers. Plants or animals may take mineral sub- 
stances from the earth or water, which upon their 
death are accumulated into beds, and thus made to 
again enter into the construction of rocks. 
Any rock may disintegrate either by mechanical or 
chemical means, though some do so more readily than 
others. Let us take granite as an instance of a rock 
exposed to the disintegrating action of the air. When 
it first became a solid, as a result of the cooling of 
melted lava, it had a temperature certainly many 
hundred degrees above the boiling-point of water. So 
when finally exposed to the air and to percolating 
water, the minerals of the granite find themselves 
existing under conditions of temperature different from 
those that were present when they developed. Some 
of them are ready for change. 
