114 ELEMENTARY GEOLOGY 
as soon as the sun sets, they cool rapidly and contract. 
This may cause the brittle minerals to snap at the sur- 
face, so that fragments fall off. 
In early times, before mining was a science, this fact . 
was utilized. Fires were built beside the rock which 
was to be mined, and this was then suddenly cooled. 
One may see this action of expansion and contraction, 
in any stone building that has burned. The great heat 
causes the rocks to splinter and crack, or the water that 
is thrown on the warmed walls, brings about the same 
result by producing contraction. On a much less 
noticeable scale, nature is constantly at work in the 
warmer lands, causing the. rocks to crumble by this 
change of temperature. 
Effect of Moisture and Dryness. Some rocks which 
are open to the air, crumble when dry, while others are 
made to crumble by the addition of water. Since 
rocks in exposed places are subjected to changes in 
the amount of water which they contain, they are often 
slowly caused to break, and fall into bits, by alternate 
wetting and drying. 
Action of Plants. If we look at any granite ledge, 
we find the surface rough, and here and there little 
bits of gravel are seen, that by some means have been 
worked off from the solid rock. Upon its surface we 
find another agent of destruction in the form of plants, 
either lichens (Fig. 49) or mosses. Attempting to pick 
