24 THE OPEN BOOK OF NATURE 
and break layers of them away in thin six-sided 
plates. Further examination will show you that 
the crystals are set in a glasslike substance : this is 
quartz. But I must not tire you with too many 
details ; what I want to do now is simply to arouse 
your interest in the rocks, give you some general 
ideas about them, and encourage you to study books 
that are devoted entirely to the science of geology. 
I ought to add that plutonic rocks are named after 
Pluto, the mythical god of the lower regions. 
It may be asked why plutonic rocks, such as 
granite, so often appear on the surface. If they were 
formed at great depth you might naturally think 
they would always be found deep down in the earth. 
A little thought will soon remove your difficulty. I 
have spoken of earth movements by which land 
surfaces are elevated, and sometimes depressed. — 
When land surfaces are raised they are, of course, 
exposed to heat and cold, frost and wind, chemical 
action, and the ravages of running water. By 
gradual upheaval rocks which were formed thou- 
sands of feet beneath the surface of the earth may 
be raised to a great height above sea-level. Then 
the stratified rocks covering, say, the granite, in the 
course of ages of wear and tear are removed, and the 
underlying granite is laid bare. Large areas of 
granite and other plutonic rocks have been un- 
covered in this way, and they in their turn are being 
gradually worn away. | 
The igneous rocks called Volcanic differ from the 
plutonic in that they have issued from the bowels 
of the earth as lavas, and have become solid at or 
near the surface. Both volcanic and plutonic rocks 
