PLUTONIC ROCKS 23 
The Plutonic rocks are those which have cooled 
and become solid under the surface of the earth at 
great depth. Plutonic rocks are highly crystalline. 
Rocks which have cooled slowly are always more 
crystalline than such as have cooled quickly. In 
this respect they are like some substances which 
form large crystals if their solution is subjected to 
slow evaporation, and small crystals if evaporated 
quickly. You might make a very strong solution 
of alum, and illustrate the principle for yourselves. 
Apply the heat of a spirit-lamp to half your solution 
so as to cause speedy evaporation ; small crystals 
will result. Allow the remaining half of your 
solution to evaporate without heat in a cool place, 
and you will, after a time, find much larger crystals. 
You may rest satisfied that a highly crystalline 
igneous rock, like granite, may always be called 
“plutonic”; it has been formed at great depth 
beneath the earth’s surface. Granite is a very good 
and fairly easily known example of a plutonic rock. 
It is composed chiefly of the minerals known as 
quartz, mica, and felspar. 
Secure a specimen of granite, and examine it 
carefully ; look at it through a good magnifying- 
glass, which no serious naturalist is ever without. 
You will see some oblong, white or pinkish 
crystals. These are composed of felspar—a word 
derived from the German feldspath, meaning rock- 
spar. See how these crystals glisten. Now look 
in the little mass of rock for mica crystals. These 
may be silvery white or bronzy black, and they 
are hexagonal—d.e., six-sided—in shape. With 
the point of your knife you can split these crystals, 
