Rocks. 
77 
molten matter is forced to the surface along cracks and 
fissures, and may be poured out as a lava stream from 
the mouth of a volcano ; such rocks are on that account 
called Volcanic. Igneous rocks do not occur in any 
particular order, and are not confined to any par- 
ticular age. Wall-like masses of Igneous rocks are 
found in Aqueous rocks of all ages. They are known 
as dykes, and represent fissures or lines of weakness, 
along which the molten matter was forced from below. 
Some of these great cracks may have been produced 
by the alternate subsidence and elevation of the land. 
METAMORPHIC ROCKS. — There is yet another 
group of rocks called Metamorpliic that go to make up 
the crust of the earth. These are rocks that resemble the 
Aqueous, inasmuch as they show some signs of stratifi- 
cation, whilo in their internal structure they resemble 
the Plutonic Igneous rocks. Modern research has 
shown that the Metamorpliic rocks were originally 
either Igneous or Aqueous, but have, since they were 
first formed, undergone changes that altered their 
structure. In this way some rocks that were formed 
under water have been so altered by heat that they 
show some features characteristic of Igneous rocks, 
and rocks of undoubted igneous origin are found 
occasionally to have developed a bedded and banded 
structure suggestive of stratified Aqueous rocks. 
The rocks, then, which go to make up the crust 
of the earth may be divided, according to their origin, 
into three groups — the Aqueous, the Igneous, and the 
Metamorpliic. The Aqueous rocks were formed under 
