ROCKS 33 
changes since the time of their formation, so that it is not 
always possible to tell what their original character may have 
been. We have thus three more or less well-marked types of 
rocks, which may be termed Igneous, Derivative, and Meta- 
morphic rocks respectively. 
L-IGNEOUS TROEKS 
This division includes all masses which owe their origin 
to the operation of eruptive and volcanic forces. Some of 
these rocks consist either wholly or in part of crystalline 
ingredients, while others are composed of fragmental materials. 
Hence we have two groups, viz.:—A. Crystalline, and B. 
Fragmental or Clastic Igneous Rocks. 
A. Crystalline Igneous Rocks 
The rocks of this group vary much in character. Some 
are thoroughly crystalline, while others consist partly of 
crystalline minerals and partly of non-differentiated matter— 
the relative proportion of crystalline and non- crystalline 
ingredients varying indefinitely. 
All these rocks have consolidated from a state of igneous 
fusion—the general character of each having been largely 
determined by the conditions under which the original molten 
matter or magma has cooled and solidified. That magma 
has a complex chemical composition, but may be said to 
consist essentially of a mixture of several silicates and 
oxides, with water and various gases. As soon as the 
temperature of this mixture begins to fall, the commingled 
ingredients commence to separate out successively—in other 
words, molecules of a like kind gather and group them- 
selves together to form crystals. Sometimes the cooling 
process is so protracted that all the several compounds 
constituting the magma have time to become thoroughly 
crystallised. In other cases solidification takes place more 
rapidly, so that crystallisation is only partially effected, and 
the resulting rock then consists of a mixture of crystalline 
ingredients and glassy or non-differentiated matter. Occa- 
sionally, indeed, cooling and consolidation proceed so promptly 
that no crystals have time to form before the whole mass 
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