UNSTRATIFIED OR IGNEOUS ROCKS 289 
The following are some of the headings under which an 
examination of rocks, such as we propose to undertake, 
may be carried out. 
Colour. 
General Structure.—i.c. whether crystalline or frag- 
mental, whether close-grained or the reverse, whether 
particles are compactly or loosely held together, large or 
small, rounded or angular. 
Hardness. Specific Gravity. 
Cleavage. _ Taste. 
Fracture. Odour. 
XI.—UNSTRATIFIED OR IGNEOUS ROCKS. 
The unstratified or igneous rocks are those which have 
been erupted from the heated interior of the earth, and 
have consolidated from a state of fusion. They include 
both plutonic rocks or those deep-seated masses which 
have cooled slowly and under great pressure, and volcanic 
rocks whose lava streams have reached the surface, 
where cooling has been relatively rapid. It is not pro- 
posed however to make any distinction between plutonic 
and volcanic rocks—but to include them both in the term 
igneous. 
It is not within the scope of this work to do more than 
briefly refer to volcanic action ; for the causes which lead to 
its development, and for a more detailed account of its 
manifestations, the reader must consult some work on 
dynamical geology. 
The existence of a high temperature beneath the earth’s 
crust has now been placed beyond all doubt by the 
evidence furnished by voleanoes, hot springs, and borings. 
T 
