46 VOLCANIC ROCKS, 
structure and composition of their crystalline 
mineral ingredients. In every particle of matter 
to which crystallization has been applied, we 
recognize the action of those undeviating laws of 
polar forces, and chemical affinity, which have 
given to all crystallized bodies a series of fixt 
definite forms and definite compositions. Such 
universal prevalence of law, method, and order 
assuredly attests the agency of some presiding 
and controlling mind. 
A further argument, which will be more in- 
sisted on in speaking on the subject of metallic 
veins, may be founded on the dispensation 
whereby the primary and transition rocks are 
made the principal repositories of many valuable 
metals, which are of such peculiar and indis- 
pensable importance to mankind. 
Chapter V. 
Volcanic Rocks, Basalt, and Trap. 
In the state of tranquil equilibrium which our 
planet has attained in the region we inhabit, 
we are apt to regard the foundation of the solid 
earth, as an emblem of duration and stabiUty. 
Very different are the feelings of those whose lot 
is cast near the foci of volcanic eruptions ; to 
