CAUSES OF VOLCANIC ACTION* 187 
plate such as hava been produced by the agency of 
fire. 
Volcanic action has been defined to be the influ- 
ence exerted by the heated interior of the earth on its 
external covering. Under this definition, therefore, 
may be included all the subterranean phenomena, 
whether of volcanoes, or earthquakes, or those in- 
sensible movements of the land by which, as Mr. 
Lyell very plausibly supposes, large districts may 
be depressed or elevated without convulsions. 
As we have already described the nature of vol- 
canic products, it is unnecessary to allude to them 
at present; a few remarks, however, may not be 
out of place in relation to the different theories 
which have been formed for the explanation of vol- 
canic phenomena. 
The prevailing hypothesis in relation to volcanic 
action is that of Leibnitz, which is supported by 
Humboldt, Cordier, and, with some modifications, by 
Philips, and the most eminent geologists of the 
present day. This regards volcanic action as the 
necessary result of the influence exerted by the 
heated interior upon the cooled exterior masses of 
the globe. " If the earth," says Philips, " be now 
generally hot within, it must formerly have been 
hotter ; in the process of cooling, the exterior so- 
lidified part and the interior fluid parts contract un- 
equally, a ^general pressure and tension result, and 
the crust breaks locally to restore the equilibrium. 
Hence earthquakes and fissures, on some of which 
volcanic vents are established, which serve more 
or less to relieve the subterranean pressure, as 
earthquakes also do. If, in addition to this general 
view, we suppose the admission of water through 
fissures to particular parts of the ' ocean of melted 
rock,' it is easy to see that the observed mechani- 
cal phenomena of volcanoes and earthquakes will 
result as the effect of a local excitement superadded 
to a general operation." From extensive observa - 
