586 Proceedings of the Royal Society 
thrust up. Two causes there are which so operate — the general 
subsidence of the crust of the earth, thrusting it in particular 
places over regions of elevation, into huge anticlinal wrinkles, and 
the expansive force of the heat from below, when once liberated by 
local relief from pressure. The former, as I have explained above, 
is what determines igneous action in any particular place ; the 
latter accounts for all violent action, such as highly inclined and 
overturned strata ; without it there would be no mountains, but 
only gentle elevations. 
A volcano is formed whenever the igneous matter from below 
either forces its way to the surface, or (what is perhaps a possible 
case) has way made for it through a crack in the superficial strata, 
formed as they bend into wrinkles. But the fact that volcanoes 
are found only in regions of elevation, proves to my mind that the 
first determining cause of volcanic action is the elevation, causing 
local relief from pressure, and enabling the solids and vapours to 
expand from the heat they contain. 
Proof has been found of the existence of active and recent 
volcanoes in the heart of the Asiatic and North American con- 
tinents (see Scrope on Volcanoes, pp. 405, 453), but they are very 
much more numerous on islands and coasts. There must be some 
reason for this, and, I believe, the reason is, that the ocean is the 
great agent of denudation and deposition ; consequently changes 
in the weight pressing on the various positions of the bed of the 
ocean are constantly taking place, and such changes cannot fail to 
cause changes in the limits of the areas of elevation and subsid- 
ence, and to produce irregularity in the wrinkling action ; and the 
more of such changes and irregularities there are, the more multi- 
plied will be the chances of local diminution of pressure sufficient 
to cause volcanic action. Over great continental areas, on the 
contrary, the wrinkling action will be undisturbed, and will go on 
for the most part more evenly, though it will still produce igneous 
action in some cases. 
