Vulcanism and Compressive Movements. 499 
waters of superficial origin. If this water were superheated 
before absorption it might help to give liquidity to the magma, 
When the magma nears the surface a part of the occluded 
water may separate as steam bubbles. The energy spent in the 
formation of steam bubbles, and consequently in expanding 
and lifting the lava, is derived from the heat of the rocks 
solid or liquid. So far as this heat is mechanical, as suggested 
by Mallet, its source is gravity. If the heat be derived from 
the stores within the earth, its source is as certainly gravity, 
only in this case a very long time interval has elapsed between 
the accummulation of the heat during the segregation of the 
earth by gravity, and its transfer to the water. Thus in any 
case the energy of the steam used in vulcanism is indirectly 
obtained from the force of gravity. 
In the process of steam bubble development in magma the vol¬ 
ume is increased, and the specific gravity of the liquid mass is 
thereby lessened. Consequently the magma may reach the surface 
at places where gravitation would not have carried it but for 
its decreased density. It is well known that some 
intrusive rocks are amygdaloidal. Moreover, such rocks are 
known in regions which have undergone considerable denuda¬ 
tion since the intrusions occurred , 1 thus showing that steam 
bubbles may begin to form at considerable depths, and there¬ 
fore may greatly help the magma to reach the surface. The im¬ 
portance of the phenomenon doubtless largely depends upon the 
quantity of water occluded. If it be supposed that this process 
of deep steam bubble formation .does extensively occur, great 
outflows of lava might take place rather high upon the ridges, 
and inundate large areas of a region. 
The facts in some of the great lava regions of the world cor¬ 
respond with the above explanation. Geikie 2 has shown that 
the great Silurian, Carboniferous, and Tertiary regional erup¬ 
tions in the British Isles mainly occurred through comparatively 
low-lying fissures. Corresponding with this rule, the vast 
masses of lava of the Keweenawan of Lake Superior seem 
1 Ancient lava flows of Great Britain, by Sir Archibald Geikie: Vol. II, 
1897, pp. 31 and 130. 
8 Loc. cit., p. 468. 
