MR. ROBERT MALLET OX VOLCANIC ENERGY. 
215 
have been already urged to Hopkins’s lakes. Either the lakes or the liquid zone lie far 
too deep to be in communication with the surface-waters ; and if once such a commu- 
nication be supposed established, no reason can be assigned why the volcanic eruption 
thus brought about should ever end before either the limitless supply of water were 
ended or the reservoir of liquid lava completely pumped out. 
205. Whether coming from liquid spherical shell or lake of lava, it is hard to see why 
the lava of the same volcanic vent drawn from the same reservoir should not always be 
the same. 
Every thing indicates that the actual focus, where fire and water contend and produce 
volcanic action, is at no great depth below the volcanic vent. 
The directions of the shocks felt during eruptions by observers not far removed from 
the axes of volcanic vents conclusively establish this. The centre of impulse of these 
shocks is coincident (on the whole) with the volcanic focus. Now, were this at a great 
depth, the emergent wave-paths around the base of the volcanic cone, and for considerable 
distances from it, must be almost vertical ; houses &c. shaken down must show that they 
were so by forces suddenly throwing them upwards and letting them fall again in lines 
not very far from vertical. But such are not the facts even in the close neighbourhood 
of the great South- American and Oriental volcanoes ; the shocks near the base are felt 
to approach nearer to horizontality than verticality, as is also the case with the best 
observed European eruptions. 
206. But the writer is enabled to produce direct proof of this in the case of the greatest 
of European cones. In 1864, while exploring Etna, he noted and measured the directions 
of the cracks produced in a large number of more or less ancient church-towers and 
other buildings by the shocks of successive eruptions at various periods. 
These observations were made at different towns or places, extending round a very large 
arc of the total circumference of the mountain. In every instance the masonry fractures 
pointed to a wave-path coming from near about the axis of the cone, and from a centre 
of impulse situated not very many miles below the level of the sea. This is conclusive 
as to the focus being not very deep; were it 800 miles deep, or half that, the injury 
done to towers and buildings must present a wholly different character, and the apparent 
verticality of the shocks could not escape universal notice. 
207. But if the volcanic foci lie in lakes or perennial spheric sheets of liquid lava, 
then they must be, ex hypothesis below the thick crust, and at an immense depth, and that 
nearly the same everywhere. 
But water must reach that depth, great as it may be ; and assuming the possibility of 
its access, it is difficult to see how, unless (notwithstanding the high temperature) it is 
compressed to a greater density than the liquid lake of solid lava, it can so get beneath 
or mixed up with the latter as to be able to blow it up in a hoursoufle condition, and 
enable it to reach the surface through ducts or fissures of 500 or 800 miles in length, 
the walls of which are comparatively cold. 
208. Recognized phenomena of different volcanoes indicate that they do not all come 
