120 CHARLES DAVISON 
Ischian earthquakes of 1881 and 1883 the depth was estimated, 
from the inclination of the fissures in walls, to be 3 mile. Again, 
the duration of the preliminary tremor of an earthquake increases 
with the distance from the focus, and the practical absence of 
any such tremor therefore points to the nearness of the focus to 
the surface. 
We may thus conclude: (a) that the foci of volcanic earth- 
quakes are situated at a very slight depth below the surface; 
(b) that the foci are usually small and seldom more than 4 or 5 
miles in length; (c) that the after-shocks, when they occur, originate 
chiefly within the focus of the principal earthquake; and (d), from 
the discussion of the Etnean and Alban earthquakes, that, while 
the majority of volcanic earthquakes originate along radial frac- 
tures of the mountain, some, and by no means the least important, 
originate along perimetric fractures. 
Ill. THE ORIGIN OF VOLCANIC EARTHQUAKES 
The earthquakes which occur in the neighborhood of active 
or dormant volcanoes are naturally attributed to the processes 
which sooner or later tend to result in an eruption. As any sudden 
displacement of material within the earth’s crust may give rise 
to an earthquake, the processes which have been suggested as 
possible causes may be grouped under the following heads: 
a) The formation of new fractures, or the reopening or exten- 
sion of old fractures, in the mountain mass, due to the pressure 
of the column of lava or of gaseous materials in their progress 
toward the surface. 
b) Explosions of any kind, such as those from the sudden 
generation of steam within the volcano by the access of water to 
the highly heated rocks below. 
c) The sudden injection of fluid rock into fractures or cavities 
formed in the mass of the volcano. 
d) The slipping of the rock surfaces adjoining a fracture due 
to subterranean movements of the magma. 
All of these are possible causes of volcanic earthquakes, and 
all four processes may be in action during or near the time of an 
eruption. The question we have now to consider is which cause 
