124 CHARLES DAVISON 
fault, or, possibly, by the pumping of water from the mine.t In 
other words, the Pendleton and other similar earth-shakes are 
of natural origin in so far as they are due to the growth of faults, 
but of artificial origin in that the slips are precipitated by mining 
operations. 
An active volcano is coursed by many fractures, the majority 
of which are radial, but a few perimetric. If the rock mass or 
masses were in some way to be deprived of support, one mass 
would slide against the other, and the friction of the grating sur- 
faces would give rise to an earthquake, not to be distinguished, 
except by its scale, from a true tectonic earthquake. In what way 
or ways could the rock mass be deprived of its support? Professor 
Platania suggests by underground movements of the magma,’ 
and this might evidently be the case beneath active, and possibly 
beneath dormant, volcanoes. In all kinds of volcanoes, and 
especially in extinct volcanoes, another cause may be in operation, 
though acting much more slowly and giving rise therefore to infre- 
quent shocks like those of Ischia. This is the gradual cooling 
of lava or heated rock beneath the fractured mass. And it should 
be noticed that the intensity of the resulting shock would not 
depend so much on the distance to which the support is withdrawn 
as on the area of the focus and on the weight of the rock displaced. 
This theory, it will be seen, accounts for all the known phe- 
nomena of volcanic earthquakes—for their close connection in 
space and time with many eruptions, the shallowness of their 
foci and the great intensity of the shock within a limited area, the 
small size of the foci and the brief duration of the shocks, the 
occurrence of series of fore-shocks and after-shocks limited in 
duration and in space chiefly to the original focus, and, lastly, 
for the occurrence of volcanic earthquakes beneath dormant and 
extinct, as well as beneath active, volcanoes. 
If this is the correct explanation of the cause of the more 
important volcanic earthquakes, it follows that such earthquakes 
are of tectonic origin in so far as they are due to the growth of 
faults, but of volcanic origin in that the slips are precipitated by 
present or past volcanic operations. 
t Geol. Mag., Vol. III (1906), pp. 171-76. ° 
2 Pubbl. dell’Ist. di Geog. Fis. e Vulcan. della R. Univ. di Catania, No. 5 (1915), 
pp. 1-2, 41. 
