122 : CHARLES DAVISON 
intensity, for less than 1 per cent of those recorded on the Asama- 
yama were sensible to the observers on the mountain side. Here, 
again, we have no doubt a true cause of volcanic earthquakes, but 
one of unproved competence to produce the destructive shocks 
that visit the flanks of Etna and Monte Epomeo. 
The sudden injection of lava into fissures and cavities would © 
seem to be more effective and was regarded by the late Dr. H. J. 
Johnston-Lavis as the cause of the Ischian earthquakes. The 
theory may be held to account for the extreme localization of the 
earthquakes, for their repetition time after time in the same region, 
and for the brevity and sudden onset of the shock.* 
Within the last few years the view has been gradually gaining 
ground that many of the stronger volcanic earthquakes are in 
reality tectonic earthquakes precipitated by volcanic action, that 
they are due not so much to the actual formation of fractures 
within the mass of the mountain as to slipping along pre-existing 
fractures. ‘This view has the merit of connecting the earthquakes 
under extinct volcanoes such as the Alban Hills, with those, from 
which they do not in reality differ, under active or dormant 
volcanoes. 
Professor G. Platania has shown that the Linera earthquake 
of May 8, tor4, was connected with a pre-existing fracture of 
the volcano, and he urges that subterranean movements of the 
magma have produced powerful tensions in the eastern region of 
Etna, by means of which the strata have been fractured, or pre- 
existing fractures have been enlarged, or the strata have slipped 
along them, and that these are the causes of the earthquakes.” 
In some of the mining districts of Great Britain local earth- 
shakes occur which resemble the Etnean earthquakes so closely 
in their nature, and probably also in their origin, that it may be 
useful to give a brief description of one of them. 
In Figure 9 are shown the isoseismal lines of an earth-shake 
which occurred at Pendleton (near Manchester) on November 
25, 1905. The intensity of the shock was as high as 7 according 
to the Rossi-Forel scale (or 6 according to the Mercalli scale). 
The shock was felt over an area of 144 square miles. As the average 
« Monograph of the Earthquakes of Ischia (1885), pp. 89-95. 
2 Pubbl. dell’Ist.di Geog. Fis.e Vulcan. della R. Univ. di Catania, No. 5 (1915), p. 41. 
