1909.1 THEIR CAUSES AND EFFECTS. 243 



Naples, however, only twenty-two miles away, the shock was felt 

 by but few people, and the seismographs in the observatory on Mt. 

 Vesuvius did not record it at all, though the instruments at Rome 

 and Florence showed the passage of some extremely light vibrations. 

 The depth of the focus has been calculated at about a half mile and 

 Casamicciola received the vertical shock. The latest eruption of 

 Mte. Epomeo, Ischia's great volcano, occurred in 1302. 



Many other instances of volcanic earthquakes might be cited, but 

 perhaps none within historic times have been more severe than those 

 which have been mentioned. All show extremely restricted areas 

 of disturbance, a fact which indicates a comparatively slight depth 

 for the origin of the shocks and a far smaller expenditure of total 

 energy than is developed in connection with the great tectonic 

 quakes. It must not be overlooked, however, that some earth- 

 quakes, the origin of which is doubtful, may rightly be assigned to a 

 volcanic origin. Furthermore, the intrusion during past geologic 

 time of countless dikes, sills and laccoliths of igneous rock, the 

 occurrence of which is known from exposures all over the world, 

 must have been accompanied by sudden dislocations, causing earth- 

 quakes. Such quakes would be of combined volcanic and tectonic 

 origin. It cannot be asserted positively that they are not occurring 

 at the present epoch. 



This brings me now to the consideration of the third and most 

 important class of volcanoes, viz., tectonic quakes, or those which 

 are caused by dislocations in the earth's rock crust due to the action 

 of mountain-building forces. Mountain regions of high geological 

 antiquity, like the Appalachian protaxis and the Scandinavian 

 Peninsula, have had time to adjust themselves to the crustal strains 

 due to their elevation and hence are rarely the scene of great earth- 

 quake shocks. In the younger mountain systems, however, such as 

 the Apennines, the Japanese archipelago. Central America and those 

 of California, where young strata abut unconformably against old, 

 the adjustment to strains is still going forward, the cumulative 

 effect being followed by sudden and irregular release of pressure, 

 producing the vibrations which we know as earthquakes. Some of 

 these tectonic quakes have sensibly affected enormous areas. That 

 of Lisbon, 1755, was felt from northern Africa on the south to 



