§ 7. STRUCTURE OF VOLCANIC MOUNTAINS. 813 



exhibited the first symptom of internal change, in an earth- 

 quake which occasioned considerable damage to many 

 neighbouring cities, and of whose effects traces may yet be 

 witnessed among the interesting memorials of the awful 

 catastrophe which soon afterwards took place.* After this 

 event, slight shocks of earthquakes were frequent, when on 

 the 24th of August, in the year 79, a tremendous eruption 

 of the long pent-up incandescent materials of the volcano 

 burst forth, and spread destruction over the surrounding 

 country, overwhelming three cities, with many of their 

 inhabitants, and burying all traces of their existence beneath 

 immense accumulations of ashes, sand, and scoriae. All the 

 fearful circumstances connected with this event, and the 

 attendant physical phenomena, are so well known, that it is 

 unnecessary to dwell upon the subject. 



From that period to the present time, the internal fires of 

 Italy have resumed their ancient focus, and Vesuvius, with 

 occasional periods of tranquillity, has been more or less 

 energetic. 



7. Structure of Volcanic Mountains. — As the 

 present active volcanoes for the most part emit streams of 

 lava, showers of ashes, cinders, and scoriae, and floods of 

 mud or tuff, their cones consist of erupted materials dis- 

 posed more or less concentrically ; and where sections are 

 exposed, the beds have what is called a qua-qua-versal dip ; 

 that is, they regularly incline on every side of the moun- 

 tain. These are termed Craters of Eruption ; they con- 

 sist of successive strata of volcanic matter poured out from 

 a fissure or vent, communicating with the deep-seated focus 

 of igneous action. 



The central crater of another class of volcanic moun- 

 tains, is formed of pre-existing horizontal rocks and strata, 

 that have been forced into highly inclined positions by a 

 sudden and violent upburst of incandescent mineral matter, 

 * Daubeny on Volcanoes, p. 152. Scrope on Volcanoes. 



