104 



VOLCANIC PHENOMENA. 





Fig. 184. Volcan of Jorullo. 



snith half, which was much more arched, and separated from the other by 

 a diametrical split, only offers now a trace at the east, and an indication at 

 the west by the pumice tuta of Salvatore. The mountain, which LA proba. 

 bly represented in fig. 18b', was, says Strabo, very fertile on it* slopes; its 



Fig. 185. View of Vesuvius Fig. 186. Vesuvius in the time 



as it now is. of Strabo. 



summit was truncated, in a. great port united, entirely sterile, of a burnt 

 aspect, exhibiting cavities filed with cracks and calcined stones ; from which 

 it may be conjectured that these, places were formerly burning craters. All 

 leads to the belief that the cone, which alone bears the name of Vesuvius 

 now, all the products of which differ from the rocks of the somma, was not 

 formed till long afterwards, and probably at the time of the famous eruption 

 in the year 79, which cost the life of the Roman naturalist; it then, with, 

 out doubt, formed a permanent conduit in the midst of the matters which 

 are raised in form of a dome, and which has been enveloped by subsequent 

 scorise. This catastrophe seems to have produced but little lava, but a hor- 

 rible upheaval, which precipitated a great part of the mountain into the sea 

 (Pliny the younger), and buried Herculaneum and Pompeii, not under tor- 

 rents of melted matter, as commonly said, but under avalanches of pumice 

 which previously existed on the slope of the mountain, for Vesuvius itself 

 has never produced an atom. If the whole south slope turned towards the 

 sea is now occupied by lava, it is evident that before the formation of the 

 permanent volcan it was covered with pumice tufa, traces of which are still 

 heen at different points, the same as now on the external slope of the sornma, 

 and in all Campa'nia. 



11. Definition of a Volcan. In those events, it often happens 

 that the rent, which has given rise to observed effects, rs obstructed 

 or closed at a considerable depth, and tranquillity is entirely re- 

 stored, a^ at Monte-nuovo. Under other circumstances, on the 



1 1. What is a vo'can, or volcano ? 



