OX ITS EXTERIOK. VOLCANOES. 215 



there any permanent channel of communication open with 

 the interior; it is only rarely that any traces of modern 

 volcanic activity are found either within them or in their 

 vicinity. The force capable of producing such considerable 

 effects must have been long accumulating in the interior, 

 before it acquires sufficient strength to overcome the resist- 

 ance of the superincumbent mass, and is enabled, for example, 

 to raise new islands above the surface of the ocean, by 

 breaking through granular rocks and conglomerates (strata of 

 tufa containing marine plants). The strongly compressed 

 vapours escape through the crater of elevation, but the great 

 upheaved mass again falls back, and recloses the opening 

 thus momentarily produced by a vast effort. No volcano 

 could in such case be formed." ( 212 ) 



A volcano, properly so called, exists only where a per- 

 manent communication is established between the interior of 

 the earth and the atmosphere : the reaction of the interior 

 upon the surface is in such case continued during long 

 periods of time, and although interrupted for centuries, as 

 in the case of Vesuvius ( 213 ), it may afterwards be re- 

 newed with fresh energy. In the time of Nero there was 

 a disposition to class Mount Etna amongst the burning 

 mountains which 'were gradually becoming extinct; ( 214 ) 

 and at a still later epoch JUlian even affirmed that the 

 summit of the mountain was subsiding, that mariners could 

 no longer discern it at so great a distance from the shore as 

 formerly. ( 215 ) Where traces of the first eruption exist, or 

 where, if I may so express myself, the primitive scaffolding 

 is still preserved entire, the volcano rises from the middle 

 of the crater of elevation, and the isolated cone is surrounded 

 by an amphitheatre of lofty precipices, composed of greatly 



