192 Vesuvius. [April, 



the entire conjoint mountain sliould be blown np by a more than 

 ordinarily vioieut paroxysm, and the crater of Somma re-formed.* 



Although many of the early writers have preserved for us much 

 of interest which relates to Vesuvius, yet their ideas of telluric 

 phenomena were always so involved with the supernatural, that we 

 are seldom able to reach the germ of truth which produced the 

 great mass of mythological romance.! 



Strabo, Yitruvius, Diodorus Sicnlus, and Tacitus, all speak of 

 the indications of the igneous origin of this region of Southern 

 Italy : Seneca also detected the true character of Somma (b.c. 1 to 

 A.D. 64), and pointed out that its crater had thrown out more than 

 its own mass of earthy matter, " being, in fact, a channel for the 

 fire, but not its food." 



By a careful study of volcanic mountains, we are enabled readily 

 to perceive that they possess a marked similarity, their character- 

 istic form being a cone, truncated at the summit by a funnel- 

 shaped cavity or crater. The cone is found to be built up of ashes, 

 scoriae, and other loose materials ejected from the crater, and 

 deposited in layers, which are thickest near the rim, and conse- 

 quently attain a steeper angle as the cone increases in height. 

 These loose materials are often bound together by outflows of 

 liquid mud, or lava, and the percolation of rain-water, acting on the 

 light tufls and loose volcanic sand, converts them into a heavy and 

 compact rock. 



These alternating layers of ashes, lava, scoriae, and mud, afford 

 an interesting illustration of suhaerial stratification resembling 

 marine and fresh-water deposits, and, like them, often rich in 

 organic remains. | 



But no marine or fresh-water deposits, however formed, could 

 have preserved for nearly two thousand years the rich treasures of 

 Eoman painting and ceramic art, in all their freshness and beauty, 

 which the buried cities of the Campania have revealed to us. 



Liquid lava streams seldom issue from the cone of eruption, 

 more frequently they make their escape from lateral vents at low 

 levels. When lava rises and overflows the crater, it usually breaks 

 down a portion of the wall, and so makes its escaj^e. 



Fragments of melted lava ejected from the cone harden instantly 

 on their surface, while the interior becomes vesicular from the ex- 

 pansion of the gases and minute particles of water disseminated 



* Scropc, 'Qu Volcanoes,' p. 189. 



t The contest between Earth and Sky whioli the Creek falile of Typhtviis 

 brin<:;3 before us, is locally assoeiiitcil with volcanic euer<,'y in the region ruuml 

 Naples, yEtna, and elaewht-re. — l'hillii)s, p. 2. 



+ An crni)ti()n which ocenvrrd in the island of AiTan in the Carboniferous 

 period, has covered and enclosed a forest of erect frees in volcanic matter. 

 I'rofessor Phillips (p. 129) nieutious seeing carbonized trees in lava near the 

 Hermitiige, Vesuvius. 



