G40 



HERCULANEUM AND POMPEII. 



[Ch. XXV. 



have been wider than those which I saw in 1857, shooting 



down slopes of 30 . 



In the escarpment of the Atrio, the stony lavas form no 

 more than a seventh part of the whole mass, the rest consist- 

 in^ of tuff or volcanic sand and fragmentary scoriae, 

 observed at many points ropy lavas in this section, and some 



•ottos which indicate tunnels like those in which modern 

 lavas often flow (see above, p. 626). For a short distance, 



I 



i-3 



some 



1857, and I have no doubt that it contained within it a stony 

 layer having a dip of at least 40°. 



Such gibbosities are 

 3COus streams, which 



stop at different heights on the flanks of the cone for want 



matter 



them 



proceed farther. 



H 



in the description of fig. 



mm and Pompeii.— I have spoken 

 68, of the alluvial matter which 

 foot of the mountain. Aqueous 

 vapours are evolved copiously from volcanic craters during 

 eruptions, and often for a long time subsequently to the dis- 

 charge of scoriae and lava : these vapours are condensed m 

 the cold atmosphere surrounding the high volcanic peak, and 



heavy rains are thus caused. 



The floods thus occasioned 



lm 



rent of mud is produced, which is called in Campania * lava 

 d' acqua,' and is often more dreaded than an igneous stream 

 (lava di fuoco) 



moves 



On the 27th of October, 1822, one of these alluviums de- 

 scended the cone of Vesuvius, and, after overspreading much 

 cultivated soil, flowed suddenly into the villages of St. Sebas- 

 tian and Massa, where, filling the streets and interior of 

 some of the houses, it suffocated seven persons. It will, 

 therefore, happen very frequently that, towards the base of 









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enti 



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v 





M 1 



intli. 





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stony layers in the Atrio have so steep a dip as to 

 resemble dikes ; but I doubt whether even these have been 

 tilted for some of the lava of 1857, on the N.N.E. side of the 

 cone near the summit, flowed down a steep slope, reaching 

 at one point an angle of 43°. I observed that its surface Jt ; 



was ropy and root-like, and had evidently consisted of very .^ 



viscous lava. It formed what was called the ' gibbosity ' of 



n 





run 

 lie in 



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befille< 



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5 and tl 



*onta]. 







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