240 THE ERUPTION OF VESUVIUS IN APRIL, 1906. 



deafening sound, were a netAvork of incessant lightnings, some 

 straight, some zigzag, and some in the form of tremendous arcs pro- 

 longed from the sunmiit of Monte Somma to that of Vesuvius. The 

 observations of Paola confirm those of Palmieri. The electric 

 potential of the ash that falls is always negative, whereas that of the 

 water vapor on the way to condensation, which rises, is positive. 

 The production of lightning in the vapor emitted from the crater 

 implies a rapid ascent ; that is to say, a great violence of explosion 

 and abundant solid material. This is why the electric phenomena 

 cease with the paroxysmal period, although the density of the clouds 

 remains great for days. 



THE MUD TORRENTS. 



The customary epilogue of the A^olcanic eruptions of Vesuvius con- 

 sists in the formation of mud torrents, the " lave d'aqua " and " lave 

 di tango " of this volcano being celebrated. The recent eruption has 

 furnished much nuiterial for detailed study by the localization of 

 the ejected products — thick beds of lapilli in the northeast sector, 

 the breccia with coarse constituents formed in the dry way upon the 

 cone and carried down by avalanches, and the fine dust scattered 

 everywhere on the volcano. 



The mechanism of the production of nuul torrents in the course of 

 an eruption is not complicated. The loose material freshly ejected, 

 always more or less porous, absorbs the rain water from the showers 

 of the upper slopes. If these are A^ery intense, as was the case in the 

 Antilles, a torrent can form immediately; but on Vesuvius, in the 

 first days of the eruption, the rainfall was localized, intermittent, 

 and slight, and imbibition took place gradually. When it had pro- 

 ceeded sufficiently far, the entire mass commenced to move upon the 

 slopes as a thick mud, which advanced rapidly down the valleys, 

 eroding and transporting much material. The mud lava finally 

 ceased to flow and b}^ its consolidation formed a conglomerate with 

 a chaotic structure. The most simple case is realized when the quan- 

 tity of rain is not great, but if the rainfall continues there comes a 

 more liquid flood, which erodes the nlud that has gone before, cuts 

 into the underlying material, and deposits on the lower lydvts of the 

 mountain sediments Avith torrential stratification. 



The mud lava on the northeast side of the mountain consisted of 

 rather even-grained lapilli united by fine mud. The flow occupying 

 the bed of the torrent of Ottajano, on becoming dry, formed a black 

 mass 8 meters wide by 0.75 meter thick at the front end. The surface 

 was cohered with concentric wrinkles like those of viscous lava and 

 longitudinal furrows, indicating the different levels of the stream. 

 It bore a remarkable resemblance to true lava floAvs. On drying, the 



