160 Jounsron-Lavis— The Eruption of Vesuvius in April, 1906. 
great fluidity in consequence of the small cooling surface, for it must have been 
for a considerable distance from 5 to 12 or more metres in depth. The flow 
through this narrow valley was very instructive (see figs. A, 8, 9, Pl. IX.). The 
ravine at one moment was filled above the level of its banks; but the cohesive, 
viscous nature of the lava, and the quantity of scoria it cast off at its sides, kept 
it together. The scoria moraines are very striking in some parts (figs. 8, 9, 
Pl. IX., fig. 18, Pl. XI, and aa fig. A), especially at curves. The fluid rock 
left smears (BB) of itself, in consequence of the cooling and shearing as it flowed 
onwards. When the flood had passed, the main mass drained out of the ravine, 
leaving only, as it were, a small ingot (Cc) at the bottom. This was an exception 
to the lava in general of this eruption: in some parts it showed the corded-folded 
‘* nahoehoe”’ surface, instead of a rough, scoriaceous aa structure. This was, no 
doubt, due to the lower portion having time to liberate the major part of its 
vapour more gradually from the slower cooling, and the carrying on of the 
surface scoria by the falling, advancing flood. 
Figure A.—Dracrammaric Section or VALLEY THROUGH WHICH THE LAVA FLOWED ABOVE BoscorREcase. 
LL.—Level of lava at full flood. CC.—Residual ingot of lava remaining after flood 
AA.—Scoria moraines formed at sides at full flood. had drained away. 
BB.—Smears of cooled lava on ravine walls during full 
flood. 
The great fluidity of the lava when newly poured out, and in considerable 
bulk, permitted it to flow with great facility in very narrow channels (figs. 8, 9, 
Pl. IX., fig. 12, Pl. X., fig. 14, Pl. XI., fig. 16, Pl. XII.). At one spot it filled and 
followed the Circum-Vesuvian Railway-cutting (fig. 15, Pl. XI.), passing under 
a stone bridge with ease. As the flow slackened, the rapid loss of heat made it 
so viscous that the obstruction of the arch, compared with the cutting in which 
it was flowing, eventually arrested its advance. It then collected behind the 
bridge, and had already built up such a pile that the whole bridge had started to 
be pushed bodily on. At this point, however, the lava supply failed, and left 
things as seen in fig. 19. 
