Jounston-Lavis—The Eruption of Vesuvius in April, 1906. 177 
ejecta would reach lesser and lesser distances. At the same time, more hot and 
dry dust would be mixed with the coarser materials, rendering their repose on an 
incline more and more unstable. The mountain would then be sheathed in a 
thick mantle of a loose, incoherent collection of blocks and masses of different 
size, ranging down to flour-like dust. As the propulsive energy still further 
diminished, the fragmentary materials would only just fall without the crater- 
rim. Gradually the increasing pile of materials on the outer-lip of the crater 
would reach the limit of stable equilibrium, and would begin to slide down, 
starting other materials below them, so that the moving mass would rapidly 
acquire increased volume and momentum, and would sweep down, ploughing out a 
ravine just as a snow-avalanche does. This, no doubt, occurred again and again, 
all round the crater-rim, until the whole mountain-slopes were scored into ridges 
and furrows. 
The mechanism may be seen in an ordinary heap of dry sand; but by mixing 
any fine, light powder with different-sized dry gravel on a slope of about 27°, 
the process may be imitated to perfection. 
The gigantic size of many of the blocks ejected may have frequently been 
individually sufficient to have initiated the starting-point of a big slip. Such a 
mass as that seen in fig. 3, Pl. VII, and which weighs aé least 30 tons, has 
travelled over a kilometre from the crater, and denotes a very powerful impulse 
to have sent it so far with the high trajectory necessary to propel it from the 
crater-bottom. Had it pitched on the crater-edge, we can quite comprehend what 
a powerful initiator of an avalanche it would have been, not to speak of its 
function as a plough. The force with which projectiles struck the surface of 
the cone is witnessed by the pieces of metal gouged out of the steel-rails that 
remain of the funicular railways. 
In my first attempts to reach the crater after the eruption, the ascent along 
one of these barranco bottoms was most instructive. It required all the skill and 
nerve of un experienced mountaineer to ayoid converting oneself into the starter 
of an avalanche of stones, in which one stood of course every chance of being 
overwhelmed. Minor slips constantly occurred every few paces, and others were 
progressing in the neighbouring ravines. 
No doubt, in the period of great volcanic activity, the continuous quaking 
and tremor of the whole mountain must have vastly helped the shaking down 
of the unstable material from its slopes. In addition, the great pseudo-fluidity 
of fine dust, when very dry, is known to everybody. The powdered rock, 
mixed with coarser materials, acted the part of a solid lubricant, just as tale 
or plumbago-powder would of course do in a greater or less degree. 
Furthermore, all was at a very high temperature, and the convection air- 
current set up around each particle would diminish its effective weight, and 
