158 Jounsron-Lavis—The Eruption of Vesuvius in April, 1906. 
formation as part of the great cone and their breaking up and ejection, to intense 
and continuous fumarolic action. The result is that the dominant and prevailing 
tint is brick-red, which tint characterizes the bulk of the enormous breccia deposits 
at the toe of the great cone, as well as more than two-thirds of the stratum that 
buried Ottajano, San Giuseppe, and the neighbourhood. 
The escaping vapour that at first carries the accessory breccia and lapilli high 
into the atmosphere, will gradually become less and less, till any coarse material 
can reach little beyond the crater-rim, and most of the remaining explosive 
energy is employed in the grinding-up, by the issuing gases, of the fragmentary 
debris tending to fall down the chimney. The escape of these would still be 
sufficiently strong to carry upwards, in the highly heated column, large quantities 
of the fine resulting dust, to be wafted to one side or the other by the prevailing 
air-currents. 
This product is represented by the cocoa-coloured dust that was ejected from 
10 p.m. on April 8th till April 13th. The dust then quite suddenly changed 
colour to a light-grey, that gave to the mountain the peculiar snow-covered effect. 
Here I have only one explanation to offer, and that one I can only qualify as 
purely hypothetical. ‘Till the 13th it is probable that the chimney was still 
patent between the surface and the top of the magma, that the explosions were 
still able to deal with the main part of the falling materials, so that the passage 
remained open. About the 13th, probably, the continuous slipping of the crater- 
edges finally blocked the channel for the free evolution of gas, which was only 
able to escape from time to time, carrying with it the finer portions of the debris 
slipped in from the crater-edges, and which, at the line of truncation of the cone, 
have been little reddened by fumarolic action. It is also possible that the colour 
may have been changed by the filtering through the plug of loose materials by 
the volcanic vapours, producing a slight fumarolic action on each grain, so 
converting the peroxide of iron into chlorides, sulphates, or even sulphides. The 
analyses of the soluble parts of these dusts afford us uncertain information; but 
only small traces of acid vapours would be sufficient to change the colour, and this 
seems to be the case, as indicated in the analysis by Casoria (p. 174). 
Before leaving this general survey, there is one other slight difficulty that 
requires consideration. After the principal outpours of lavas had practically 
stopped, and after the great paroxysm by which we should have thought the lava- 
level had fallen below the level of the lateral outlets, we find recorded that about 
midnight of the 10th, the ‘“ bocca” above Terzigno again emitted some lava, that 
flowed over that already solidified. The explanation of this phenomenon that 
offers itself to one’s mind is that that lava was the remnant left in a radial dyke, 
or sensibly horizontal sill, which acted as a reservoir, the contents of which was 
squeezed out by settlement of the mountain. 
