188 Jounston-Lavis—The Eruption of Vesuvius in April, 1906. 
volcano to the west of the north and south axis was more or less thickly coated 
with the fine red and grey dusts which are very slowly permeable, and when 
washed into the pores of permeable materials, make the latter equally resistant 
to the penetration of water. On the upper slopes of Somma, at the end of April 
and beginning of May, the greatest care was required, after rains, to walk 
anywhere but on the crest, for the moment the feet touched the slope, the wet 
dust became like the softest clay, or “slip.” Once started, one would have slid 
down hundreds of feet and been overwhelmed by a perfect mud-avalanche or 
stream. 
We should expect that, in all the region where, as we have said, this dust- 
covering existed, disaster would result from the heavy rains. Such was, in fact, 
the case. Cercola, S. Sebastiano, Barra, Portici, Resina, Torre del Greco have 
been from time to time overwhelmed with great mud-flows, or lava d’ acqua, 
as the people of the district call them, burying the streets and invading 
the houses, leaving in a few hours a metre or more of mud. 
To prevent this, since the eruption occurred, a sum of about four millions of 
francs has been spent in constructing walls or dams across any depression that 
during storms may form a water-course of any importance. ‘These uncemented 
walls or drighe are built of loose blocks of lava and scoria, and have the faces 
whitewashed, so that they are visible as horizontal streaks of white scattered over 
the western slopes of the mountain. 
Much valuable agricultural land has been destroyed where covered by the new 
lava-streams, which, from their very rough and rugged surface, will remain for 
centuries before they will again become fertile, unless some new eruption covers 
them with fragmentary materials. 
In the area where the fall of lapilli was thick, a vast amount of damage was done 
to the towns at the foot of Monte Somma. Most of the houses in Ottajano and San 
Giuseppe were wrecked by the enormous weight of material that collected on the 
flat (dastrico) or low-pitched roofs. The walls remained intact, except where levered 
over by the bending down of the beams of the roofs and floors inserted in them. 
It was to this collapse of roofs and floors that most of the loss of life was due, as a 
number of people had collected in the churches to pray under the weak and 
ill-constructed roofs, instead of getting on to them to clean them. Even during the 
heaviest part of the fall persons could move about if they covered their heads and 
shoulders with pillows, tables, and other such improvised shields. Taking the 
average depth of the deposit to be 0°'76m., as on the Piazza San Francesco, I 
have, by weighing a cubic metre of the loose ejecta, found that it amounted to 
about 2000 kilogr. per square metre—that is, over a ton and a half—a weight 
few roofs could stand. 
The south-east sector of the volcano received so little dust that the damage to 
