Jounsron-Lavis—The Eruption of Vesuvius in April, 1906. 161 
The effect of obstructions is well illustrated in figs. 15, Pl. XI., and 16, 
PI. XII.; in one case (fig. 16), although the lava in the street without reached 
above the level of the first floor, and flowed on a great distance, yet, after the 
destruction of the wooden gates, it only advanced a comparatively short distance 
into the courtyard of the Villa Michele and Teresa Borosio. Where the lava 
followed streets, a similar effect was produced, unless the quantity was very great. 
The actual depths of the flow, on fairly level ground, can be roughly judged by 
the two figs. 12, Pl. X., and 18, Pl. XI. Another plate shows a row of houses, 
near Oratorio (fig. 17, Pl. XII.), which were of one story high, the ‘‘lastrico” roofs 
only showing, which would give an average depth of about 4 metres. When 
houses and walls were in the course of the stream, they could not long resist the 
pressure, and were soon pushed over, and the fragments carried on by the current. 
The effect of the lava where crossing a wall-lined road is shown in fig. 11, Pl. X. 
Here the wall was in the act of being bodily pushed over, when the supply of 
lava stopped. In certain spots the lava was so fluid that it flowed through the 
lower story of a house without overturning the building; but where this could 
take place, it was favoured by suitable doors and windows in the line of flow, and 
strongly built walls that were able to withstand the onslaught. 
Where trees of some bulk were enveloped, these were usually burnt through to 
the depth of the enclosing lava. Sometimes they were still to be seen standing, 
held up by the brand of carbonized wood, held in place in the thick scoria-stratum 
above the lava. In other cases they fell over, but were not burnt on account of 
the bad conductivity of the scoria-covering. Most, however, were completely 
incinerated, leaving no trace behind. 
The dry wood of houses was, of course, rapidly burnt up. Masonry was 
little altered, unless comparatively small pieces were enveloped in the lava, and 
remained there during the weeks of cooling. So far, little of such pieces have 
been recovered, as extensive quarrying operations are only now begun (Noy., 
1906). I found some interesting changes, however, in the ordinary red tiles, 
which have become a light pea-green, and mortar also has assumed a greenish 
tint. The investigation of these changes might afford some interesting facts. 
The surfaces of the lava-streams are abundantly strewn with the irregular 
globular masses that I described as flotation-bombs. I showed many years since* 
that these are not true bombs, but only masses accidentally caught up in a 
lava-stream, and enveloped in a crust of the fluid rock that they congeal around 
them, and are carried up to the surface, partly by their lightness, as they usually 
contain hollow spaces of one sort or another, and are also floated up by forming 
a surface for the liberation of vapour, just as lemon-pips float in an effervescing 
lemonade. I think a still better term for them than flotation-bombs, as they are 
* “On the Fragmentary Ejectamenta of Volcanoes,”—Proc, Geol. Assoc., vol. ix., pp. 421-4382, 
