Jounston-Lavis—The Eruption of Vesuvius in April, 1906. 159 
Such, then, is a general review of the sequence of events that this paper is 
intended to record. We shall now proceed to examine the different products of 
the eruption, and see whether we can obtain from their physical character a 
confirmatory record of what took place. 
Tue Lavas. 
I have examined the different lava-streams to see if it were possible to find 
any marked difference between those emitted early or late in the eruption, or 
whether those that issued at a higher level or a lower one exhibited variations 
in character that might afford us any valuable information. Specimens were 
collected of the extreme scoriaceous crust, both at the top and bottom of the 
stream and one or more from the interior—that is so far as circumstances 
would allow. There is in all lava-flows a considerable difference between the 
crust that cooled quickly and the heart of the stream that may have taken 
months to completely solidify. ‘That difference is that the surface shows an 
abundance of residual glass, whereas practically all of it has become converted 
into a microlitic base in the interior of the stream. 
Such variations are to be met with in all the streams of Vesuvius, and the 
observer has to be careful that any particular flow is not considered different from 
another by the examination of too limited a number of specimens. In some of 
the upper flows it is quite impossible to examine the real interior; but large slabs 
can occasionally be met with and broken up—the most central part being used 
for study. 
The general character of these lava-streams is, that they possess an extremely 
rough scoriaceous surface (Pl. XIII.), quite of that kind which Dana called the 
aa type—a name that I have since applied to other lavas than those of the 
Sandwich Islands, and which is being now adopted by other geologists. These 
new lavas are very similar to many others emitted from Vesuvius in late years. 
Where flowing over a fairly level surface, they have the usual steep sides, and 
in a few spots have left the usual scoria-morainic borders. The depths vary 
from less than a metre to three or more, according to the angle of slope and the 
fluidity of the lava. In depressions, near obstructions, or where several streams 
are piled one above another, naturally the depths are much greater. Some 
interesting phenomena were displayed in the flow of the lava. 
Near its source the lava seems to have shown great fluidity. One observer 
credits it with a speed of 67 metres per minute, and another 134 metres per 
minute. Admitting that this last estimate is exaggerated, it must be conceded 
that the fluidity was very great. When it had cooled somewhat, and burdened 
itself with scoria, its progress was of course much slower. As the eastern main 
branch that destroyed Boscotrecase flowed through the narrow ravine, it retained 
