Jounsron-Lavis—The Eruption of Vesuvius in April, 1906. 175 
chloromanganokalite, apthitalite, gypsum, and other minerals of this class, which, 
we shall see, were ejected in a state still unpulverized at the same time. It is 
probably the presence of a little free acid that has changed the red peroxide of 
iron, making the colours differ. In fact, as seen in the table, Casoria found these 
dusts to contain free acid, which he determined as double in the grey dust, com- 
pared with that in the red dust. 
During the suspension in the atmosphere of these dusts, small local showers or 
rain-drops fell, and produced at certain spots great numbers of pisolites. These, 
though very small, exhibit all the characters of those I described in the Plinian 
dusts and other similar ones ;* that is, there is a minute central cavity formed by 
the jamming together and arching of the first coarse particles caught by the rain- 
drop. Around the coarser-grained nucleus come layers of finer and finer material, 
as the available water for wetting got scarcer, so that, at the surface, it was only 
the most impalpable powder that could adhere. 
The thin stratum of black sand at the base of the deposit at Cancello di Cook, 
and the Eremo, although shown in section as a and 4, represents a good deal of fine 
accessory or even essential material also, which could reach these localities quite 
early in the eruption, probably corresponding, as I have already said, to the first 
ejection of black sand that fell in Naples on the evening of the 4th of April. 
Prof. Semola, of the R. Observatory of Capodimonte, found that the black 
sand that fell there on April 4th, 5th, and 6th weighed 1170 grammes per 
litre or decimetre cube, that is, in bulk slightly heavier than water. The 
reddish dust of the following days he found weighed 0°880 grammes per 
decim. ¢., or in bulk lighter than water. Casoriat gives the specific gravity of 
the grey dust of the 4th, 5th, and 6th as 2°7159, and that of the cocoa-coloured 
dust 2°7706; but his was collected much nearer the eruptive centre. 
THE Cone AnD ITs MopIFICATIONS. 
The dominant features of the eruption of April, 1906, of Vesuvius were the 
extensive truncation of the great cone, and the large amount of fragmentary ejecta 
spread over the mountain and region around Vesuvius. If we go back over a 
century, we can give many examples of as extensive outflows of lava, but we find 
that the only parallel for paroxysmal energy and degradation of the cone is that 
of 1822. 
From whatever aspect an observer looked at Vesuvius for many years before 
the recent eruption, the striking characteristic of it was the symmetrical form of 
the great cone, only broken here and there by the humps on the 8.8. W. and. E. 
slopes. These were most apparent as interfering with the symmetry when the 
* “Geology of Monte Somma and Vesuvius.’’-—Quart. Journ. Geol. Soc., vol. xl, 1884, p. 83. 
| Op. cit. 
