Jounston-Lavis—The Eruption of Vesuvius in April, 1906. 187 
rains in the intervals of large eruptions. The vast thickness of volcanic alluvial 
breccias that extend outwards from the base of the mountain was shown to 
be due to the erosion of the upper, and especially middle, parts of the slope. 
At that time I attributed all these breccias to an alluvial origin, and, no doubt, 
the largest proportion is due to that cause. The recent experiences of the 
eruption of Vesuvius have demonstrated that vast piles of volcanic rubble, or 
breccias, may be collected at the foot of the cone by the dry slips of fragmentary 
matter deposited on the upper slopes of the volcano, provided the angle is 
sufficiently high to allow of this movement of descent. No doubt, were we 
able to dissect the base of the old cone of Somma, when it attained 2300 metres 
or 7546 feet in height, we should encounter great deposits of such breccias, now, 
of course, buried under more recent deposits. That magnificent pile of breccias 
(Phase 11., period 4), 55 metres thick, displayed in the Vallone Grande, may well 
owe their existence to this cause.* One other explanation has offered itself to 
my mind, namely, that it is the result of an outburst on the west slopes of the 
great Somma cone, possibly represented by the Fossa Faraone and the Fossa 
Vetrana. To that possible conclusion I was led by the extraordinary accumu- 
lation at one part only of the surface of the cone. But this may well be a 
localization of accessory ejecta to a N. W. sector of the cone, such as occurred 
in the direction of Ottajano in 1906. 
This breccia resembles far more that collected around the foot of the great 
Vesuvian cone during the late eruption than the thick deposits around the foot 
of Somma, which usually show sorting and false bedding, characteristic of alluvial 
sorting. I have often, during big rain-storms, made a tour of the Lagnos on the 
north toe of Somma, and watched the great mud-streams pouring down along 
their bottoms, the finer and lighter scoriaceous or pumice materials, pushing along 
the heavier blocks, until by the diminishing declivity the water disappeared, 
in great part by sinkage, leaving the rock-burden as a future addition to the 
alluvial tuff. Hardly a year passes without some of the towns around the 
voleano having their streets covered by quite thick deposits brought down 
by the floods of rain. The whole question hangs on the ratio between the 
rainfall and the permeability of the surface. The moment the former exceeds 
the latter, a stream starts, and, from the high angle of its bed, and the in- 
coherence of its bed-materials, soon removes and carries these on downwards 
to lower levels. 
Though the north-east sector of Vesuvius and Somma suffered terribly from 
the showers of lapilli, little dust fell; and the rain will have to be very great to 
do much damage, as it will easily disappear by sinkage. That part of the 
* H. J. J.-L., op. cit., Quart. Journ. Geol. Soc., 1884. 
TRANS. ROY. DUB. SOC., VOL. IX., PART VIII. 2G 
