OF MONTE SOMMA AND VESUVIUS. 1138 
The erosive action of water may be easily comprehended by any 
one who pays a visit to the mountain after a heavy rain. In Sep- 
tember 1881, after a downpour of a day and a night, I saw the result 
of a few hours’ erosion just above Resina, where the slope of the 
ground is less than 10°, and covered with cultivation-works such as 
terraces, banks, &c., which tend to break the impetus of the water. 
Many walls were completely overthrown, and in one place the water, 
after bursting through two high well-built ones, with drainage-holes 
through them, then traversed a vine-garden, where it excavated out 
of smooth nearly level ground a ravine 43 metres deep and 33 broad 
at the top. The débris of this excavation, consisting in part of large 
blocks of lava, &c., was spread over the surface of the ground, 
tearing up and breaking off the vines, and removing the bark of the 
trees in the torrent’s course. 
The roads around the base of the mountain are often covered 
with a deposit of sand, which in some cases, after much rain, renders 
them impassable. In 1881, men were employed many days in 
carting away this alluvium near Torre del Greco and at Resina, the 
site of Herculaneum; this year, 1882, I am told that the Castella- 
mare railway traffic was suspended for some days fromthe same cause. 
Jt is this mass of material continually being deposited after each 
storm that accounts for those large beds of alluvial breccias and 
tufas that have been formed since the end of Puasz VI. 
The valleys we have been discussing extended higher up the 
original cone. This conelusion is forced upon us by the peculiar 
serrated outline of the ridge of Monte Somma. LHach depression 
usually corresponds to the basin of one or more valleys, and is often 
cut through so many thick and tough lava-streams that we can only 
explain the eroding away of such rock by water derived from greater 
heights, which at this line must have already formed a torrent of 
enormous excavating power. ‘The serrations may be regarded, 
therefore, as so many natural sections of the valleys which have 
been sheared from above downwards, and from within outwards, 
by the enlargement from time to time of craters of paroxysmal 
eruptions. 
EXPLANATION OF PLATE II. 
Fiqure l. 
Twelve sections taken at different points around Monte Somma to show the 
correlation of one deposit with its equivalent in a neighbouring 
section (see next page). 
Figure 2. 
A tangential, or more correctly circumferential section taken at the end of a 
radius of 3160 metres from the centre of the i872 crater. This ex- 
hibits the change in position of the valley at different epochs and the 
variability in thickness of each bed. 
Nors. In both figures the colours show the age of the deposit, whilst the 
shading represents its lithological characters. Where the figure shows the bed 
only as a thin line its characters are indicated by index letters for colours, and 
numbers for shading. 
Q.J.G.8S. No. 157. I 
