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H, J. JOHNSTON-LAVIS ON THE GEOLOGY 
Puase VI., Period 3. 
General Description—The products of this eruption that go to 
make up its representative deposits are very variable in amount, 
but often reach a very considerable development. At one part of 
the Vallone Sanseverino they attain a thickness of 13 metres ; also 
in the Cupa di Pallarino, where they seem to have collected at the 
bottom of the valley, they attain a maximum of 7 metres. 
Where the deposit is well developed it forms a striking contrast 
to the usual yellow and brownish ash-beds between which it is 
intercalated, consisting as it does of masses of a very light white 
pumice, often as large as a cocoanut or larger. 
Mixed with the primary material are a number of rounded leuci- 
litic lapilli, most abundant near the middle, where they form a 
band and increase in proportion near the top. In the Vallone 
Sanseverino many thin ash-bands, varying from 2 to 10 centimetres 
in thickness, occur at intervals, and are often persistent and uni- 
form for considerable distances. Very few and minute fragments 
of foreign ejected blocks are with difficulty to be found. 
In general characters this pumice, both in external and micro- 
scopic structure, is identical with, or at least undistinguishable from, 
that forming the deposit of Puasz IIL, Period 1. The description 
of the latter will serve equally well for the former as far as the 
microscope is concerned. It may perhaps be worth recording that 
in the more recent deposit crystals of pyroxene may be met with 
enveloping those of amphibole (?), which latter mineral would appear 
to be the most abundant. 
Remarks.—The general characters of this deposit show it to be 
the result of a violent paroxysmal eruption; but it would appear 
that the crater was sufficiently large to allow an easy escape of the 
igneous products. This fact is demonstrated by the comparatively 
few leucilitic lapilli. The band of these old lava fragments occu- 
pying the middle of the deposit may probably be derived from the 
slipping-in of part of the crater walls, which would be immediately 
re-ejected. 
The practical absence of all metamorphic rocks from amongst the 
ejectamenta would rather point to little eroding action exerted on 
the walls of the vent or apex of the crater. 
The amphibole we may suppose to have been formed at great 
depths, and then enveloped in pyroxene when the magma reached 
a lower pressure and temperature. 
Overlying the white pumice are deposits that vary within certain 
limits. In the Vallone Pietri Pomice, immediately above the pumice, 
occurs a concretionary bed 30 centimetres in thickness, of a yellow 
or buff colour when dry. Stratification-lines are distinctly visible in 
general coarseness or fineness of structure. The fracture is earthy 
or irregular. The internal structure shows the mass often to be 
made up of a blending together of pisolitic concretions ; other speci- 
mens are more uniform in texture, and are filled with a number of 
vesicular cavities, which are usually irregularly spherical, ovoid, or 
