56 H. J. JOHNSTON-LAVIS ON THE GEOLOGY 
This layer of pumice, of comparatively little importance as to 
thickness, is yet at the same time one of the most persistent of the 
group of deposits to which it belongs. It is rarely absent when de- 
posited on scoria, due, no doubt, to the open structure of the latter 
allowing waiter to trickle through between the fragments. Where 
deposited on lava it has commonly been removed, floated away in 
fact. 
Rarely has it left behind it any of its débris; nevertheless, in the 
left lower branch of the Cupa di Pallarino, we see a mass of yellowish 
dusty earth, containing water-rolled fragments of this pumice, over- 
lain by the products of the eruption immediately following. It ap- 
pears that this alluvial deposit was derived from the disintegration 
and decomposition of the pumice-bed, which was collected at the 
bottom of a ravine which we now see cut through in section. 
This alluvial material tells of a quiescent period, probably of ro 
great length, during which subaerial denudation had been going on, 
and during which preparation was being made for ‘another great 
outburst of terrestrial forces. 
No metamorphosed erratic rocks are found amongst the materials 
composing this deposit, although the leucilitic lapilli derived from 
the mechanical disintegration of old lavas are so abundant as to 
form an important element. The two facts show that a consider- 
able crater must have been excavated, but that it did not extend 
into the subjacent sedimentary rocks. 
Puass ITI., Period 2. 
General Description.—It was remarked in speaking of the pre- 
ceding deposits, that the alluvial materials resulting from them 
apparently only represented a short interval of tranquillity. ‘This 
short interval seems to be demonstrated by the passage in other 
localities of pumice of Period 1 into that of the present one, which 
consists of a heavy, hard, dark scoria, without any intervention of 
alluvial matter. To the inattentive eye the line of demarcation is 
very distinct; but on closer examination we may see at the line of 
junction pieces of rock of different gradations between the upper 
more crystalline pumice of Period 1 and the pumiceous scoria of 
Period 2. 
This transition, which may be well seen in the Vallone Sanseve- 
rino, is visible not only in the external, but also in the microscopic 
characters, which exhibit this passage from a more or less vitreous 
through a half microlithic to a crystalline rock. 
This pumiceous scoria, since it still retains somewhat of the pumi- 
ceous facies about it, occurs as fragments varying in size up to that 
of a human head. It is of dark chocolate-brown, black, or dark 
reddish colour. Usually more compact at the surface, and redder 
than in the interior, it has an appearance of having cooled from a 
plastic condition. The density is considerable; it breaks with a slight 
tap of a hammer into a certain number of fragments along old 
cooling-fissures, the resulting pieces being rather tough and resis- 
