50 H. J. JOHNSTON-LAVIS ON THE GEOLOGY 
Let us follow the description given by Von Buch, who says: —“ The 
remainder of the volcano, formed of black beds singularly contrasting 
with the white ones of tufa, rises out of this envelope like a ripe fruit 
which protrudes from its calyx. It is approximately in this manner 
that one can represent Somma piercing this envelope of tufa.” We 
here see that Von Buch had a perfectly clear idea of stratigraphical 
sequence, a fact unfortunately lost sight of by some later writers. 
Prof. Scacchi * also alludes to the superposition of the tufas upon the 
lava on the surface of Somma. Scropey describes the lavas as 
overlain by ‘‘ whitish or yellowish trachytic tuff composed of pumice 
and lapili. This extends up the external flanks of the mountain to 
nearly two thirds the height.” 
Lyell = makes various allusions to tufa, but it is often not clear 
whether reference is made to the leucilitic or pumiceous beds. 
PuassE III., Period 1. 
General Characters——This bed varies in thickness from 0-10 to 
1:00 metre, and consists of a white pumice and leucilitic lapilli. 
The latter, which may form 50 per cent. of the deposit, are chiefly 
broken fragments of lava, subangular in form and exhibiting eyi- 
dence of short but violent attrition. There is in addition a small 
quantity of dust derived from the abrasion of both materials, but 
which is not nearly sufficient to fill the interspaces between the larger 
fragments. This fact we shall see to be partly the reason why this 
bed has been so little disturbed by denuding agencies. 
The pumice fragments are angular in form, falling apart when 
removed from their place in situ, so as to appear traversed by 
cooling fissures. Their original external surface is more compact 
than the interior, having an irregular knotty aspect, presenting all the 
appearance of having been cooled from fusion. The pieces float 
with ease in water, owing to their very spongy structure. The 
vesicles are seen to be small but moderately regular in size, some- 
times pulled out in one direction so as to give the pumice a fibrous 
appearance. Rarely do the vesicles coalesce to form an irregular 
cellular cavity. 
The dirty white mass is rarely seen to be interrupted by a black 
spot, which if removed proves to be a scale of black mica, or crystal 
of pyroxene or amphibole, so far as the unaided eye can judge. 
Sparsely distributed also throughout the mass are a few small sani- 
dine crystals porphyritically enclosed. 
Microscopical Kxamination.—This light spongy pumice is one of 
the most difficult rocks to sectionize from its vesicularity, the walls 
of the cells being so exceedingly thin and delicate. The section 
must be very thin, otherwise the number of unopened air-cavities, 
from their refractive peculiarities, will prevent any examination. 
The greatest difficulty arises, however, from the disproportion in 
resistance to grinding action of the matrix and the porphyritically 
* « Tezioni di Geologia’ (Napoli, 1843), p. 171. 
Tt § Volcanos,’ 1868. ¢ ‘ Principles.’ 
