96 H. J. JOHNSTON-LAVIS ON THE GEOLOGY 
Atrio del Cavallo or great crater, which must then have been an 
annular valley. Neither does the appearance of the ejectamenta 
denote them to have been associated with lava. 
This bed, of no inconsiderable thickness, is seen to cover over 
alluvial breccia of earlier but uncertain date, in which is enveloped 
the Roman house we have described (p. 87) as occurring in the 
Lagno di Trocchia. 
Puase VIL., Period 5. 
General Description.—Superposed on the black pumiceous scoria 
at the Canale di Arena are about two metres of another scoria in small 
fragments, ranging from the size of a hazel-nut to that of a walnut. 
The pieces are exceedingly light and spongy, being filled with large 
cellular cavities, haying smooth glossy black shining walls, giving 
to the deposit a recent appearance, quite obsidian-like at a short 
distance, and entirely different from any of its predecessors. Some- 
times a slight rusty crust has been formed, giving the bed a dark 
brown or yellowish tint. 
There is totally absent any thing but the primary or essential 
eruptive material. Interstratified are two thin bands of a fine pinkish 
coarse sand, which seems to be composed chiefly of isolated and 
rounded crystals, with fragments of rock-particles abraded in the usual 
manner. These are mixed and covered with a fine pinkish dust, 
the result of the powerful and continuous attrition to which they 
had been subjected. Overlying these materiais we have a thin bed 
of yeliow-brown dusty matter, of about 30 centimetres thickness. 
These peculiar deposits, from their position and facility of recog- 
nition, can be traced at many parts of the mountain. They forma 
mantle over nearly all the upper part of the northern slope, espe- 
cially where vegetation has helped to retain them im their present 
position. They may be well seen in the section of the Valloni 
Pietri Pomice and St. Patrizio on the north ; and I have recognized 
the same materials from a well sunk in the quarry of,.and beneath, 
the lava of a.p. 1631, near Camaldoli della Torre, on the south. 
Microscopical Examination.—The crystalline element that reaches 
greatest development as regards size is the pyroxene. It is of a very 
light pea-green colour, enclosed in an outer coat of uniform thick- 
ness of the same matter, but of a much darker tint, the separation 
between the two being usually very distinct and well marked. 
Wedding remarked the same thing in some Vesuvian lavas; and the 
same was seen to occur in the products of the preceding eruption. 
There are usually many and large enclosures of a very granular 
dusty brown glass. 
Next in abundance are beautifully pure and well-formed large 
crystals of biotite, highly polychroic, ranging from an ochre tint to 
black. 
Forming a most important constituent of therock, but rarely, if ever, 
reaching beyond the size of microscopic erystals, is the leucite. This 
mineral, from its small size, rarely transmits light between crossed 
_—_————— eee 
) ae TY 
. 
. 
