60 H. J. JOHNSTON-LAVIS ‘ON THE GEOLOGY 
to the space between the Vallone Pollena and the two flanks of the 
Vallone Grande (Massa di Somma), and it rapidly thins out late- 
rally from this district. 
Remarks. — Lithologically this deposit is the termination of 
Puase III. of Monte Somma, and is characterized by the production 
of fragmentary rocks only. It is followed immediately, if not con- 
tinuously, by lavas differing little from what had been produced 
by the old volcano in its younger days. 
This outpour of lavas, really the outcome and result of Puassz III., 
we shall find it convenient to separate, under the heading of another 
phase. 
Puase LV., Period 1. 
General Description—Overlying the group of beds that we have 
just been studying in some sections, we meet with one or more lava- 
streams or beds of scoria. 
In the Vallone Sanseverino and in the Vallone Von Buch we en- 
counter beds of lava having characters almost identical both macro- 
and microscopically. In the former valley the flow varies from 
half a metre to two or more metres in thickness, and in sztw presents 
the following features :— 
The lava forms a moderately regular bed on both sides and floor 
of the valley for a considerable distance. It is traversed pretty 
abundantly by cooling-jointage-planes, which are usually at right 
angles toits surfaces. It is covered with a very thin layer of ragged 
spongy scoria. In addition to the above-mentioned fissures, the 
rock cleaves easily along planes parallel with its surfaces, to such 
an extent in some places, where this lava occupies the bed of the 
ravine, that one might almost mistake it at first sight for a sedimen- 
tary rock. 
In colour it is grey, with a pinkish or violet tinge. Scattered 
throughout the mass, especially at its upper part, are a great number 
of rather small vesicular cavities, generally irregular in form, and 
having a granular surface covered with minute crystals. The rock 
breaks with a somewhat dull earthy fracture, due to a certain 
amount of decomposition that has taken place. 
The uniform colour of the matrix is only broken occasionally by 
a few very small crystals of black mica and dark green pyroxene ; 
there are also seen many minute white spots, especially by using a 
lens, which turn out to be felspar. . 
Microscopical Examination.—The porphyritically enclosed minerals 
are those we have already mentioned as discernible by the naked 
eye. 
The pyroxene crystals are of a light pea-green colour, commonly 
showing slight alteration along their cleavage-planes, and often con- 
taining large octahedra of magnetite. The biotite, or black mica, 
is of light sepia-brown colour in thin sections, very polychroic, 
varying from the above tint to complete black when rotated above 
the polarizer. It often encloses crystals of sanidine! The sanidine 
itself is very abundantly distributed throughout the rock, and seems 
