OF MONLE SOMMA AND VESUVIUS. 59 
especially to the pyroxene, which is very abundant both as large 
and small crystals, many of which have as a nucleus magnetite. 
Plates of biotite are much finer, and often stand out from the 
walls of the cells into the cavity, attached only by one edge or 
angle. There are also some peculiar large lozenge-shaped plates (?) 
quite transparent, which, rotated between crossed nicols, transmit a 
faint brownish-grey light. From the absence of any characteristic 
peculiarities, it is difficult to know to what rock-forming mineral to 
assign them. 
Puask III., Period 4. 
General Descryption.—The last-described deposit sometimes passes 
up into, or may be represented by, a bed of very coarse leucilitic 
breccia. 
In some localities we find this breccia reaching a comparatively 
great thickness, so that in the Vallone di Pollena and Vallone 
Grande, above Massa di Somma, the sides of these valleys show a 
thickness of it varying from 2 to 55 metres. When reaching the 
latter comparatively enormous development, it is seen to be composed 
of blocks of lava, with more or less rounded angles, very variable in 
structure and size, ranging from the finest lapilli up to masses some 
tons in weight. These are enveloped in a dirty purple or slate- 
coloured ash-like dust*. This powder proves to be composed of 
fragments of minerals and a fine granular amorphous matter, pro- 
bably the result of decomposition of the constituents, especially 
leucite. The coarser grains only contain those minerals which 
would be derived from the disintegration of the lava-fragments. All 
the minerals are in a fragmentary condition. Pyroxene was most 
abundant, besides which was recognizable olivine (rare), sanidine, 
plagioclastic felspar (rare), and leucite (abundant), but having lost all 
its optical properties, being also rounded and eroded on the surface. 
The large blocks of lava have only undergone slight decomposi- 
tion. An odd fragment of much-rolled pumice can occasionally be 
found by diligent search. 
To determine the position of this bed involved some difficulty, 
and its true horizon is only to be learned high up the valley, where 
an opening occurs in the superficial vegetable soil, and the other 
members of Past III. are seen to underlie the breccia, which is 
overlain by the deposits of Puasz VI. 
The ten metres of materials resembling this, which he between 
the deposits of Puasr III. at the Vallone St. Patrizio, and other 
similar deposits in that region, must be correlated with this bed. 
Its peculiarly excessive development in one particular locality 
certainly deserves attention. This is still more the case when we 
remark how high up the slopes of the mountain this deposit retains 
its great thickness. The area where this occurs is confined chiefly 
* This powder was sifted, and levigated so as to separate it into different sizes. 
The products of each sifting were examined by the microscope before and after 
a short treatment with hydrochloric acid. In this way the calcareous and 
limonitic coating of the particles was removed. 
