OF MONTE SOMMA AND VESUVIUS. 57 
tant. The included cavities, of large size, are irregular in shape, and 
not rounded or cellular. Broken crystals, and fragments of lava 
and other rocks are exceedingly common as enclosures. 
Mixed with the fragments of this primary eruptive matter are a 
number of very small pieces of altered limestone and its derivatives. 
By far the most abundant, however, are a number of fragments of 
marl and clay, very slightly altered and rich in organic remains. 
The matter is still plastic in the interior ; and often adhering to the 
surface is a layer of fine lapilli, showing the small amount of heat 
to which these masses were subjected, and the short time they were 
exposed to it. These remnants of prevolcanic Tertiaries are most 
abundant at the bottom of the bed, whilst the dolomitic limestone is 
found higher up. There are also a good number of leucilitic lapilli. 
These substances united form a deposit that rarely exceeds a 
metre in thickness. 
Microscopical Examination.—In the thinnest section this pumi- 
ceous scoria makes a very dark, opaque, brown preparation. ‘The 
essential elements are a few crystals of pyroxene, amphibole (?), 
biotite, in small irregular plates, moderately polychroic, ranging 
from dark amber or sherry-colour to light lemon, and crystals of 
sanidine. Some of the latter are large and broken around their 
edges, fissured throughout, and their cleavage-planes infiltrated by 
an earth-brown amorphous substance, which gives them a very 
peculiar appearance, but has in no way altered their optical proper- 
ties. Most show Carlsbad twinning very distinctly. They seem 
to be crystals that have undergone vicissitudes of temperature ; 
others have a clear nucleus and dusty exterior. The included frag- 
ments of lava have their sanidine crystals altered and fissured as 
those belonging to the rock proper. Magnetite is to be found rather 
plentifully, in large unaltered crystals. 
The matrix appears under a low power as a granular brown net- 
work of opened vesicles, the walls of which are irregular and rough- 
ened by the projection inwards of many microliths. When a higher 
power is used the granular mass proves to be composed of at least 
two kinds of microliths, which form the principal portion ; but there 
seems to be some interstitial glass. The first and commonest are 
very small transparent rods, which are probably pyroxene; a few 
larger ones seem to be sanidine. They are doubly-refractive, clear, 
and colourless, except where divided along their longer axes by a 
septum of dark brown matter. This intercalated material is narrow 
at the centre and widens as it approaches the terminations of the 
prism, and seems to be identical with that portion of the crystal of 
sanidine included by the peculiar curved lines described by Mr. F. 
Rutley*. The opposite halves generally extinguish alternately 
between crossed nicols. ‘The other microliths, or small crystals, are 
polygonal in form, showing apparently quadrilateral and six-sided 
outlines, although, from their small size, it is difficult to see such 
distinctly. With a very strong light, some of the smaller ones 
* Quart. Journ. Geol. Soc. 1876, p. 479, and ‘Study of Rocks’ (London, 
1879), p. 96. 
