OF MONTE SOMMA AND VESUVIUS. 75 
Puase VI., Period 4. 
General Description.—Probably none of the components of Monte 
Somma so far discussed has given rise to somuch attention and investi- 
gation as that we are about to consider. This arises from different 
reasons, such as the extent and uniformity of the materials and also the 
richness of the ejected blocks of metamorphosed foreign rocks, which 
have yielded some of the most interesting and rare of mineral species, 
besides an abundance of common ones. Last, but not least, this 
deposit Las been brought into the discussion of the physical history 
of the volcano by many eminent writers. From this it will be 
evident that this bed of pumice and the associated materials possess 
so many characters of the utmost interest that a more extensive con- 
sideration of them is called for. 
Hardly a single valley scoring the slopes of Monte Somma, does 
not show some sections of this interesting deposit. Its usual 
thickness, where no denuding or other causes of disturbance have 
been at work, varies usually from one to three metres, but may range 
from only a few centimetres, as at Cisterna, to seven metres in the 
Cupa Pallarino. 
The whole deposit is capable of separation into three subdivisions. 
The lowest of these is composed chiefly of a white highly sanidinic 
_ pumice and leucilitic lapilli, with few or no fragments of foreign 
rocks. This, the earliest product of the eruption, in some situa- 
tions, as in the Vallone di Pollena, is terminated by a band of very 
fine material, an ash of the same origin as the coarser pumice and 
lapilli. Where, however, this band does not exist this inferior 
member passes into the middle division without a perfectly abrupt 
line of demarcation. This middle portion is composed of a brown 
or greenish-grey compact pumice, leucilitic lapill, and a great num- 
ber and variety of ejected blocks of erratic rocks not belonging to 
Monte Somma proper. The third and uppermost subdivision is a 
fine grey ash-bed, always characterized by the number and perfec- 
tion of the pisolitic concretions it contains. 
To the naked eye the primary or essential eruptive matter of the 
lower division is a pumice of a beautiful white or cream-colour, as a 
rule, very vesicular and cellular in structure. It is moderately tough 
and filled with great numbers of large well-developed sanidine 
crystals, small plates of black mica, crystals of amphibole, pyroxene, 
and garnet (melanite). 
Microscopical Hxumination.—Sanidine occurs abundantly in large 
well-formed crystals, variously twinned, usually clear, but sometimes 
containing slightly dusty glass-cavities with vacuoles &c., or enve- 
loping crystals of amphibole. One peculiarity worth remarking is_ 
the large number of amphibole microliths arranged along the planes 
of growth of some of the sanidine crystals. 
The principal of the dark spots that were discernible by the 
unaided eye prove to be amphibole. It 1s usually of a dark bottle- 
green colour, polychroic to brown, the cleavage and crystalline form 
well developed. 
