OF MONTE SOMMA AND VESUVIUS. 67 
entiated from the base, presents itself as a multitude of globulites 
appearing as small opaque spots, which are scattered about between 
the microliths and trichites, in the same manner that magnetite occurs 
in some of the more highly crystalline structures already described. 
If we may judge by analogy, the globulites appear to be nothing 
more than minute microliths of magnetite. When subjected to very 
high powers these globulites seem to transmit a reddish-brown 
light; and in those specimens in which the larger crystals of mag- 
netite are much decomposed these minute specks are absent, which 
would be expected from their smallness and the ease with which 
they are consequently destroyed. 
It may be taken as a general rule, that the pumice which is 
richest in well-formed magnetite, and unaltered, or only slightly 
changed, is the darkest in colour. 
The next constituents, and the ones that form the greatest part of 
the deposit, are the usual rounded and subangular lapilli of ancient 
leucilitic lava. The only thing worth remarking about them is the 
evidence of the violent attrition to which they have been subjected, 
differing very much from what one observes in water-worn pebbles. 
The third set of materials contained in these ejectamenta, but of 
less importance as to volume, are the various erratic fragments of 
metamorphosed sedimentary rocks. The most abundant of these are 
pieces of limestone, varying in size, but rarely larger than a mode- 
rate-sized orange. One peculiarity about them is that they are 
least common at the bottom of the deposit. Other varieties, and 
of more interest, are masses of marl and marly clay, often crammed 
with fossils, chiefly marine. These pieces of a shallow-water sedi- 
mentary rock, are slightly altered on their exterior, and vary from 
the size of a walnut to that of a large cocoanut. In some places 
they are very abundant, especially about the middle of the deposit. 
There are also a few small pieces of those extraordinary series of 
metamorphic rocks that form the major part of the ejected foreign 
blocks, shot out at later eruptions. 
These three various materials, so different in their origin, together 
make up a stratum varying usually from 4 metre to 4 or 5 metres in 
thickness. This stratum generally presents a certain characteristic 
arrangement of its materials. The lowest quarter or third is very rich 
in pumice, mixed first with leucilitic lapilli, then with Tertiary marls, 
and above these with limestone fragments. None of these erratics 
are absent from any part, but they predominate at certain horizons. 
The remaining and greater bulk of the stratum consists of a breccia 
with fine ash, in rather regular alternation, and, near the top, more 
pumice is again encountered. The breccia is most variable in 
coarseness, and composed chiefly of leucilitic lava-fragments. The 
old lava-blocks are much decomposed towards the top, as if they 
had been much exposed to the vicissitudes of weather before being 
covered over by the products of the subsequent eruption (?). The 
upper subdivision, or breccia, is often false-bedded, and it is no easy 
matter to say whether such an arrangement is due to deposition 
upon an inclined surface or the effect of aqueous influences. 
/ F2 
