OF MONTE SOMMA AND VESUVIUS. 73 
even multilocular, and sometimes seem to form the centre of a 
piselite. 
When sectionized it proves under the microscope to consist of the 
same minerals as the other primary ejectamenta of this volcano, all, 
however, broken, abraded, and partly decomposed. Curiously 
enough a large number of microliths still retain their form and 
optical properties, and appear to be pyroxenic. These microliths, in 
proportion to the mass, are much more abundant than in the subja- 
cent pumice. The whole is blended together by a brownish amor- 
phous interstitial cement. 
When a fragment is treated with acetic or dilute hydrochloric 
acid the rock effervesces only at points where it contains calcareous 
fragments, and does not break down even after prolonged action of 
these agents, or of concentrated hydrochloric acid in the cold, though, 
when boiled, the mass gradually falls apart. It would therefore 
appear that this rock is either a mechanical accretion or a concretion 
in which the cementing material is some silicate. 
In the Vallone Pietri Pomice this concretionary bed is followed 
by 60 centimetres of fine rounded and false-bedded white pumice 
and yellow ash, succeeded by another concretionary bed of the 
same type as the lower one. In the ridge cut through by the 
path that runs between the Vallone di Pollena and Vallone Grande 
these rolled pumice and dust deposits may be two, three, or more 
metres in thickness where there is only one concretionary band. 
In the Lagno di Trocchia the deposit is represented in part by a 
coarse leucilitic breccia mixed with pieces of the underlying white 
pumice, the upper portion only being a fine yellow dusty soil. 
There seem to be a number of questions, not easily solved, arising 
out of the physical features and constitution of these deposits. 
The peculiar strata of yellow earth are so persistent and so slightly 
variable over the whole of the mountain that we might be led to 
conclude that we had to deal with simple deposition of a volcanic 
ash falling into its present site. On the other hand the rounded 
pumice, the false bedding, the eroded surfaces of underlying deposits, 
and the peculiar sweeping curves of stratification around large stones, 
show distinctly the interference of water. If these beds were simply 
re-sorted by water from the higher parts of the mountain we ought 
to find the constituents of them to be materials derived from all 
manner of sources; but, on the contrary, the white pumice seems 
only to have been derived from the bed immediately below. 
Then, again, the peculiar structure and composition of the concre- 
tionary bands bring in another difficulty. Supposing these to be 
simple segregations in a fine water-sorted ash, how are we to account 
for the vesicular cavities and the persistence of these bands occupy- 
ing nearly always the same relative position over large areas? 
Looking at the question from a more general point of view, it would 
seem that both igneous and aqueous action had played a part in 
their formation. It has been shown that the lowering of the apex 
of the crater below drainage-level might result in the simultaneous 
ejection of both primary and secondary volcanic products with 
