74 Dr. H. J. Johnston-Lavis—Trachyte at Naples. 
VII.—On a RemarKasBie SoparitE TRACHYTE LATELY DISCOVERED 
In Napuszs, ITALY. 
By H. J. Jounston-Lavis, M.D., B.-és-Se., F.G.S., ete. 
N the last two British Association Reports of the Vesuvian 
Committee, I drew attention to some very large masses of 
trachyte traversed, at the back of Naples, by the tunnel of the 
Cumana Railway. <A full account of the stratigraphy of these will 
be deferred until some further excavations, now in progress, are 
completed. I propose at present merely to describe the earliest- 
discovered and most striking of these masses. 
This trachyte is cut through on the E.N.E. side, at 1890 metres 
from the terminus end of the tunnel at Monte Santo, in a slightly 
oblique manner, which traverses it for 1980 metres on the southern 
side, and 2000 on the northern. Thus, it will be seen that the 
tunnel forms the chord of an are, the circumference or boundary-line 
of the trachyte being directed southerly, or towards the present sea- 
coast. Both contacts are old escarpments, and at the W.S.W. end 
a talus is cut through of fragments of the trachyte and other 
materials, on which is superposed a series of pumice, ash, lapilli 
and vegetable soil-beds, representing many successive eruptions ; 
the latter are also met with at the opposite escarpment, placed 
at a high angle of rest. We thus see that this mass was no 
doubt erupted as a subaerial lava-stream, probably cut back by the 
sea, or other denuding agents, to a vertical cliff, and subsequently 
covered by the different tufts that constitute the hills above. 
The rock is divided into rather large blocks by irregular cooling 
cracks, very similar to the Mont Olibano trachyte, which it most 
resembles in general characters. It is of light grey colour, with a 
somewhat bluish tint and earthy fracture, is harsh to the touch, 
adheres slightly to the tongue, and a drop of water falling on its 
surface is immediately absorbed. These latter characters are probably 
dependent upon actual spaces between the crystals and microliths, 
whilst the colour can be seen to be due to the admixture of glassy 
felspar with numerous minute dark particles. The sanidine crystals 
are mostly very thin, tabular in form, and show sections up to 
nearly two centimetres in length, in which Carlsbad twinning can 
be seen. A few rare but large individuals are compacter in shape, 
and prism-like, resembling those of the Drachenfels trachyte. 
Microscopically they are much fractured and fissured, very clear, 
with a few glass cavities, whilst in each fractured fragment much 
internal strain is evidenced by the ring of colours just within its 
outline. The sanidine is decidedly the most important constituent 
of the rock, and next to it we have the amphibole, which exists in 
two or three varieties. One is usually in larger crystals, attaining 
a maximum measurement of two or three millimetres or more, and 
commonly occurring as irregular crystalline grains, with sharp edges, 
as if fractured shortly before consolidation in consequence of the 
internal friction of the semi-solidified pasty mass. Some sections 
exhibited typical cleavage. The colour of this variety is yellowish 
