404 REPORT—1890. 
far as cut, still remains in the upper yellow Neapolitan tuff, and one point 
is 20 m. lower than the floor of the new tunnel, and close to the elevator 
shaft, which has proved the upper yellow tuff to be 60 m. thick above the 
causeway, and therefore giving an aggregate thickness for this deposit 
of at least over 80m. This deposit, if due toa single eruption, as its 
homogeneity would seem to indicate, is certainly a very remarkable fact. 
Where this tunnel crosses the plain from Fuorigrotta to Bagnoli 
-great uniformity of materials has been met with, well-stratified beds of 
varying thickness of yellow argillaceous dust, including more or less 
white pumice, as well as brownish violet pozzolana and lapilli, with frag- 
ments of dark buff or brown pumice like that of the hills to the 8.W. of 
the Solfatara predominating. Occasionally a band or two bands of dark 
brown scoriaceous trachyte lapilli are met with. These beds are nearly 
horizontal except where they approach the slopes of Posilippo. Most of 
these strata seem to be either subaerial deposits, or laid down in very 
shallow water. Fragments of lignitised wood are occasionally met with, 
and one overseer told me he had met with the impression of a fern leaf, 
but that it had crumbled to pieces. 
As the tunnel approaches the outer toe of the slopes that encircle the 
Lago d’Agnano, at a spot more than a kilometre from the celebrated 
Grotto del Cane, a very serious escape of carbonic acid commenced to 
take place in the workings, which soon caused operations to be suspended, 
and such was the output of irrespirable gas that only with very powerful 
ventilators could the work be carried on. The length of the tunnel 
along which this escape takes place is very considerable, but cannot be 
determined until more advance is made; but, altogether, the area over 
which the exudation of carbonic acid gas is going on must be a very wide 
one, and must represent the diurnal escape into the atmosphere of an 
enormous quantity.! 
The working shaft near the road to Agnano presents a section some- 
what different from the others in the Bagnoli plain, since here the toe of 
the outer slopes of the Agnano crater is cut through. The following 
was the section obtained from above downwards :— 
Greenish buff, pozzolana, and rolled pumice, the whole stratified hori- 
zontally but with false bedded details . : : : : : . 540 
Light brown pozzolana, black and vegetable soil at top . : 5 . 3°80 
Chocolate tuffs like Bagnoli and Solfatara down to : : : - 300 
The work, on account of the above-mentioned difficulties, has not yet 
been more than commenced near Bagnoli and Pozzuoli, where many 
technical difficulties present themselves, and geological facts of the greatest 
importance are likely to be brought to light. The ‘Societa degli 
Ingegneri Costruttore,’ before facing these difficulties, requested me to 
study the region thoroughly and report thereon ; and as those studies are 
of no uncommon interest, the substance is included in this report. 
The main sewer will be a tunnel which perforates the mountains 
from the thermal region of Bagnoli under the slopes of the Solfatara 
Monte Olibano, and thence to Pozzuoli, a region where the very embers 
of the Forge of Vulcan have to be traversed. 
"In 1886 an attempt was made to excavate a well by the roadside where the 
road from Agnano joins that from Fuorigrotta to Bagnoli, and here much ‘Mofetta’ 
was also encountered, indicating that the area of escape extends at least another 
half-kilometre eastwards. 
