Al4 Geological Society :— 
has given rise to some difference of opinion as to its nature and 
origin. The author is disposed to regard it as a trachytic lava with 
schlieren, the dark lenticles being made up of such minerals as 
augite, egirine, and magnetite, while the lighter matrix is fel- 
spathic (anorthose) with a spherulitic structure and microliths of 
eegirine and augite. 
The second phase (b) of the first eruptive period is represented 
by ashes, lapilli, pumice, and sands, intercalated with marine shell- 
bearing clays and marls, and also with conglomerates and breccias, 
these coarser kinds of detritus overlying them and varying in 
thickness according to their proximity to, or distance from, the 
vents whence these materials were ejected. The sccumulations of 
this epoch were pierced through in the artesian boring at the Royal 
Gardens, Naples, where they were 330 teet thick. 
II. Above the records of the first volcanic period lie those of the 
second—the yellow tuff, which forms the most widespread and 
most characteristic of al] the volcanic formations of the Phlegreean 
Fields. It is a yellow or cream-coloured, compact, well-stratified 
aggregate of trachytic detritus, through which are scattered frag- 
ments of tuff and Java. Its average thickness exceeds 300 feet. 
That it was a submarine accumulation is shown by the occurrence 
in it of Ostrea, Pecten, and other organisms. Owing to the general 
uniformity of its lithological characters, the yellow tuff has not 
furnished any satisfactory evidence of a definite order of succes- 
sion in the eruptions to which it was due. Despite prolonged 
denudation and successive later volcanic vicissitudes, it is still 
possible to recognize some of the separate vents from which the tuff 
was discharged, such as the islet of Nisida, the hills of Posillipo, 
Vomero, Capodimonte, Camaldoli, and Gauro. 
III. After the discharge of the yellow tuff from numerous cones 
and craters scattered over the sea-floor where the Phlegrzean Fields 
now extend, the volcanic tract appears to have been upraised into 
land, and to have been thereafter exposed to a prolonged period of 
subaérial denudation. But volcanic activity was not extinct, for 
a number of vents made their appearance, and discharged a succession 
of fragmental materials, which differ from the yellow tuff in showing 
both macroscopically and microscopically a greater variety of com- 
position, and in the proofs which they furnish of a succession of 
eruptions both in space and time, and a gradual southward shifting 
and diminution of the vigour of the eruptive energy. The largest 
and most ancient of the volcanoes of this latest period is that of 
Agnano, the crater of which is built up of layers of pumice, ashes, 
lapilli, soft grey tuff, and beds of scoria. Not improbably it was 
from this eruptive centre that the trachy-andesitic lava of Caprara 
issued. Other volcanoes of the same series are Astroni, Solfatara, 
the two small vents of Cigliano and Campana behind the north- 
western slopes of Astroni, the last-named example showing three 
concentric rings, within the innermost of which a beautifully- 
perfect little crater marks the last efforts of this vent. The crater- 
lake of Avernus belongs likewise to the latest group, and perhaps 
it was the water percolating from this basin to the thermal springs 
of Tripergole which, in September 1538, gave rise to the explosion 
that built up Monte Nuovo—the youngest of the cones of the 
Phlegreean Fields. 
