Vol. 60. VOLCANIC ACTION IN THE PHLEGRJEAX FIELDS. 307 



On the other hand, it is no easy matter to trace the original 

 vents or apertures whence were derived the outcrops of yellow 

 tuff which are to be observed around the Piano di Quarto up 

 to its extreme northern boundary (where this is cut by the Via 

 Campana) ; or those which crop up here and there along the beach 

 from Bagnoli to Pozzuoli. Subsequent geological changes have 

 obliterated every vestige of the original craters. 



Between the shore and the Piano di Quarto, however, the 

 volcano of the Gauro, the finest in the Phlegraean Fields, and one 

 of the best-preserved of those built up of the yellow tuff, towers to 

 a height of 10S2 feet above the sea. The cone, unbroken on the 

 north, was torn open on the east and west by two subsequent 

 outbursts, and has been worn down on the south by the rains, the 

 winds, and the waves of the sea, which beat against it at the time 

 of its emergence. If we ascend the slopes of this great cone, we 

 see on reaching the summit a vast crater yawning below us (hence 

 the epithet, which Juvenal applied to the mountain, of Gaums 

 inanis), nearly 5000 feet wide and more than 650 feet deep. In 

 dimensions and majesty it challenges comparison with the later, 

 neighbouring crater of Astroni. 



Of uncertain origin, again, are the outcrops of yellow tuff which 

 occur along the western portion of the Phlegraean Fields, at Arco 

 Felice, Bacoli, etc. up to the Monte di Cuma, near the so-called 

 Temple of Apollo, and to the Monte di Procida. the eastern 

 shoulder of which is capped by them. 



On the other hand, the crateriform character of the Porto di 

 Miseno and Cape Miseno is sufficiently obvious : they are both 

 made up of yellow tuff, overlain by pozzolana and scoriae of 

 later eruption. The crater of Porto Miseno is all but drowned 

 by the sea, its upper rim only emerging in part. Cape Miseno, long 

 famous for its internal structure, laid bare on its broken-down south- 

 western flank, emerges to the height of 544 feet above the wave-, 

 while its roots plunge down to 330 feet below them. Thus, 

 both in dimensions and in form, it is strikingly similar to the 

 crater of Xisida, which may be regarded as the other southern 

 outpost of the Phlegraaan Fields. 



Moreover, the form of Miseno and Xisida is paralleled by that of 

 Monte Gauro and the other yellow-tuff volcanoes, and this paral- 

 lelism of form is associated with similarity of structure and petro- 

 graphic composition. All of which fits in to a certain extent with 

 their common mode of origin, namely, submarine eruptions 

 taking place almost simultaneously over the entire 

 Phlegra?an area. 



A much greater, though not perhaps an extreme, diversity is 

 found among the later volcanoes, which arose at different points 

 and at different times, and almost all on land in the open air. 



(3) Third Period. 



It would seem that the eruptions of yellow tuff which had 

 fashioned almost the entire framework of the Phlegnean Fields 



