Vol. 60.] VOLCANIC ACTION IN THE PHLEGR^AN FIELDS. 305 



For these reasons, in the following rapid enumeration of the 

 various volcanoes built up of yellow tuff, it is thought best to 

 adopt a topographical arrangement. 



The Volcanoes of Yellow Tuff. 



The ashes, lapilli, and pumice of which this tuft' is composed, 

 as they were erupted from various orifices scattered about the 

 Phlegmean area, were heaped up around these vents in obedience 

 to the laws of projective energy and gravity, forming therefore so 

 many cones, rather broad and flat than otherwise, with wide and 

 deep craters. In these cones, the material, being uniform in 

 character, accumulated in layers, the major portion of which 

 coincided in inclination with the external slope of the cone ; while 

 the remainder was stratified conformably with the internal crater- 

 slope. This structure is in accordance with the law of the 

 formation of detritic volcanoes, whereof Monte Xuovo is a notable 

 example. 



After their formation, these cones of yellow tuff, together with 

 the underlying rock-platform, were upheaved above the waves of 

 the sea, and each in turn fell a prey to the destructive agencies of 

 the atmosphere, which proved to be more relentless in their attack 

 at some points than at others. Moreover, within the area of the 

 Phlegraean Fields later vents of eruption opened, (with which I 

 shall deal when describing the Third Period), and by their explosive 

 energy rent and dislocated such of the cones of yellow tuff as lay 

 in their way, and overwhelmed and covered them with fresh erupted 

 material. Thus it is that what we now behold are the mere frag- 

 mentary ruins of the volcanoes of yellow tuff, and yet they form 

 the most important and conspicuous feature in the scenery of the 

 Phlegmean Fields. 



The southernmost, perhaps, of these volcanoes of yellow tuff 

 still submerged beneath the sea are represented by the two shoals of 

 Mezzogiorno and Penta Pal u rutin o, which, at distances of 

 1^ and '2h miles respectively from Cape Miseno, rise from a depth 

 of some 300 feet to within 164: and 98 feet respectively from the 

 surface of the sea, and betray by their conical form their volcanic 

 origin. But we may pass on from these to the consideration 

 of those unmistakable volcanoes which rise above the waves. 



First among them is the little Islet of Xisida, which attains a 

 height of 330 feet or so above the sea, while the roots of the 

 volcano certainly go down to at least an equivalent depth below 

 sea-level. The crater, into which the waves flow by a narrow 

 breach open to the south-west, is 300 feet deep, measures 1312 feet 

 round its upper rim, and 490 feet round its lower orifice. These 

 dimensions are almost identical with those of the crater of Monte 

 Xuovo, which it also resembles most strikingly in shape. The 

 typical yellow tuff of which it is built, is unmistakably stratified 

 with a quaquaversal dip along the external slope of the cone and 

 along the inner declivity of the crater. The tuff is crowded with 



