Vol. 60.] VULCANIC ACTION IN THE PHLEGRJiAN FIELDS. 309 



some distance from the eruptive vents, and where the material has 

 been spread out and distributed uniformly by the winds and the 

 dynamic force of the outbursts, it is only possible to speak of ashes, 

 lapilli, pumice, and tuffs as promiscuously derived from the central 

 volcanoes. In a few instances only, as, for example, in the case of 

 a small layer of mangauiferous purplish ash, which occurs on the 

 summit of the Hill of the Camaldoli and at some other localities in 

 the Phlegrasan Fields, can it be said that this particular deposit has 

 been derived from the Astroni eruptions : the evidence for this 

 identification being the existence, in the walls of the Astroni crater, 

 of a thicker band of the same ash. 1 In the same manner, we may 

 conclude that the loosely-textured grey material which almost every- 

 where caps the hills of yellow tuff (and is known to the quarrymeu 

 as mappamonte) is no product of disintegration or aqueous 

 erosion, but is on the whole directly derived from the central grey- 

 tuff volcanoes of the Phlegraean Fields. 



Not always, however, did the eruptive vents of the Third Period 

 discharge a quantity of material sufficient to build up true crateri- 

 form cones, on the type of Agnano, Astroni, Cigliano, Monte 

 Nuovo, etc. Sometimes the outbursts merely rent asunder the 

 ancient deposits of yellow tuff, forming in them 'craters of 

 explosion,' round the rims of which the scanty products of the 

 outburst accumulated. Such are the circular or semicircular cavities 

 which occur dispersedly in the Phlegraean Fields. Thus, from the 

 colossal example of the Piano di Quarto, with a maximum 

 diameter of 2| miles, we may pass to the Piano di Torre 

 Poerio (north of Astroni and east of the Craters of Campana), to 

 the Piano di Teano (south-west of Monte Gauro), and thence 

 to yet others, until we reach Avernus, the most typical of all 

 these craters of explosion. This too, almost alone among those 

 of the Phlegrsean Fields, has furnished, besides the authigenous 

 material erupted from it, scoriae and blocks of leucotephrite which 

 now form a small band among the layers of fragmental material 

 on its northern flank. 



Of course, pari passu with the shifting of the axis of eruptivity 

 the craters of accumulation have occasionally alternated with those 

 of explosion. Consequently, if we endeavour to establish a chrono- 

 logical sequence among the central volcanoes of the Phlegrooan 

 Fields, we must take account of both categories. A sequence of 

 this kind, as I have said before, can only be determined with a 

 comparative amount of relative certainty. One series, for instance, 

 is exemplified by the contemporaneous craters of Posillipo, Soccavo, 

 or Pianura, with which are successively and concentrically asso- 

 ciated the craters of Agnano and Astroni, and the internal cone of 

 the latter. 



Another sequence, concentric also, may well be represented by 

 the crater of Pianura, the explosion-crater of Torre Poerio, the 



1 G. de Lorenzo & C. E-iva ' II Cratere di Astroni nei Cainpi Flegrei " Atti 

 R. Accad. Sci. Napoli, ser. 2, vol. xi (1902) no. 8, pp. 22-23. 



