Vol. 60.] VOLCANIC ACTION IN THE PHLEGR^EAN FIELDS. 299 



many fault-blocks, subsequently sculptured by atmospheric agencies: 

 so do the Island of Capri and the Peninsula of Sorrento themselves 

 in reality form part of a single great calcareous mass limited by the 

 submarine contour of 3280 feet, and measuring at least 7870 feet 

 from top to bottom, of which thickness only about 4590 feet now 

 emerges from the waves (see PI. XXVI). 



This great mass is followed on the north-west by a second, which 

 rises from the 3280-foot submarine contour to about 650 feet 

 below sea-level (see PI. XXVI & PI. XXVII, section) ; and this 

 is divided from the first by an embay ment, which in all probability 

 corresponds to a valley of transverse fracture, analogous to all the 

 others known elsewhere in the Apennines. The character of the 

 contours, more precipitous on the south and more gentle on the 

 north, suggests the inference that in this submarine massif, just as 

 in the Peninsula of Sorrento, the strata dip from south-east to north- 

 west, and are cut off by a great fracture on the south-east. In this 

 second mass the marine Tertiary and Quaternary sediments have 

 been naturally better preserved, because they were more protected 

 from erosive agencies. The blocks ejected from Monte Somma 

 have furnished abundant evidence in favour of this conclusion. 



To sum up then, the bottom of the Bay of Xaples, originally 

 moulded by the orogenic post-Eocene folding and by the post- 

 Pliocene uplift, is chiefly composed of two great masses of lime- 

 stone and dolomite, intensely fractured and dislocated, the con- 

 stituent strata of which dip en masse towards the foci of eruption. 

 This synclinal dip of the strata towards the volcanic centre, observ- 

 able also in the Monte Vulture, 1 and at other localities, is doubt- 

 less favourable to eruptive phenomena, perhaps for the reason that 

 it carries deeper down the dislocated material, which then becomes 

 subject to powerful thermal agencies with the consequent produc- 

 tion of an igneous magma. This hypothesis is, to some extent, 

 confirmed by the fact that in the neighbouring Gulf of Salerno, 

 where dislocations are both more considerable and more numerous 

 than in the Bay of Naples, but in which the arrangement of the 

 strata is anticlinal, there is not the faintest trace of eruptive 

 phenomena. Whatever may be the origin of the deep-seated 

 magma, certain it is that the Pleistocene submarine eruptions 

 emerged above the sedimentary masses, dislocated and folded into 

 a basin, in the Bay of Xaples. First came those of Ischia and of 

 other crateriform vents, which built up the whole Campanian 

 plain with sanidine -bearing materials, which are also found 

 heaped up at Sorrento and Capri, and along the range of the 

 Southern Apennines. These discharges were followed by a less 

 widespread phase of vulcanicity, represented by the trachy-andesitic 

 rocks of the Phlegrgean Fields and the leucotephritic material of 

 Vesuvius. \Vhile, however, at the Vesuvian vent the eruptions 

 have discharged fragmental and lavaform materials from one single 



1 G-. de Lorenzo ' Studio geologico del Monte Vulture ' Atti E. Accad. Sci. 

 !N;ipoli, ser. 2, vol. x (1901) no. 1. 



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