308 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [Feb. 6, 



pretend to determine the period of the upheaval of certain masses of 

 these Neapohtan tuffs near the sea, by consulting the marine strata 

 which alternate Ayith them, even in those which contain shells perfectly 

 identical with those now living in the adjacent Mediterranean. In 

 my last memoir I have shown that species of sea-shells still in being 

 have existed previously to terrestrial species now enthely extinct (see 

 Quart. Journ. Geol. Soc. vol. v. p. 237). And if we thus fail in 

 measuring the relation of previous epochs to the historical sera, by 

 comparison of things marine with things terrestrial, still less ought 

 we to venture to approximate all the diversified subaqueous volcanic 

 operations of the Phlegrsean Fields, or the Campagna di Roma, to 

 our historical atmospheric volcanos, by any reasoning from a few 

 species of marine shells only. 



Conclusion. — In my method of accounting for the origin and pre- 

 sent form of the volcanic tract of Rocca Monfina, I shall probably 

 neither satisfy those who contend that it is an elevation-crater, nor 

 those who would explain it by reference to any knoT\Ti example of 

 modern subaerial volcanic action. I do not admit that in this case, 

 tufaceous deposits, once horizontal, were raised by the sudden protru- 

 sion into the atmosphere of a dome of trachyte into highly-inclined 

 positions ; for I cannot suppose that this was ever the site of a true 

 terrestrial volcano. In the Latian or Alban Hills, however (if my view 

 of the succession of their ei-uptions be correct), the formation of a 

 subaerial cone and crater may, as it was the last volcanic operation 

 of that tract, be very well supposed to have raised and thrown off the 

 surrounding dejections of previous and subaqueous origin, with more 

 or less of an eccentric dip of shght inchnation. In those parts of the 

 Colles Ciminii .where there are no central trachytes, as at Rocca Mon- 

 fina, and no true central volcano of subsequent date, as at Hannibal's 

 Camp, I can see no vahd objection to the admission, that some of the 

 strata around the lakes of Bolseua, Baccano, &c., which consist ex- 

 clusively of sti'atified igneous products, may have been ejected from 

 these orifices, probably under shallow brackish or lacustrine waters. 

 On the other hand, I can see no reason for doubting that subsequent 

 efforts towards eruption, whether accomyjanied by upheaval of soUd 

 rocks or not, may have operated from the same centre to raise up and 

 throw off still more the suiTounding stratiform accumulations. If we 

 deny the possibihty of all stratiform qua-qua-versal arrangements 

 havhig either resulted from, or been increased by, upheaval or eleva- 

 tion, when apphed to scoriaceous and tufaceous submarine dejections, 



respect to some of the dejections ^Yhich suiTound the trachyte of Rocca Monfina, 

 M. ScaccM suggests that several of the mounts may have been separate points of 

 eruption, and possibly had not reference to the great central crater. All the trachyte 

 of Ischia, he shows,' must have been formed under submarine conditions, as sea- 

 shells alternate with it up to 1500 feet in height. The craters which supplied the 

 materials of the Campi Phlegrsi arose, he conceives, in the sea, however their ac- 

 companying cones may have been raised into the atmosphere ; and those felspathic 

 tuffs of the Campania* which are far removed from their original vents, were, he 

 su<^gests, not formed under the sea, but upon pebble-beds formed by rivers (and 

 ireat lakes ?), after the partial emersion of the laud. 



