Notices of Memoirs — Br. Johnston-Lams on Vesuvius. 509 



. The foundation of the Cumean hill is the well-known trachyte, 

 rich in inclusions of sodalite, of aniphibole, and I have detected, not 

 uncommonly, crystals of fayalite. Were it more vesicular it would 

 very much resemble the western mass of traclij'^te of the Cumana 

 railway tunnel at the back of Naples. 



Above this come some pumice and dust beds, which are probably 

 the equivalent of the Rione Amedeo tuffs. Superposed on this we 

 find a dirty grey pipernoid tuff which shows much remaniement. 

 1'he Museum breccia is well represented in patches, and is overlaid 

 by a bed of vitreous trachyte produced by the resoldering of the 

 falling masses into one solid stratum, where the surface was flat, 

 but where on an incline the fragments have remained separate. The 

 whole is capped by the compact yellow tuff. There are also some 

 minor pumice and dust beds which require further working out. 



The trachyte seems to have oozed forth in a highly pasty con- 

 dition, breaking up its scoriaceous surface, which rolled down the 

 sides of the dome-shaped mass, and by pressure and heat fi'om the 

 main mass become again soldered together — in fact, a sort of 

 regelation. The brecciated structure is undiscernible in hand speci- 

 mens or under the microscope, but is well etched out by meteoric 

 agencies. Each of the deposits mentioned above shows moi'e or less 

 unconformability, which correspond as they themselves do with 

 those beds of the Monte Santo Funicular Railway, the Cumana 

 Railway, Pianura, Soccavo, Monte di Procida, Nocera, Castellamare, 

 St. Agata, Capri, Caserta, etc., that I have described in other reports 

 and papers. 



These are the principal sections which record the later geological 

 history of the Phlegrean Fields, and from which I have been able to 

 unravel the stratigraphy of the highly complex Neapolitan volcanic 

 region. So far it has been explained only in these reports and other 

 disjointed papers, but before long I hope to be able to place before 

 the scientific world a far more detailed description of one of the 

 most interesting as well as the most classic and accessible volcanic 

 regions of the world. 



Before quitting the subject, however, I wish to call attention to 

 the confirmation that the sections mentioned in this Report afford of 

 my explanation of the piperno and pipernoid structure in general. 

 "We see distinctly that the variation in colour and texture of the two 

 constituents of the piperno, which chemically are identical, is simply 

 due to the greater saturation with H2O of one portion of the magma 

 than the other in the old chimney of the volcano at the time of the 

 eruption. The consequence was that the more aquiferous part was 

 erupted as a fine dust, and the less aquiferous, more coherent magma 

 was ejected in large fragments or more or less scoriaceous cakes, 

 which lost their heat the more slowly, in proportion to the less 

 water they contained. The densest, and at the same time the slowest 

 to cool, fell near the eruptive mouth, flattened out, squeezed out 

 those beneath them, and were squeezed out by those above them, 

 forming, with the included dust, the compact piperno in which the 

 foliated structure is most developed towards the west end of the 



