HARBWICKE'G SCIENCE-GOSSIP. 



219 



alopinj? steeply on to a small flat plain, now a culti- 

 vated field, at the bottom. 



Piom the highest point of the crater-edge, ou the 

 east, a fine view is obtained towards Pozzuoli and 

 Vesuvius, the top of which appears above the 

 Solfatara, while immediately below is the old line 

 of sea-cliff and the strip of level land upheaved at 

 the time of the formation of Monte Nuovo. From 

 the crater-edge on the west a view is obtained of 

 Lake Avernus, with its crater-walls, and of the vol- 

 canic mountains of Ischia beyond. 



A piece of plain country, covered with vines and 

 famed for the first victory of the Romans over the 

 Samnites, separates Monte Nuovo from Monte Bar- 

 baro, alofty volcanic mountain, its outersidescovered 

 ■with vineyards, and having a deep crater of an oval 

 form about three miles and a half in circumference, 

 the inner slopes clad with wood and the inclosed 

 plain rich with cultivation. Here again no lava- 

 flows are to be seen, but on every hand beds of 

 pumice-tutT with the usual outward dip, though the 

 ash is somewhat harder than that of some of its 

 neighbours. The eastern end of the crater seems 

 to liave been partially destroyed, perhaps by the 

 opening out of a neighbouring vent, and here 

 sections are disclosed showing an inward dip of 

 some of the ash-beds. Now in the case of a cone 

 formed by ejections round a central orifice, those 

 materials which fall back within the crater will have 

 an inward dip, or towards the orifice, while those fall- 

 ing outside will have an outward dip. The former 

 will have less chance of preservation than the latter, 

 since a volcano in action is continually blowing or 

 clearing out its crater by sudden outbursts. To 

 most minds the sight of outwardly-dipping beds, 

 formed of material which at some time must have 

 been ejected, lying at the angles of repose for such 

 material, and arranged round a central orifice 

 •which had the power of ejecting vast quantities of 

 ash and scorise, is sufficient evidence that the cone 

 is one of eruption, and not of elevation or upheaval 

 of originally flat beds round a central orifice in a 

 dome-shaped fashion ; but if there is the additional 

 evidence of some of the inwardly-dipping beds like- 

 wise, then assurance is made doubly sure. In the 

 case of a small part of the eastern side of Monte 

 Barbaro, it would seem as if most of the outwardly- 

 dipping beds had been destroyed, and those having 

 the inward slope in great measure left. * 



On a line with this last mountain, but farther to 

 the east, is the perfect crater of Astroni, four miles 

 in circuit and full of most luxuriant wood. Its 

 crater-walls present the same appearance, and are 



• An objection to evidence deriyed from inward dip in this 

 case may be urged thus. The destruction of the outwardly- 

 dipping beds, if caused by the opening out of a neighbouring 

 vent, may have tilted the beds inwardly. I do not, however, 

 think that it is so in this case. 



' formed of similar materials, to those of the other 

 volcanic mountains, and no sheets of lava seem 

 intermingled with the ash. About the centre of the 

 northern side strata with an inward dip are seen 

 among the trees ; a similar dip is to be seen near 

 the entrance, caused, however, in this case by a large 

 landslip. In the centre of the crater are bosses 

 of a trachytic lava, which probably possessed very 

 imperfect fluidity, and, as suggested by Mr.'Scrope, 

 very likely formed the last phase in the eruption. 

 It is certainly quite inconsistent to regard this as a 

 crater of elevation (round the central bosses) and 

 the other neighbouring ones as craters of eruption ; 

 and a thorough examination of such a district as 

 that of Naples, unbiassed by pre-conceived notions* 

 will lead inevitably to the conclusion that here at 

 any rale exist only craters of eruption, and tend to 

 diminish faith in even the existence of elevation 

 craters at all.* 



The special points then to be gleaned from this 

 slight glance at the volcanoes of the Phlegrsean 

 Fields are : — 



1. The general evidence they afford of being 

 purely volcanoes of eruption. 



2. The great scarcity of lava-flows. 



3. The trachytic nature of all the ashes, and of 

 the lavas of the Solfatara. 



4. The state of the ejected material; as fine 

 muddy ash often finely laminated, or dry ash and 

 scorice. 



Vesuvius. — This, as is well known, is a double 

 mountain, the old Monte Somma, and the recent 

 Vesuvius proper. That part of Monte Somma now 

 remaining is but a portion of the old crater-walls, 

 the south-west side having been blown away. 

 Within the centre of this old crater-line stands the 

 modern cone of Vesuvius, a deep trough called the 

 Atrio del Cavallo separating it from Monte Somma. 

 Previously to A.D. 79, Monte Somma presented the 

 form of a large truncated cone, its sides clothed 

 with vegetation, and with more or less of.a plain at 

 the summit. In the year 79 occurred that notable 

 eruption which, blowing great part of the old 

 mountain away, overwhelmed Pompeii with ashes 

 and Herculaneum with torrents of ash-mud, and 

 since that period its activity has never entirely 

 ceased, and the modern cone of Vesuvius with a 

 central crater has grown up so as even to overtop 

 the highest point of Somma. 



That Monte Somma was a cone of eruption is 

 clearly seen by examination of the outwardly- 

 dipping beds of lava and ash in the cliffs above the 

 Atrio del Cavallo, these beds being pierced by 

 dykes running in various directions, which were the 



♦ It is certainly much to be deplored that in such a gene- 

 rally excellent work as Murray's "Handbook for Naples," 

 the old crater- elevation theory of Von Buch should be 

 adhered to in some instances so persistently. 



l2 



