April 21, 1898] 



NATURE 



585 



(i) If a hill is convex towards one side and concave 

 towards the other, it is assYimed that the crater which it 

 formerly surrounded was situated on the concave side ; 

 and the radius of the crater at any level is indicated by 

 the radius of the arc formed by the horizontal projection 

 of the horizontal curve of the crater slope at the same 

 level. 



(2) A further indication of position is afforded by the 

 fact that in almost every case the natural crater slope is 

 much steeper than the corresponding cone slope. 



(3) If arc-shaped hills be so situated that a curve 

 approaching a circle can be drawn so as to pass along 

 all their ridges, it is an indication that the hills are parts 

 of one and the same crater wall. 



to have once formed part of the walls of volcanic craters- 

 whose position is sufficiently well-indicated by the criteria 

 enumerated above, are shown in the accompanying map. 

 The map was constructed by drawing lines along the 

 highest ridges of the hills enclosing undoubted volcanic 

 craters, and then along those of less certain origin. The 

 lines were then thickened according to a definite scale in 

 which the breadth of line was made proportional to-the 

 height of the hill above sea-level. The highest hills in 

 the north-east are over 1500 feet above sea-level, and are 

 represented by the broadest lines. 



One of the most striking features brought out in this 

 map is that several of the crater-ring walls end abruptly 

 in the circumference of others. W here this is the case 



Explanation of Map. 



I. Capo Miseno 

 II. Porto Miseno 

 III. Bacoli 

 IV., V. Fondi di Baia 

 VI. Baia 



VII. Monte Nuovo 

 VIII. Lago d'Averno 

 IX. Moute Grillo, Monte Rosso 



X. Cuma_ 

 XI. Campiglione 

 XII. Montagna Spaccata' 

 XIII. 3 Cratere di Campana 

 XIV. Solfatara 

 XV. Astroni 

 XVI. Archiagnano 

 XVII. Cigliano 



XVIII. Quarto 

 XIX. Pianura 

 XX. Soccavo 

 XXI. Naples 

 XXII. Chiaja 



XXIII. Fuorigrotta 



XXIV. SanStrato 

 XXV. Santa Teresa 



(4) In the Phlegr^an Fields there is nothing more 

 common than to find volcanic cones denuded on the side 

 facing the sea. It is, therefore, probable that the last 

 vestiges of a volcanic crater ring will be on the land side. 



Examined with especial regard to these points, the 

 hills of Baia, Monte Grillo and Monte Rosso, Cuma, 

 Montagna Spaccata, and many others admit of inter- 

 pretation as segments of volcanic crater walls. Lastly, 

 there are some hills which may possibly be remnants of 

 volcanic cones ; but if they are, they are so worn and 

 denuded that I have not been able to assign any position 

 on satisfactory evidence to the vent from which they have 

 been erupted. 



The hills of the Phlegrasan Fields which are considered 



NO. i486, VOL. 57] 



it is assumed that the crater wall ending in the other is 

 the product of a more ancient eruption. If this assump- 

 tion prove to be correct, we have a very easily observable 

 indication of the relative ages of the intersecting hills. 

 Confirmatory evidence is often supplied by the physio- 

 graphical characters of the hill slopes themselves. The 

 slopes of the older crater walls seem to be, as a rule, less 

 steep, and to exhibit the weather-worn scars of erosion 

 and denudation to greater perfection than the steeper 

 gradients of the younger crater slopes. 



In the table given on p. 584 are set forth the volcanic 

 structures so as to indicate their chronological relation- 

 ships, so far as I have been able to make them out. The 

 only crater of known age is Monte Nuovo : the most 



