PRODUCTION OF VOLCANIC DYKES. 475 



the results of the Neapolitan earthquake of 1857, and who is, I 

 believe, still in Xaples. 



After my return to England my survey was to a great extent 

 plotted ; and the results showed themselves as concordant as 

 could have been expected from geodetic work carried on upon 

 excessively uneven and elevated ground, and under the disad- 

 vantages of weather described. The necessities of professional 

 engagements and other circumstances, however, obliged my laying 

 aside for a considerable time the completion of my work, until 

 ultimately failure of sight and the impossibility of my note-books 

 and sketches being deciphered by another have compelled me to 

 lay it aside altogether. Unwilling, however, that the results of so 

 much labour and care should be wholly lost, especially as some 

 of the facts ascertained are new and appear not devoid of im- 

 portance, I have thought it best to note down some of the more 

 salient results, which others more fortunate than myself may per- 

 haps accept as a basis for further research. My inability to pro- 

 duce the whole of my work, together with the triangulation, in a 

 fit state for publication is now the more regretted by me since 

 most of the features, the surface-levels &c. of the Atrio del 

 Cavallo, as they existed in 1864 have been to an immense extent 

 altered by the great eructations of lava that poured into it during 

 the great eruption of 1872, so well described by Siguor Palmieri. 

 It is also probable that the scorching heat then produced in the 

 Atrio del Cavallo, and extending westward far beyond the Ob- 

 servatory, may have partially, if not wholly, obliterated most of 

 the red numbers painted by me upon the dykes, and perhaps even 

 consumed by fire the wooden cross of the Salvatore, which was one 

 of my marks. 



On examining with care, and aided by the telescope, the entire 

 surface of the escarpment of Somma, it is seen to consist of a pre- 

 cipitous but nowhere actually vertical bank, with a highly irregular 

 or serrated sky-line rising at some points 800 or more feet above 

 the floor of the Atrio. Its exposed face consists to a preponderant 

 exteut of beds of volcanic conglomerate, composed of fragments, 

 subangular or more or less perfectly rounded, of various sizes, 

 from masses occasionally of a cubic yard or more in volume to 

 nodules of the size of a human head, or of an orange, mixed 

 with pebbles of various smaller sizes and with gravel or lapilli. Of 

 the latter there are also great beds. Intercalated with all these are 

 many beds of lava, which, though now for the most part cross- 

 fractured and twisted, had flowed in a state of fusion from the 

 ancient crater of Somma and consolidated upon its slopes. All 

 these beds, though wildly confused in many places, yet preserve a 

 general horizontality round the amphitheatre. Crossing all these 

 in very various directions, forming different angles with the 

 horizon, and scattered at very irregular distances from each other, 

 existing sometimes singly but often several in close proximity, and 

 sometimes inosculating with each other, and with their edges highly 

 irregular and broken, projecting more or less from the abrupt face of 



