474 R. MALLET OS THE MECHANISM OP 



seen from the floor of the Atrio, nor the latter from it, because no 

 station-mark "of a permanent character, or offering a reasonable 

 chance of not disappearing in perhaps the very next eruption, could 

 be found in or nearer the Atrio del Cavallo. At this point, very- 

 near the head of the Fossa di Vetrano, the first prominent dyke of 

 the escarpment of Somma occurs, which I called No. 1 ; and having 

 perambulated the entire base of the escarpment, thence going 

 eastward and choosing out for geodetic determination, from the in- 

 numerable dykes of every size which intersect it in almost every 

 direction, twenty-seven of those which seemed largest, highest, and 

 generally most characteristic, I numbered the whole of these with 

 red oil paint in figures five or six feet long upon the projecting 

 edge or face of each near the base or level at which these dykes 

 generally disappear below the floor of the Atrio del Cavallo, taking 

 care that all these figures should be discernible by the telescope of 

 my theodolite from the point of observation chosen at the base of 

 Vesuvius as already mentioned. I however examined carefully as to 

 their lithological circumstances a number of other dykes intercalated 

 at various points between those numbered, amounting with the 

 preceding to thirty-four in all. 



Having in connexion with the line of triangles by which the 

 observing-point was fixed, measured a short base of 374 feet (Eng- 

 lish) upon a level part of the Atrio del Cavallo extending in a S.E. 

 direction, the N.W. extremity being nearly in a line between the 

 Punta del Naso and the centre of the Yesuvian cone, I then 

 extended my triangulation so as to fix, in reference to my observing- 

 point, and to the then existing cone and crater of Vesuvius, the general 

 line of the escarpment of Somma from west to east, extending along 

 the curve of the escarpment to a distance of rather more than 2% 

 English miles. This line has in reality much less of the form of a 

 regular circular curved amphitheatre than it appears to the eye as 

 viewed from the northern side of the cone of Vesuvius ; and I found 

 it was by no means exactly laid down upon the engraved sheets 

 which contain Vesuvius and Somma, issued from the topographical 

 office at Naples by the former Government of the King of the Two 

 Sicilies. I was thus in a position to fix trigonometrically the 

 orientation of the twenty-seven numbered dykes in reference to my 

 observing-point, and to the axis of the cone of Vesuvius ; and then, 

 again perambulating the whole line of dykes from west to east, I 

 observed and noted the characteristics of each, the lithological 

 character of its mass, the horizontal direction in which the plane of 

 the dyke penetrated the bank of the escarpment, the " hade," or 

 dip, or departure in a vertical direction from perpendicularity in 

 the plate of rock, its curvature, if any, and other particulars to be 

 hereafter noticed. These operations and observations occupied me, 

 along with a staff of men hired at Naples and Resina, for about a 

 fortnight in November, during several days of which there was 

 continuous rain and so boisterous a wind as rendered work with a 

 theodolite difficult. My chief assistant was Antonio d'Antonini, 

 who had been my faithful and intelligent attendant in exploring 



