PRODUCTION OF VOLCANIC DYKES. 473 



and of those supposed to have existed in the Yal del Bove. In the 

 latter region the orientations or lines of horizontal direction of the 

 numberless dykes are so various and discordant as to resemble 

 in very many places nothing more than the chance directions which 

 might be assumed by a number of short straws or sticks cast at 

 random upon the ground. Lines drawn through them might be 

 made to create great numbers of points of intersection; and yet 

 great uncertainty would attend attempts to infer from any of them the 

 former existence of a crater there. These perplexed phenomena in 

 the Yal del Bove are upon too large a scale for easy disentangle- 

 ment, and are far less suitable for the study of the laws of production 

 of injected dykes than are those found so abundantly in the escarpment 

 of Monte Somma, where the whole scale is smaller, the dykes gene- 

 rally closer together round the amphitheatre, the phenomena less 

 perplexed than in the Yal del Bove by the circumstances which pro- 

 duced that immense depression, and where there exists the main 

 chimney or axis of the cone of Yesuvius, the position of which is 

 not presumed to have materially changed from that before occupied 

 by the crater while Somma was in existence, and to which therefore 

 it might be presumed that the directions of all the existing dykes of 

 Somma should show a general convergence. Eeserving therefore 

 the formation of any conclusion as to the facts I had observed upon 

 Etna, I resolved upon my return to Naples to make a careful study of 

 the dykes of Somma, and to endeavour to decipher therefrom the laws 

 of production of these and other similar volcanic dykes. Por though 

 these dykes of Somma have been again and again described in their 

 general aspects, and though almost all writers on volcanoes have as- 

 sumed without question that their directions do really converge at 

 the axis of the existing cone of Yesuvius, I could not find that any 

 careful determination by actual survey had ever been made of the 

 directions of these dykes ; nor had sufficiently exact observations been 

 made as to many points in the dykes themselves tending to elucidate 

 the exact nature of their origination. Accordingly, in November 

 1864 I made arrangements for determining with instrumental ac- 

 curacy the positions and directions, both horizontal and vertical, of a 

 considerable number of these dykes of Somma. Having, through 

 the kind assistance of Professor Palmieri, been permitted to lodge at 

 the Observatory above the Hermitage upon Yesuvius, I ran a line 

 of trigonometric triangles from the south-east corner of the Obser- 

 vatory, passed the wooden cross existing at that time, and known 

 as La Croce del Salvatore, and into about the centre of the Atrio del 

 Cavallo, whence I was enabled to fix the position of a point chosen 

 for that of observations upon one of the slight elevations at the 

 northern base of the cone of Yesuvius, which commanded an unin- 

 terrupted view of a very large portion of the entire escarpment of 

 Somma, extending from the Canale del Inferno, or thereabouts, on 

 the eastward, and passing round the amphitheatre westward to a 

 point nearly opposite La Croce, and a little to the west of north with 

 respect to the same. I was obliged to choose the quoin of the Ob- 

 servatory as an initial station -mark, although the building cannot be 



