PRODUCTION OF VOLCANIC DYKES. 477 



cano ; for a great depth of the loose material which mainly consti- 

 tuted the original cone of Somma, a depth which may in some places 

 have reached perhaps 200 or 300 feet, has been removed and swept 

 down towards the plain by the action through long ages of rain and of 

 pluvial torrents. In passing, it may be remarked that the rapidity 

 with which the incoherent matter deposited upon the flanks of vol- 

 canic mountains is thus swept away by rain is well exemplified upon 

 the flank of Etna by the condition of the old oak trees which con- 

 stitute the Bosco della Casa, which is crossed, three or four miles 

 above Nicolosi, by the usual mule-path towards the summit. The 

 slope of the mountain here, the inclination of which generally is very 

 moderate, not exceeding perhaps 10°, is composed almost wholly of 

 loose black pyroxenic sand, in which and probably in old tufaceous 

 beds beneath the oak trees grow. 



On examining the trunks of these, it is at once apparent that 

 the level of the surface during their earlier life was from two or 

 three to (in several places and over large areas) five feet above what 

 it is at present, the lowermost part of the bole proper, or trunk, of the 

 tree, with its rough bark and spreading-out base, marking the original 

 ground-line, while the tree now stands, as it were, upon its toes, 

 the great tap-roots, one or several, forming its only connexion with 

 the present ground (1864), which they penetrate so as to support 

 the tree. None of these oaks are probably more than one hundred 

 or one hundred and fifty years old ; within that time therefore 

 a depth of from three to five feet of material has been denuded from 

 from this part of the mountain over a very large area, and this not- 

 withstanding that relays of like detritus have during the interval 

 been brought down by rain from still higher parts of the slope. 



The results of my survey may be classed into : — 1st, Observations 

 on the directions in space and in reference to the material of the es- 

 carpment of the dykes. 2nd. The lithological and other circum- 

 stances of the dykes themselves and of the matrix adjacent to them, 

 which throw some light upon the nature of their production. 3rd. 

 Conclusions to which these data appear to point. 



The apparent horizontal direction in which a dyke observed at the 

 level of the floor of the Atrio plunged into the bank of the escarpment, 

 I have called the strike. This direction, when referred to the axis 

 of the cone of Vesuvius, I have called the orientation of the dyke. 

 The divergence, if any, from verticality of the plane of the dyke I 

 have called its dip or hade. Curvature refers to bending round some 

 imaginary line other than vertical, while twisting means torsion round 

 some imaginary line exterior to the dyke and not far from vertical. 

 The dykes, at their exposed edges, rise to very various heights, some 

 to the top of the escarpment, generally thinning out as they ascend ; 

 others only rise sixty or seventy feet above the floor of the Atrio, 

 and taper rapidly, coming to an edge from a thickness of perhaps 

 five or six feet at the visible base, and without any observable fissure 

 extending beyond the summit into the matrix. Some are so curved, 

 twisted, and often dislocated into fragments of greater or less length 

 that no deduction as to orientation is possible ; but many others^ 



