478 R. MALLET ON THE MECHANISM OF 



though not free from twist or curvature or even from cross fractures, 

 still admit, within moderate limits, of the deduction of their orienta- 

 tion as well as of their general dip. One source of possible error 

 which may affect some of my observations I must not omit to men- 

 tion. From the smallness of the exposed surface and the necessity 

 of observing eacli dyke at but a few feet from its base, the only practi- 

 cable method of determining the strike was by means of the compass. 

 But as many of these dykes are, like most volcanic products, mag- 

 netic, and sometimes with polarity, the indications as to horizontal 

 direction afforded by compass are somewhat uncertain ; and the 

 amount of deviation from this cause, if it exist, is not easily determi- 

 nable. I do not think, however, that any very large error due to 

 these causes exists in my observations ; for on comparing the directions 

 shown by two different compasses, one carrying a heavy and power- 

 ful pivoted bar-magnet, the other a very light needle, both success- 

 ively placed in the same position and within a few feet of the base of 

 some of the largest of these dykes, and on also comparing the direction 

 of both compasses when removed a hundred yards or more from the 

 base of the same dykes and still in the direction of the plane of the 

 dyke, I did not find a notable difference of indication, though some- 

 times reaching a degree or two. The observations of strike .thus 

 made, when combined with the position at the escarpment and level 

 of the floor of the Atrio, of the terminal edge of each dyke, com- 

 pleted the elements necessary for the determination of its orienta- 

 tion or line passing horizontally through its general plane when pro- 

 duced through the axis of the existing Yesuvian cone. 



The orientation of five or six of the dykes thus obtained is such 

 as to pass, though only approximately, through the axis of the 

 cone — all the others observed presenting wide disparities in direc- 

 tion of orientation, which in most cases passed outside the confines 

 of the cone itself at the level of the Atrio. Indeed in some cases 

 two dykes of different ages cross one another ; and while the orien- 

 tation of one roughly approximates to the Yesuvian cone, that of the 

 other passes wide away from it. The divergence from axial orien- 

 tation increases much as we approach nearer to the eastern end of 

 the escarpment ; and as we begin to descend from the level of the 

 floor of the Atrio along the prolongation of the escarpment towards 

 where it is lost in the eastern slope of the mountain, we find a large 

 number of dykes whose orientation is seen at a glance to be wide 

 away from any part of the existing cone of Yesuvius, and whose dip, 

 or hade, is towards the west and north, and rapidly increases as we 

 descend, so that several of the most easterly of these dykes present to 

 the eye the character of old beds of lava inclined at angles steeper 

 than the slope of the mountain, and dipping into the latter, in which 

 they are partially buried, rather than of dykes at all. I did not de- 

 termine the exact strike of any of these dykes or beds eastward 

 of my number 27, partly by reason of the facts just given, but mainly 

 because their bases could not be seen from my observing-station at 

 the base of the cone and could only be approached over a bed of 

 hugely scoriaceous lava which had overflowed from the Atrio at this 



