PRODUCTION OF VOLCANIC DYKES. 493 



so that at a distance of a mile from its crater origin, the orientation 

 of the fragments, as well as the dip of the dyke now observable in 

 the escarpment, may be wide away from those at the point whence 

 it is supposed to have started. Further, this change of direction 

 may be repeated in opposite directions, producing planes of double 

 curvature such as were actually observed by Lyell and Hartung in a 

 dyke in Madeira, and as observed by me in some of the twisted 

 dykes of Somma. 



Without disputing the fact that the horizontal coincidence at or 

 near a single point of the apparent orientation of a considerable 

 number of the fragments of dykes observed only at considerable 

 distances, may afford a presumptive probability, greater or less, in 

 proportion to the greater number of the coincidences and to the 

 generally straight and unshattered character of the fragments of the 

 dykes observable, I must yet come to the conclusion from what has 

 preceded that the concurrence or intersection at a single point of 

 the apparent orientation of dykes is too unsafe a guide to warrant 

 our fixing the position of an ancient crater upon such a basis alone, 

 and that such a mode of determining crater-position, though it may 

 attain to probability, can never amount to certainty. 



In stating this conclusion I wish not to be misunderstood as 

 casting any doubt upon Sir C. Lyell's views expressed in his paper 

 to which I have already referred, as to the ancient existence and 

 position of a second crater to Etna. The evidence which he has 

 adduced as to such a crater at Trifoglietto does not rest alone upon 

 the convergence there of the greenstone dykes observed by Yon 

 Waltershausen and himself, but is also sustained by the synclinal 

 position, as adduced by him, of the lava and conglomerate beds 

 towards some line between the existing crater of Etna and that 

 concluded to have existed at Trifoglietto ; and to this evidence as 

 supporting the former existence of a second crater there is no objec- 

 tion to offer. At the same time I must remark that the position 

 of the lowermost beds of any volcano upon the vast scale of Etna 

 must have greatly changed in the course of ages by the depression 

 beneath the central parts produced by the enormous superincumbent 

 weight, which is constantly being added to, of the cone itself, and 

 from the erosion constantly proceeding irregularly under the base 

 of the mountain. 



If Etna, after having broken through the Tertiary strata, de- 

 posited its materials on a nearly level surface, that, as the mass of 

 the mountain has been heaped up through long ages, must from the 

 causes mentioned have become depressed towards the central parts 

 into a shallow saucer-shaped dish ; so that the lowermost beds with 

 the non-volcanic strata upon them, could we now see them, might 

 be found to dip more or less from the circumference of the moun- 

 tain downwards towards its axis, the same phenomena here oc- 

 curring upon a colossal scale that are well known to happen when 

 heavy earthen embankments are formed over even very firm soil, 

 which is depressed beneath the weight, and sometimes more or less 

 raised at considerable distances from that, but often not sensibly 



