472 K. MALLET ON THE MECHANISM OF 



52. On the Mechanism of Pkoduction of Volcanic Dykes, and on 

 those of Monte Somma. By E. Mallet, Esq., C.E., E.E.S., 

 E.G.S.* (Eead June 21, 1876.) 



In the year 1864 I was enabled to employ some time in the 

 study of some of the chief volcanic phenomena presented by the 

 cone of Etna, devoting my attention principally to the laws which 

 govern the flow of lava currents, the formation of the parasitic 

 cones which abound upon certain portions of the surface of the great 

 mountain, of which Monte Bosso and Monte Peleri in the neigh- 

 bourhood of Nicolosi are amongst the largest examples, and in 

 examining the Yal del Bove and the many so-called dykes which 

 intersect its surrounding escarpment in so many places. These last 

 objects were examined by me with the able memoir of Sir Charles 

 Lyell, published in the Philosophical Transaction for 1858, in hand, 

 the title of which is " on Lavas of Mount Etna," &c. 



In the second part of that memoir the author rests part of the 

 evidence upon which he concludes that an ancient great vent existed 

 in the Yal del Bove at Trifoglietto, upon the convergence at about 

 that point of the prolonged lines of direction of thirteen dykes 

 existing in the surrounding escarpment, in accordance with the 

 views previously promulgated by Von Waltershausen (Phil. Trans., 

 Part ii. 1858, page 703). I have no intention of casting any doubt 

 here upon any part of the above able memoir of Sir Charles Lyell, 

 in all the main conclusions of which, indeed, I concur, my pre- 

 sent object being limited to remarking upon some of the conditions 

 which affect the formation of volcanic so-called dykes of injection, 

 such as those existing in the escarpment of the Val del Bove, and 

 of that of Monte Somma, and to pointing out the extent to which 

 the orientation or direction of such dykes can be safely employed as 

 a means for determining, with any thing approaching to certainty, 

 the existence or position of a central chimney or crater, from which 

 such dykes may be supposed to have emanated, by means of the in- 

 tersection of lines or planes prolonged in the direction of such dykes. 

 On going round nearly the entire escarpment of the Val del Bove, 

 with Von "Waltershausen's map in hand, the dykes, which are almost 

 innumerable, when examined at many points both from above and 

 below the escarpment, presented phenomena in many other respects 

 as well as in direction, so perplexing as to raise in my mind much 

 doubt as to whether they could be at all accounted for upon the 

 commonly accepted theory of their production, and suggested the 

 inquiry whether that theory which assigns their production to the 

 injection of fissures in crater walls by the liquid lava within the 

 crater might not need considerable modification, and if so, whether, 

 in its imperfect state, the orientation of dykes could, by intersec- 

 tion, be safely employed at all for determining the position of craters 

 which have disappeared, as in the case of that of Monte Somma 



