488 R. MALLET ON THE MECHANISM OP 



localities ; I venture to commend it to the attention of volcanic ex- 

 plorers. Some light may be thrown on the subject by the remarks 

 about to be made on the mode of production and of injection of the 

 fissures of these dykes, a subject which has engaged but very slight 

 and insufficient attention hitherto. 



So far as my reading extends, I do not find that any writer on 

 geology or volcanoes has attempted to frame a physical theory profess- 

 ing to account for the mode of production of the fissures which, when 

 filled, form volcanic dykes, nor for the positions they are found to 

 occupy, i. e. their apparent general fan-like arrangement when 

 viewed horizontally. Nearly all that Dr. Daubeny has to say upon 

 the subject is comprised in the following sentence : — " Whatever 

 theory we may adopt with respect to the formation of the beds in 

 which these dykes occur, it is probable that we shall all concur in 

 considering them produced by the injection of a leucitic lava into 

 cracks which previously existed in the containing rock " (Daubeny, 

 < Volcanoes/ 2nd edit. 1843, p. 232). 



The phrase " previously existed " is indistinct ; no crack can be 

 filled until after it has been previously opened ; but how opened, or 

 how long previously to filling, we are afforded no light by this 

 passage. 



Most English and several foreign text-books make reference to 

 volcanic dykes in so casual and incomplete a manner that the 

 reader can gather little more than the general notion that they 

 appear to radiate from some central axis or axes ; even this is very 

 often not stated, but left to be inferred from the description of the 

 dykes given. In the late Professor Phillips's generally admirable 

 little book upon Vesuvius, an ideal diagram is given at page 132 of 

 the aspect of a part, at least, of the dykes of Somma ; but throughout 

 the work, where a most complete description and discussion of these 

 dykes and of the mode of their production might have been looked 

 for, we do not find a single paragraph which treats of them. 



De la Beche, generally so exact as to questions of physical geology, 

 is little more explicit (Geol. Observer, 2nd edit. 1853, p. 378), 

 except that he distinctly affirms that fissures formed in the mass of 

 volcanic cones are filled by the liquid lava, which exercises hydro- 

 static pressure upon the interior walls of the crater. 



Sir Charles Lyell, in his paper already referred to, " On Lavas of 

 Mount Etna " (Phil. Trans. 1858), following Von Waltershausen, by 

 employing the apparent convergence of thirteen of the dykes of the 

 Val del Bove as the means of fixing the position of the second axis 

 or ancient crater which he assigns to Etna at Trifoglietto within the 

 Val del Bove, tacitly assumes that the clefts in which these dykes 

 were produced were so formed as all to emanate radially from 

 Trifoglietto as an axis. He however assigns no reason why this 

 should be so ; and from the words (page 760) " near the principal 

 crater, where earthquakes rend the mountain, and where lava is ever 

 ready to flow into rents," he appears to be of opinion that the clefts 

 of these dykes were rent open by earthquakes — admitting which, it is 

 difficult to conceive why they should pass outwards from the 



