506 Transactions. — Geology. 



has proposed another theory, namely, that the principal cause of vnlcanicity 

 is to be sought in the compressing and crushing action taking place beneath 

 the crust of the earth, and by which such a great amount of heat is 

 generated that a fusion of rocks, often on a large scale, is easily produced. 

 This theory would so far explain Very well the difference in the composition 

 of the rocks varying according to the depth where the crushing action was 

 actually taking place ; thus, if the same action were to act upon granites, 

 trachytes, and other acidic rocks, the result would be the production of 

 trachytes, whilst if basic rocks were fused, basalts would ascend towards or 

 to the surface. Here, however, another great difficulty presents itself in the 

 fact that, although the number of volcanic eruptions during which the 

 caldera walls were built up must have been very great, no trachytic lava 

 streams, with one single exception, have made their appearance, the whole 

 series being of a basic, whilst most of the principal dykes are of an acidic 

 nature. In such a case, the crushing of acidic rocks would have exclusively 

 taken place when the dykes were being formed, and never when lava-streams 

 issued from the crater's mouth, which is altogether improbable. 



Although I have carefully read every work accessible to me in English, 

 German, French, and Italian, treating on vulcanicity, I have not been able 

 to find either any account of the existence of dykes in other volcanic regions 

 converging so regularly to a few centres close to each other, or continuing 

 over such a large area (always keeping the general direction with which 

 they set out), as do those of the Lyttelton caldera; or again, offering an 

 explanation for the difference in the composition of the dyke rocks when 

 compared with the lava-streams or agglomeratic beds through which they 

 pass. Mr. E. Mallet's excellent paper on the " Mechanism of Production 

 of Volcanic Dykes,"* and of those of Mount Somma, in which an 

 exhaustive account of the physical features of the dykes in the old caldera 

 wall of Mount Vesuvius is given, unfortunately does not contain any 

 physical theory to account for the mode by which fissures are produced, 

 forming, when filled, volcanic dykes. If we take the heterogeneous nature 

 of the material of which the caldera wall has been built up into account, it 

 is astonishing that the dykes show such a remarkable regularity, always 

 starting from a few points not far from each other, from which they radiate 

 in aU directions. It is still more remarkable to observe that all dykea 

 which are cu.t by the Christchurch and Lyttelton railway tunnel have such 

 a constant direction that they all, with one or two exceptions, appear to 

 converge to one single axis behind Quail Island, a fact worthy of note if we 

 consider the distance, which is more than four miles, measured to the most 

 distant dyke in that tunnel. The only dyke with which I am acquainted, 



* (Quarterly Journal of the Geological Society of London, No. 128, Nov., 1876, 



