342 Geology of 



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 pro- 

 duction 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, Grerman, 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. R. Mallet's 

 excellent paper on the " Mechanism of Production of Yolcanic Dykes," 

 and of those of Mount Somma, in the " Quarterly Journal of the 

 Geological Society of London, No. 128, Nov. 1876," 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 all directions. It is still 

 more remarkable to observe that all dykes which are cut by the Christ- 

 church 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 



