* 
506 Transactions.—Geology. 
has proposed another theory, namely, that the principal cause of vulcanicity 
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. R. 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, voleanic 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 Christchurch and Lyttelton railway tunnel have such 
8, 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., 1870. 
