340 Geology of 



In studying the position of the dykes it becomes manifest that they 

 have been formed at different times ; however, the altitude of their 

 uppermost portion does not indicate their age. I have no doubt that 

 many of them, which scarcely reach above high-water mark, are not older 

 than others of the same petrological nature, which reach to the very 

 summit of the caldera wall. In the present state of our knowledge it is 

 impossible to solve this interesting question in all its bearings, and I can 

 therefore only suggest that dykes containing rocks of exactly the same 

 lithological character have most probably been formed during the same 

 eruption. It is also evident that a number of dykes were formed long 

 before the whole of the caldera wall was built up, and that they were 

 partly destroyed during one of the next eruptions. One clear instance of 

 the occurrence of such older dykes is to be found near Cliff's Cove in 

 Lyttelton Harbour, where several trachydoleritic dykes were injected 

 when the rest of the caldera wall was at least 1000 feet lower than at 

 present. They pass through a basaltic lava-stream, which latter was 

 afterwards partly destroyed along with them, the whole possessing now 

 a nearly straight surface, upon which a large bed of agglomerate has 

 been deposited. There are a great many instances of this kind. 

 However, what is of the greatest interest in the history of the volcanic 

 systems under consideration is the predominating acidic character of 

 the dykes when compared with the basic lava-streams. In Vesuvius 

 and Etna all the dykes are formed by the same kind of rock as the 

 lava-streams are composed of, but they are generally more compact, 

 having, as Lvell suggests, cooled and consolidated under greater 

 pressure. It is evident that they owe their existence to the same 

 subterranean efforts by which the lava-streams were ejected from the 

 mouth of the crater, the fissures in which they were formed being 

 evidently filled up from the same focus, and about the same time as 

 the eruption of the lava-streams took place. But such a simple 

 process cannot be admitted for the greater portion of the dykes of 

 Banks* Peninsula, which must owe their existence to paroxysmal 

 perturbations in the earth's crust, distinct from those during which 

 the caldera walls were built up. It is evident that a great portion of 

 the lava -streams and agdomeratic beds which once formed the crater 

 of the volcanic svstem of Lyttelton Harbour, must have been blown 

 awav. or at least removed during one of those violent outbursts of 

 subterranean forces necessary to clear the choked vent of the 

 volcano, similar to those by which in recent times the upper portions 

 of active volcanoes have repeatedly been destroyed under the eyes 

 of the trembling population in the neighbourhood. 



