Marshall. — On Tridymite-Trachyte of Lyttelton. 369 



seem bold to invoke the aid of such a vast explosion, we must 

 remember that even in recent times there have been cataclysms 

 that have equalled in intensity and magnitude that required 

 for the formation of Lyttelton Harbour. The force expended 

 at the Krakatoa eruption was quite as great as that required 

 by hypothesis in the present case. Even in New Zealand we 

 have had an illustration of the immense power exerted by 

 imprisoned steam in the destruction of the terraces at Eoto- 

 mahana, where a gigantic chasm was formed almost as large 

 as Lyttelton Harbour. 



Since this great spasmodic effort two other eruptions or 

 periods of activity have been witnessed in this system. The 

 first resulted in the formation of Mount Herbert in the former 

 caldera wall, where the lavas which flowed down the sides of 

 this secondary vent interfere with the symmetrical appearance 

 of the walls. 



By the last eruption Quail Island was formed. This erup- 

 tion is unimportant ; it does not seem to have been attended 

 by any explosive action, and to have attained to only extremely 

 small dimensions. 



As would be expected from the spasmodic character of the 

 earlier eruptions, a large number of radiating cracks were torn 

 open in the surrounding crater- walls, and into these the magma 

 was injected, giving rise to a well-defined system of dykes, which 

 preserve an astonishingly constant direction, width, and com- 

 position over large horizontal and vertical distances. From a 

 careful examination of these dykes it was ascertained that, 

 with a few exceptions, they can be classed in two systems. 

 Of these the most important radiates from a spot situated a 

 little to the south of Quail Island, while the other proceeds 

 from the centre of a shallow bay to the south-east of the 

 former one. 



The rocks of the Lyttelton system are, with a few excep- 

 tions, members of the basic series of igneous rocks, the com- 

 monest species being one that is best named olivine-andesite. 

 Andesites without olivine have also been found, while basalts, 

 especially in the later eruptions, are frequent, being, as a rule, 

 very finely grained. Trachytes also occur, particularly in 

 dykes, and from the prevalence of dykes of trachyte Sir Julius 

 von Haast drew an important induction in support of ' Du- 

 rocher's theory of volcanic action and the origin of the 

 eruptive rocks. Ehyolites are also found as members of the 

 very earliest eruptions, and still crop out at Governor's Bay, 

 on the shore of the harbour. 



The most anomalous of all the rocks occurs on the Lyttelton- 



Sumner Eoad, about half a mile from Lyttelton. This rock 



was discovered by Sir J. von Haast, and was called by him a 



domite. A chemical analysis was made by Professor Bickerton, 



24 



