Canterbury and Westland. 343 



acquainted, showing some remarkable irregularity, is the one in which 

 the so-called Ellis Quarry is situated. This dyke, which strikes nearly 

 east and west, goes out about 400 feet below the summit, where a saddle 

 intersects the spur. Shortly above its lower termination it sends off 

 a smaller branch in a south-west direction, also ceasing after a short 

 course. Whilst the main dyke does not appear any more above the 

 surface, the smaller south-western branch crops up again on the other 

 side of the depression, now gradually changing its direction, so that, 

 in its lower course, about 300 feet above the plains, it crosses the spur 

 in a south-east and north-west direction. The whole system of dykes 

 in the Lyttelton caldera wall is thus very different from the dykes of 

 Mount Soinma, of which, in his paper, Mr. E. Mallet gives us such a 

 lucid and suggestive account, and of which many are fractured, dis- 

 placed, and crushed, and have at the same time a wedge-shaped form. 

 We can therefore safely assume that the fissures and dykes in the 

 Lyttelton caldera were only formed after the latter had been so 

 thoroughly consolidated that, after the formation of the fissures and 

 their filling up by the principal dykes, no more changes of any import- 

 ance took place in them ; and that, moreover, the forces by which the 

 walls of the volcano were starred from top to bottom, must have been 

 far deeper seated and more effective than the agencies by which 

 Mount Somma was rent. 



Leaving for the present the Lyttelton caldera, the genetic history of 

 which will also serve to explain the mode of formation of the other 

 calderas of the volcanic system under consideration, I shall now 

 proceed to treat of another volcanic focus, either contemporaneous 

 with the former, or at least formed shortly afterwards. Of this second 

 centre of eruption, which is so greatly destroyed or hidden by lava- 

 streams and agglomeratic beds of younger age, ejected from other 

 volcanic foci, that only a portion of the western caldera can be made 

 out, the vent was situated somewhere in the valley of the Little 

 River. All the rocks are similar in character to those of the Lyttelton 

 caldera. 



The system next in age, and distinguished both by its size and the 

 splendid preservation of its nearly entire wall, is the Akaroa caldera. 

 I should only have to repeat myself were I to give a description 

 of the lava-streams, their mode of deposition, and of the beds 

 of agglomerate, ashes, and tufas interstratified with them, by 

 which this caldera wall has been built up, as it resembles closely 

 the one at Lyttelton. The lava-streams consist also of a basaltic 



