Speight. — The Geology of Banks Peninsula . 391 



inside and outside of a cone would probably arise as erosion proceeded 

 without the intervention of explosion or collapse of the interior. 



The general features of the Lyttelton crater are reproduced in Akaroa 

 Harbour. The steep interior slopes towards the entrance, the peninsulas 

 at the head with their drowned valleys lying between them, the gradually 

 deepening of the harbour and its tributary bays towards the entrance, the 

 accordance of the grade of the floors of the valleys with that of the harbour, 

 the wave-cut cliffs gradually getting higher as the outlet is approached, 

 and the perpendicular walls which guard the entrance agree exactly in both 

 cases. Akaroa bas, however, a more perfect crater-ring : it is only broken 

 where the sea enters, and its regularity is not destroyed by subsequent 

 eruptions breaking out on the edge of the crater, as has happened in the case 

 of Lyttelton. If we make due allowance for the special features which exist 

 in the case of Lyttelton and are absent in Akaroa, we cannot but conclude 

 that the same causes have contributed in each case to establish the caldera- 

 like form, and the dominating one is the effect of stream action on a crater 

 which has been enlarged by a paroxysmal explosion of moderate intensity 

 or by breaching, but not one which has produced the great cavities which 

 now form the harbours as they are. 



The stream-erosion theory of the formation of calderas has been advanced 

 by Gagel in connection with the Palmas Caldera, and I believe that he did 

 not regard explosion as a contributing agency. I have not been able to 

 obtain his paper, but have only the reference to it in Professor R. A. 

 Daly's Igneous Rocks and their Origins. Haast and Hutton, on the other 

 hand, regarded the cavity as having been produced by explosion alone. 

 If, however, explosion were entirely responsible for the formation of the 

 cavity there should be accumulations of fragmentary matter round the 

 vent approximately equal in volume to that of the cavity. The amount 

 of material thrown out to make a space five miles across and at least 

 2,000 ft. deep should certainly have left traces in the surrounding country, 

 even after allowing for denudation. There is no such accumulation under 

 the subsequent lava-flows from Mount Herbert, where it would be pro- 

 tected from denuding agents. It is not intended to deny that in the 

 early stages moderate explosions did exert some effect. When the small 

 size of the necks of old volcanoes is considered (see Sir A. Geikie's Ancient 

 Volcanoes oj Britain), the largest recorded in Fife being a mile in diameter, 

 and the average far below this, we must conclude that unless the original 

 crater is enlarged in some way water erosion by itself would in all pro- 

 bability be unable to produce a cavity of the shape usually arrived at. 



Subsidence of the floor due to faulting has been suggested by Dutton* 

 as a prime cause of the formation of calderas. especially those of the 

 Hawaiian group. When, however, we examine the Line of junction of 

 the Lyttelton lavas with the underlying greywackes at the head of 

 the harbour and the Gebbie's Pass ridge, there is no sign of dislocation 

 following the base of the crater-ring ; the long tongues of rhyolite stretch- 

 ing down therefrom into the upper part of the harbour and almost 

 reaching the actual centre show no sign of a break, such as should 

 occur were the line of fault to follow the bounding walls of the depression. 

 If such a line did occur it should certainly be found cutting across the direc- 

 tion of flow of the rhyolite lava-streams in close proximity to their junction 



* C. E. Dutton, Hawaiian Volcanoes, 4th Ann. Rep. U.S. Geol. Svrv., 1884. p. 105. 



