382 Transactioths. 



The remaining portion of this peninsula is composed of basalt penetrated 

 by trachyte dykes, with one small exposure on the eastern side of a well- 

 stratified tuff containing pebbles of rhyolite. The field relations of this 

 occurrence are obscure, but it certainly underlies basalt-flows, and it is 

 further interesting as indicating a probable extension in a south-easterly 

 direction of the Gebbie's Pass rhyolites under the covering of basic rocks. 



(c.) Little River Valley. 

 To this epoch Haast* and also Huttonf would also assign the forma- 

 tion of a caldera in the Little River Valley. Although an explosion 

 may have been responsible for the formation of this valley in its initial 

 stages, there is no actual evidence of its having been an independent 

 centre. There is no appearance of a quaquaversal dip of the lavas 

 issuing from a centre located somewhere within its basin ; rather they 

 are all disposed as if they flowed from the direction of Akaroa. The dyke 

 system is that belonging to Akaroa and radiating from the centre of that 

 volcano. There is, besides, no difference in the lithological nature of the 

 lavas from the two localities. Further, its shape, with three long trailing 

 spurs directed in some measure towards the middle of its basin, suggests 

 most strongly the action of water. The fact that in certain parts of the 

 western side cliffs face the valley and simulate to a minor degree the form 

 of the caldera is entirely explained by streams cutting parallel with the 

 strike of the beds, which must of necessity produce at times a form which 

 resembles the steep interior faces of a caldera. The same remarks apply 

 to Pigeon Bay, also regarded by Haast as a caldera, except that it is divided 

 into two subordinate valleys, and not into three as in the case of Little 

 River. I regard both as eroded bv streams on the flanks of a volcanic cone. 



o 



4. Third Volcanic Phase. 



(a.) Mount Herbert Volcano. 

 The third distinct phase in the history of the area was marked by out- 

 pourings of lava from the neighbourhood of Mount Herbert, in all pro- 

 bability from a centre in Kaituna Valley or on its north-western boundary. 

 This may perhaps be regarded as a third and subsequent vent, analogous 

 to the two great calderas but on a smaller scale. By these eruptions the 

 highest peak on the peninsula was constructed, the greatest development of 

 the lavas being on Mount Herbert Peak and the ridge which extends in 

 a westerly direction from it and forms the flat-topped mountain known 

 as Mount Herbert. The tops of these elevations are formed of massive 

 flows of lava, lying very level, individual flows being from 80 ft. to 100 ft. 

 in thickness, and exhibiting in places columnar jointing on a large scale. 

 The flows form successive tiers of ramparts round the head of the valley 

 above the Head of the Bay and Charteris Bay, and again on the southern 

 side fronting Kaituna Valley ; but between Charteris Bay and Purau they 

 form a long gentle slope with an average inclination of 10°, both the slope 

 of the ground and of the lava -flows being in agreement. The termination 

 of the flows has been cut back into a sea-cliff, about 50 ft. in height, for 

 about a space of a mile on the southern shore of the harbour between 

 Charteris Bay and Purau, and there is an outlying fragment on the eastern 

 side of Purau forming a low ridge behind the Ripa Island fort. 



* J. von Haast, loc. cit., p. 343. 



t F. W. Hutton, Sketch of the Geology of New Zealand, Quart. Journ. Geol. Soc, 

 vol. 41. 1885, p. 216. 



