372 Transactions. — Geology. 



Now, it is evident that this large dyke must have been 

 formed during or preceding such a paroxysmal eruption. Its 

 age, then, must be assigned to some period during which these 

 eruptions were still in full force. Again, it is evident that the 

 caldera of Lyttelton had already been formed when this erup- 

 tion took place, for the lavas that were emitted from this 

 dyke flowed down the face of the harbour- walls when they 

 were approximately of their present shape and form, otherwise 

 during the formation of the caldera this small accumulation 

 would have been blown out of existence. Thus its age can be 

 assigned as not earlier than the latest phase of eruption. 



Lastly, the presence of other dykes piercing this rock 

 proves that the convulsion by which the large dyke was 

 formed was not the last effort of the declining volcano, but 

 force still resided in it sufficient to crack the surrounding rock, 

 and volcanic magma was still present in sufficient quantity 

 and under sufficient pressure to fill these cracks up to the level 

 of the surface, and thus form dykes. 



The age, then, of the system must be stated as younger than 

 the most violent paroxysms of the central volcano, but in all 

 probability older than the Mount Herbert system, for these 

 lavas are in no place seen to be pierced by dykes, and the 

 eruption was therefore subsequent to the convulsions of the 

 central crater. 



The presence of sea-worn caves even near the top of the 

 hill does not help us much, as it has been shown by Professor 

 Hutton and others that within the Pliocene period great oscil- 

 lations of level have taken place. 



In macroscopic appearance the trachyte previously men- 

 tioned resembles a rhyolite in its very light colour, but no 

 quartz crystals can be seen. 



The colour is almost white in places, but generally iron- 

 oxide has segregated in cracks owing to weathering, thus 

 giving it a banded and sometimes almost spherulitic appear- 

 ance. 



Large crystals of plagioclase can be distinguished, the 

 striation often being visible with a simple lens. The rock is 

 generally vesicular, and it is in these vesicles that glass-clear 

 tridymite crystals are seen, and frequently appear to have a 

 hexagonal outline. 



The texture is porphyritic, the phenocysts being invari- 

 ably feldspars. 



There are two well-developed divisional planes in the rock, 

 one being parallel to its surface and the other parallel to the 

 direction of flow, showing that the cooling proceeded from the 

 surface as well as from the sides. 



No macroscopical difference can be seen between the rock 

 on the summit of the hill and that on the sea-level, except 



