498 Transactions. — Geology. 



with beds of agglomerates, ashes, tufas, and laterites can be traced to tbe 

 very summit. Still clearer sections are open to our inspection if we follow 

 the barranco or entrance into the harbour, forming sometimes vertical 

 cliffs of considerable altitude, and where the whole series of beds can easily 

 be followed. However, the most interesting and complete insight wa3 

 obtained in the railway tunnel passing through the caldera wall, and of 

 which, as the work gradually advanced, I prepared a careful section. The 

 succession and dip of the lava streams and the intervening beds can also be 

 made out by following the slopes of the ridges between the deep valleys 

 washed out on the outer side of the crater wall, where it will be found that 

 the lava streams forming the lip of the crater have generally a slighter 

 inclination than those lower down, the dip of the upper ones being only nine 

 degrees on the average. In the tunnel the dip is greater, an inclination of 

 twenty degrees not being uncommon. It is evident that the building up of 

 such a huge system during numerous eruptions, often of great magnitude, 

 could not be accomplished without a great destruction of portions of the 

 beds previously formed taking place, the point of eruption in the crater 

 shifting continuously about the centre. If, at the same time, we examine 

 the lava streams and the interstratified agglomerate and ash beds along the 

 water's edge, we have to come to the conclusion that all the eruptions by 

 which the caldera wall was formed from summit to bottom, occurred under 

 the same physical conditions. 



Examining into the formation of the Lyttelton caldera, and beginning 

 our observations in the harbour, we find that many lava streams have been 

 preserved which have cooled in their ascent ; others lie horizontal for a 

 short distance, and are then seen to descend, conforming to the gradient of 

 the underlying lava streams or agglomerate beds. In many instances we 

 have also clear evidence that considerable destruction of the beds previously 

 formed had taken place before new streams flowed over the lip of the crater, 

 or before beds of ashes, scoriae and lapilli, were deposited anew. The 

 tunnel section in this respect is also very instructive. Thus, in course of 

 time, the great crater wall was formed, rising to an altitude of nearly 2,000 

 feet, and having a diameter of more than five miles at its crest. It is clear 

 that close to the vent, from which scoria and ashes were thrown out in 

 large quantities, the greatest thickness of the agglomerate beds ought to be 

 formed, and this, in fact, is the case, as the largest beds, having sometimes 

 a thickness of several hundred feet, are situated within the inner side of the 

 caldera wall. The lava-^streams here between these agglomerates are irre- 

 gular in their direction, and mostly of small dimensions. The more we 

 advance towards the outer slopes of the caldera wall, the less frequent 

 )jecome these agglomei'atic or tufaceous layers, whilst the lava-streams^ 



