328 Geology of 



were mostly o£ submarine origin ; but further and more detailed search 

 has since proved, that although there are a few beds, which might 

 be of marine, at least all the older portion is of subserial origin. The 

 boulders and pebbles in question have either been rounded by attrition 

 during their ejection, or they have been rolled in brooks descending 

 along the slopes of high volcanic ridges, which were mostly destroyed 

 before or during the formation of the Mount Herbert system. More- 

 over, no marine shells or exuvias of other marine animals have ever 

 been discovered in the tufaceous deposits interstratified with the lava- 

 streams ; whilst the existence of timber which has assumed the character 

 of an altered coal, and is obtained in these deposits near Akaroa, 

 points, from the manner in which it occurs, to a land origin, which, 

 moreover, is indicated by numerous beds of laterite, representing, 

 doubtless, ancient soils. 



The oldest crater, of which the principal boundaries can be traced 

 at the present time, is the Lyttelton Harbour caldera, having a 

 general diameter of about two miles, the centre of which is situated a 

 little to the south of Quail Island. The general structure of this crater, 

 even before the Christchurch and Lyttelton Railway tunnel was 

 entirely pierced through, could easily be made out by studying the 

 numerous sections exposed in many directions, and by ascending 

 the steep escarpments of the caldera wall, where a succession of 

 streams of stony or scoriaceous lava, interstratified with beds of 

 agglomerates, ashes, tufas, and laterites can be traced to the 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 was obtained in the railway tunnel passing through the caldera 

 wall, and of which, as the work gradually advanced, I prepared a 

 careful section. The main results of this survey, together with a 

 description of some portions of the tunnel, of which sections in 

 chromo-lithography have been added, will be offered at the end of this 

 chapter. The succession and dip of the lava-streams and the inter- 

 vening 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 in the average. In 

 the tunnel the dip is greater, an inclination of twenty degrees not 



