334 Geology of 



towards Quail Island is the younger. Close to the end of the 

 Peninsula, the crossing point of three dykes of considerable size 

 is reached, the oldest pointing to the centre of the bay, the second 

 towards Kaupaki, and the third or youngest again towards Quail 

 Island. At the extremity of the same Peninsula there are such a 

 number of dykes intersecting each other in all directions, that it is 

 impossible to trace their relations in detail, without devoting consi- 

 derable time to it. However, it is evident that not far from this 

 spot all the principal centres of eruption seem to have been situated. 

 It is here also that some of the dykes divide into several branches, 

 and others anastomose repeatedly. The most striking fact in 

 connection with the system of dykes of the Lyttelton caldera, 

 and to which I have devoted considerable attention is their size, 

 longitudinal extent, and constancy in direction. Prom the researches 

 of numerous observers, it has been proved that all the dykes of 

 Mount Vesuvius and Mount Etna do not extend much beyond the 

 centres of eruption, so that they advance only a short distance, and, 

 rapidly thinning out, soon disappear, a fact which my dwn observa- 

 tions along the crater walls of both mountains have amply confirmed. 

 However, I have no doubt that other volcanoes similar in construction 

 to Banks' Peninsula, and differing as considerably from these two 

 European volcanic mountains, will be found to possess their systems of 

 dykes developed in the same manner. During a number of years, it has 

 been well ascertained by me that the dykes radiating from the several 

 centres of eruption situated not far from each other, continue in many 

 instances without notable interruption from the former mouth of the 

 crater to the outer slopes of the caldera, where they disappear below 

 the sea, or under the deposits now forming the Canterbury plains. Very 

 often the principal dykes rise nearly 2000 feet above the sea level. 

 They are well visible from the harbour to the summit of the rim of the 

 caldera wall, above which, in some instances, they stand prominently as 

 a wall, often six or eight feet high. "Where proper measurements of 

 the same dyke can be obtained for a long distance, it has been found 

 that generally, as it advances towards the outer circle, it diminishes in 

 breadth ; however, in other instances, this is not the case, as repeatedly 

 I have found some which, after narrowing on their outward course, 

 considerably enlarge again before reaching the foot of the caldera. Thus 

 to give a few examples, the large dyke of trachyte, which is crossed in 

 the railway tunnel, about 29 chains from the Heathcote end, is first seen 

 west of the town of Lyttelton, near [Naval Point, where it is nearly 

 40 feet thick. On the summit of the caldera wall, not far from the 



