378 Transactions. 



From a consideration of their average inclination, especially in the case 

 of those recorded by Haast from the Lyttelton Tunnel, some estimate may be 

 arrived at as to the former height of the Lyttelton volcano. The average 

 inclination of the flows in the tunnel is almost exactly 15°, and this value 

 corresponds with that obtained from observations of prominent flows exposed 

 on the sides of valleys eroded deeply into the flanks of the volcano. Taking 

 this value as approximately correct, and also the distance of the outer fringe 

 of the hills from the centre of the volcano as approximately six miles, the 

 height of the cone must have approached 8,000 ft. ; and if we make due 

 allowance for a probable greater inclination of the flows and an increased 

 thickness near the vent, and also for the depression of the land which has. 

 occurred since volcanic activity waned, it is possible that the cone approached, 

 if it did not actually exceed, 10,000 ft. in height. 



The present form of the cone is no doubt entirely different from that which 

 it presented at the close of this volcanic phase. Instead of the usual 

 moderate-sized crater at the top there is now a great hollow, and evidence 

 suggests that this form had developed to some extent before the next 

 phase in the volcanic history began. The actual cause of the formation 

 of these vast cavities will be dealt with later, but the following three 

 working hypotheses have been put forward to account for them : — 



(i.) They have been formed by explosion or collapse of the cone, the 



former indicating a revival of volcanic activity, 

 (ii.) The form is due to peripheral faulting causing subsidence of the 

 original crater. 



(iii.) They have been eroded by the action of streams, or by the sea, 

 or more probably, in some cases at all events, by a combination 

 of both processes. 



The Dyke System (see fig. 3). — A most striking feature in connection 

 with this volcanic cone is the dyke system. The great majority of these 

 intrusions are of trachyte, decidedly alkaline in composition, and frequently 

 containing as a consequence alkaline augites and hornblendes, though 

 common hornblende trachytes also occur, as well as ordinary fine- and coarse- 

 grained basalts and andesitic basalts. Many of the trachyte dykes are very 

 massive, such as those on the Lyttelton-Sumner Road, notably the tridymite 

 trachyte ; dykes near the head of Heathcote Valley ; the great dyke at 

 Rapaki ; those near Kennedy's Bush ; and on Dyke Hill, near Kaituna 

 Pass — the last four of which are each approximately 60 ft. in thickness. 

 There are besides many others of smaller size, so that the volume of intrusive 

 trachyte as deduced from exposures at the surface exceeds in volume all 

 other dykes. There does not appear to be any definite grouping of the 

 trachytic and basic dykes into pairs so that one might be assumed to be 

 complementary to the other, therefore any explanation of their different 

 chemical composition on the basis of a divergence from a common magma 

 requires that the differentiation took place at some depth and not near the 

 surface of the volcano. There is, however, a general tendency for groups 

 of trachytic or groups of basic dykes to occur in particular localities. For 

 example, along the Lyttelton-Sumner Road there is a special occurrence of 

 trachytes ; the same remark applies to Heathcote Valley, near the Bridle 

 Path ; then, again, there are the large trachyte dykes at the head of the 

 Rapaki Valley, and numerous smaller ones near Victoria Park and near 

 Kennedy's Bush. Basaltic dykes occur freely to the west of Dyer's Pass 

 Road, near Cooper's Knob, and in places near McQueen's Pass in greywacke 

 and rhyolite, forming well-defined groups; but, all the same, they are 



