Canterbury and Westland. 35$ 



their upper portion, the "bottom of the streams, owing to their thinners 

 and to the distance from the centre of eruption, not having been able 

 to cool to the stony compact form. I may, however, observe that the 

 boundary line between both kinds of rock is in many instances very 

 distinct and clearly denned. These scoriaceous beds occur throughout 

 the tunnel ; they are sometimes of considerable dimensions, some of 

 them being over 100 feet thick. In speaking of the formation of 

 the Lyttelton caldera, I have already pointed out that it has been 

 built up by volcanic rocks belo aging to two distinct divisions, of which 

 the basic rocks have furnished all the material for the lava-streams, 

 agglomerate and tufaceous beds, whilst the principal portion of the 

 dykes owe their origin to the acidic division. As might be anticipated, 

 the dykes are most numerous near the focus of eruption, thus we find 

 the greater portion of them near the Lyttelton side, several of them 

 not reaching to the roof of the tunnel. Of these dykes, jNTo. 29 is the 

 most important. It consists of a soft flaky and lustrous trachyte, and 

 possesses, like most of the other acidic dykes, the characteristic feature 

 that it is accompanied on both sides by a selvage of tachylite, some- 

 time two or three inches thick. This change in the character of the 

 bed rock is especially visible when the dykes pass through agglo- 

 meratic or tufaceous beds. It shows clearly that the volcanic matter 

 ascending by these fissures was in such an intense state of fusion that 

 it was able to alter the rocks on both sides so thoroughly for such a 

 distance. In some instances the dyke rocks themselves have a selvage 

 of tachylite, the bed rock being unaltered. It is worthy of notice 

 that the basaltic dykes have not produced the same effect, the rocks on 

 "both sides being generally unaltered. Large beds of loess, similar to 

 those deposited on the inner side of the caldera wall have also been 

 passed through on the Heathcote side. Of minerals of secondary 

 origin found in the tunnel, the most diffuse is sphasrosiderite which 

 usually coats the pores and cavities of scoriaceous lavas. Of others, 

 calcareous spar and aragonite are the most conspicuous. The latter 

 is younger than the former, having often been deposited upon the 

 surface of the calcareous spar in the small geodes. In a few localities, 

 hyalite fills small clefts, or is found in a stalactitic form. 



I shall close this chapter by offering a few observations on some 

 other physical features of the beds through which the tunnel has been 

 excavated, and as I noted them on the large section during the survey, 

 Forty chains from the Heathcote end, a scoriaceous lava-stream fifteen 

 feet thick, and accompanied on both sides by beds of laterite and 



