Haast. — On the Geological Structure of Banks Peninsula. 511 



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

 tliat the boundary hne between both kinds of roch is, in many instances, 

 very distinct and clearly defined. These scoriacoous 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 belonging to two distinct divisions, of which the basic rocks have 

 furnished all the material for the lava-streams, agglomeratic 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 Lyt- 

 telton side, several of them not reaching to the roof of the tunnel. Of these 

 dykes, No. 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, sometimes two or three inches thick. This change in the charac- 

 ter of the bed rock is especially visible when the dykes pass through 

 agglomeratic 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 sphferosiderite, 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 on the 

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

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



I shall close this address 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 agglomerate, was passed, 

 which was so loose and full of water that the ground had at once to be 

 heavily timbered. All the cavities in the lava are lined with sphcerosiderite, 

 on which crystals of calcareous spar have been deposited. At 40f chains 

 on the same side, in a bed of laterite, four feet above the floor of the tunnel, 

 a small spring was struck, drying up a few months after ; 35^ chains froiii 



