Haast.—On the Geological Structure of Banks Peninsula. 511 
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 defined. 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 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 voleanic 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 spherosiderite, 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 spherosiderite, 
on which crystals of calcareous spar have been deposited. At 403 chains 
on the same side, in a bed of laterite, four feet above the floor of the tunnel, - 
n small spring was struck, drying up a few months after; 353 chains from : pu 
