Haast,—On the Geological Structure of Banks Peninsula, 499 
which towards the centre have the greatest bulk, and are very stony and 
compact, become now gradually more and more numerous, but of smaller 
size and more porphyritic or scoriaceous, according to the laws by which 
the flow, dimensions and cooling of the lava-streams are regulated. It is, 
moreover, evident that many of them, owing to want of material, scarcely 
reach half way down the slopes of the caldera wall, that others rapidly thin 
out, and that many which, for some distance after flowing over the lip of 
the crater, had been of large dimensions and stony, become, long before its 
outer edge is reached, thin and scoriaceous, so that here streams of five feet 
in thickness are not uncommon. Although the tunnel does not offer us the 
necessary data to judge of the breadth of the lava-streams, we have for that 
purpose ample evidence in Godley Heads, the sea-wall near Sumner, and 
many other localities. There are streams which are 500 feet broad, others 
only 30 to 40, but all without exception are somewhat scoriaceous on the 
bottom, where the lava flowing over cold ground cooled more rapidly. In 
many instances this is well exhibited by the existence of a small bed of 
laterite, a brick-red coloured rock, sometimes only a few inches thick, which 
doubtless was a layer of soil on the decomposed upper portion of the lava- 
stream or agglomerate bed exposed for a considerable time to atmospheric 
action before the new eruption took place. The lava in the larger streams, 
and in its central portion principally, very stony and of a blackish colour, 
gradually becomes, as we approach the surface, more porphyritic, with a 
more open texture, and assumes pinkish or lilac tints, till it changes into 
scorie. The decomposition or alteration is here often so great that it is 
impossible to trace the top of the line of contact between the surface of the 
stream and the bottom of the overlying bed, both forming a layer of coarse 
agglomerate. In other instances the rough, uneven scoriaceous surface of 
the lava-streams has been well preserved, the hollow spaces being filled up 
by ashes and ejecta, in which case they resemble many of the recent lava- 
streams which I examined in Mount Vesuvius and Mount Etna shortly 
after they had issued from the crater. 
The lava of which the caldera wall under consideration has been built 
up, consists of basic rocks, changing from a dolerite to a fine-grained basalt. 
Some of the lava-streams, however, as previously pointed out, show also a 
remarkable difference in the structure of the rock of which they are com- 
posed, the central portion being a compact basalt with a few crystals of 
augite, basaltic hornblende, labradorite, whilst the upper portion consists 
of a lighter coloured porphyritic dolerite, sometimes so replete with good 
sized crystals of labradorite that the greater portion of the rock is formed 
of that mineral, 
