Haast,— Ow the Geological Structure of Banks Peninsula, 499 



•wlilcli towards the centre have the greatest biUk, and are very stony and 

 compact, become now gradually more and more numerous, but of smaller 

 size and more porpbyritic or scoriaceous, according to the laws by wliich 

 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 porpbyritic, with a 

 more open texture, and assumes pinkish or lilac tints, till it changes into 

 scoriffi. 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 porpbyritic dolerite, sometimes so replete with good 

 sized crystals of labradorite that the greater portion of the rock is formed 

 pf that mineral. 



