372 Transactions. — Geology. 



tory answer, but may perhaps venture to put forward two 

 theories, at the same time acknowledging that, in the absence 

 of actual proofs in their support, they afford but a possible 

 explanation. One is that these inclusions are fragments of 

 silicified wood which have been caught up by the lava in its 

 ascent, fused, and ejected with the scori£e. This idea sug- 

 gested itself to me on finding a specimen in which the pene- 

 trating veins of vitreous or semi-vitreous lava had taken the 

 form of thin parallel plates and threads bearing some resem- 

 blance to fibres, as if the lava had been intruded into cracks 

 or fissures formed in the direction of the grain of the original 

 wood. The other theory is that this silica represents fragments 

 of diatom earth, deposits of which occur near Auckland — as, 

 for instance, at Mount Albert. Whatever their origin, the 

 occurrence of fragments of free quartz in so basic a rock as the 

 Auckland basalt is very remarkable. 



The lava appears to have flowed rapidly — to have been, in 

 fact, fairly liquid, certainly not very viscid — for the streams 

 present, for the most part, a rough, clinkery surface, strewn 

 with loose blocks and fragments, and not, except in a few 

 places, the smooth, " ropy " surface exhibited by slowly- 

 moving lava. The surface of Eangitoto Island, for instance, 

 is exceedingly rough and uneven, being broken up by deep 

 clefts into blocks, some of them of enormous size, which are 

 covered with jagged, cindery projections. Another proof of 

 the liquidity of the lava, and therefore of the large amount of 

 steam given off from its surface while cooling, is afforded by 

 the presence on Eangitoto of small cones — miniature volcanoes 

 — of lava, thrown up on the streams, just as was the case with 

 the extremely liquid lava erupted by Vesuvius in 1872. The 

 small thickness of the streams in proportion to their length 

 and breadth, also, I think, points in the same direction. And, 

 indeed, we can well understand this lava to have been 

 thoroughly liquid, owing to its very basic composition. 



As regards its physical character, the lava from all the 

 points of eruption is very similar in general appearance, in 

 structure, and in composition ; and, though varieties of micro- 

 scopic structure are to be found in it, yet no variety charac- 

 terizes the lava from any particular hill ; on the contrary, the 

 different kinds of structure can be seen in different parts of one 

 and the same lava-stream, and are, in fact, merely the results 

 of different conditions and circumstances of cooling. The 

 texture, of course, varies according to the depth at which the 

 rock solidified, being vesicular and porphyritic near the surface 

 of the streams, becoming more granular and microscopically 

 holocrystalline the further it is from the surface of cooling. 



The olivine occurs as small crystals in the groundmass of 

 the rock, but far more abundantly as porphyritic crystals or 



