110 Transactions. 



It is evident tliat Arid Island, in its present condition — only 700 feet 

 above the sea — could not have distributed tufas at an elevation of 1,550 feet, 

 neither could lava streams flowing from it now have, at the same level as the 

 crater, and at a distance of three miles, a dip of 35°. It follows, therefore, 

 that either some other crater, in the direction of Arid Island, and much 

 higher than it, but which has now quite disappeared, was the origin of these 

 lavas and tufas ; or, that Arid Island was once at a considerably higher 

 elevation, not only above the sea, but with respect to the main land of the 

 Grreat Barrier. The depth of the intervening sea, 12 to 17 fathoms, makes 

 the latter supposition much the most probable ; and assuming that it was so, 

 we see that Arid Island must have sunk down at least 2,000 feet below the 

 level of the Grreat Barrier, because the tufa, at a level of 1,550 feet, is 

 evidently a submarine formation ; while the shape and preservation of the 

 crater of Arid Island shows that it was formed under the air, so that the 

 bottom of the crater must have been above the highest level of the tufa. 

 liow it appears most probable that the sinking of Arid Island was produced 

 either by the breaking of the roof of the cavity from which the lava and 

 ashes had been extracted, or by slipping down in mass of that part between 

 it and the Grreat Barrier. If, therefore, we suppose that Arid Island was 

 the centre of the subsided portion, we have a district six miles in diameter, 

 which has broken or sunk through by its weight into a cavity below ; and it 

 appears to be impossible that such should have been the case, unless the 

 thickness of the crust broken or sunk down was considerably thinner than 

 the diameter of the cavity. Eor if not, it would have been strong enough 

 to have resisted the fracture, and the friction along the sides would have 

 been too great to allow it to slip ; so that it seems impossible that the region 

 from which Arid Island derived its lava was so deep as six miles below the 

 surface, and therefore it would be unreasonable to infer that it was con- 

 nected with a fluid interior ; for most astronomers and geologists are now 

 pretty well agreed that the solid crust of the globe is at least a thousand 

 miles thick. 



BOTAITY. 



Our visit to Arid Island was not made under favourable conditions for 

 the investigation of its botany, most of the vegetation of the open land 

 having been burnt off by a party of Maoris a few days before we landed, 

 and our stay being limited to a few hours by the unsettled state of the 

 weather. 



Although it was impossible to make even an approximate list of the 

 plants of the. island, sufficient was observed to show it possessed a flora 

 which comprised a greater number of forms than could be collected on the 

 islets off the west coast of the Great Barrier, and that the general character 



