110 Transactions. 
It is evident that 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 
Great 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 Great 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. 
Now 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 Great 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 
thiekness of the erust broken or sunk down was considerably thinner than 
the diameter of the cavity. For 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. 
BOTANY. 
