Hurron anp Kirx.—Deseription of Arid Island. 109 
been killed off by the rats. The other birds that we saw on the island were 
the Tui (Prosthemadera nove-zealandia), the Bell-bird (Anthornis melanura), 
the Fantail (Rhipidura Jlabellifera), the Ground Lark (Anthus nove-zea- 
landie), the New Zealand Titmouse (Certhiparus nove-zealandie), the little 
Miromiro (Petroica toitoi), and the Pigeon (Carpophaga nove-zealandie). 
The greater part of the island is surrounded by high precipitous cliffs; the 
harbour, one point on the eastern, and possibly another on the northern side, 
being the only places from whence the island seems to be accessible. On 
the north and south the sea has eaten back the cliffs into mere ridges, only a 
foot or two wide in some places. 
GEOLOGY. 
The high ridge that surrounds the depressed interior of the island pro- 
claims at once that it is the summit of an old voleanic cone, and an examina- 
tion of the rocks confirms the supposition, and shows that it belongs to the 
trachytic class of volcanoes. The shape of the crater is singular, its length 
being more than twice its breadth, and the northern part being divided into 
two by a ridge running from the edge of the crater towards the centre of 
the island, and directed nearly to the boat harbour, or that point where the 
wall of the crater is lowest. This configuration is probably owing to there 
having been two craters, the southernmost of which was the last in activity, 
and filled up the northern one with ashes; and the two valleys have been 
subsequently scooped out by subaérial denudation. The direction of the 
dividing ridge, and the termination of the north-western valley in a narrow 
gorge, prove the correctness of this view. 
Nearly the whole of the island is composed of trachytie tuffs and 
breccia, generally either white or of a pale yellow or violet colour, and 
enclosing here and there fragments of trachyte and obsidian. These tuffs 
are arranged in the cliffs that formed the wall of the crater, more or less 
horizontally, although of course much confused in places, and are but little 
intermixed with lava streams. To find these latter we must go to the 
adjacent coast of the Great Barrier, about three miles distant, where, at the 
south side of Wangapoua Bay, we see thick beds of trachyte and trachy- 
dolerite, interstratified with tufa, dipping away from Arid Island at an angle 
of 35? ; and farther inland, on the top of the dividing ridge of the island, we 
find tufa and agglomerate, most probably derived from Arid Island, at an 
elevation of 1,550 feet from the sea level. 
Now these facts lead to some interesting deductions, which bear on one 
of the great questions of the day, in geology, viz., Are voleanoes connected 
with a central fluid interior of the earth, or are their lavas derived from 
eomparatively shallow depths below the surface ? 
