HuTTON AND KiBK. — Description of Arid Island. 109 



been killed off by tbe rats. Tbe other birds that we saw on the island were 

 the Tiii (Prosthemadera novce-zealandiod) , the Bell-bird {Antliornis onelami^rd) , 

 the Fantail {Bhipidura flabellifera), the Ground Lark (AntJms novcs-zea- 

 landicd), the New Zealand Titmouse (Gerthipanis novce-zealandics) , the little 

 Miromiro {Petroica toitoi), and the Pigeon {Garpophaga novcs-zealandice) . 

 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 volcanic 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 subaerial 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 trachytic 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 vdth lava streams. To find these latter we must go to the 

 adjacent coast of the G-reat 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 volcanoes connected 

 with a central fluid interior of the earth, or are their lavas derived from 

 comparatively shallow depths below the surface ? 



