144 Proceedings of the British Association. 



the porphyry at the greatest depths to which the rock has been 

 penetrated by wells or exposed in ravines. Near the south-east ex- 

 tremity of Norfolk Island are extensive beds of sand and limestone 

 resting on the porphyry ; the limestone, which is the lowest forma- 

 tion, is from 12 to 20 feet thick, and occupies about 20 acres of com- 

 paratively flat land ; in two places it has been fractured, and up- 

 heaved from an angle of 10° to absplute verticality. It is thin- 

 bedded, the laminae being usually one to three inches thick, of fine 

 quality, slightly mixed with sand, but yielding 90 per cent, of lime. 

 The sandstone appears to be entirely a modern formation, lying 

 upon and against the dislocated limestone ; the bar and projecting 

 rocks along the whole south-east front are composed of it, but it is 

 no where above 6 feet thick ; below it is found an unctuous black 

 clay full of vegetable remains, especially the leaves and seeds of 

 pines and other island trees. The sandstone is only compact on 

 the coast, where it is still forming; it contains marine shells, and 

 incrusts the boulders of greenstone on the coast. Being porous, 

 and filled with saline particles, it forms a bad building stone, the 

 houses built of it requiring to be rough-cast with lime. Opposite 

 the settlement which is placed on these beds, and about 600 yards 

 from the beach, Nepean Island rises to the height of 50 feet; it is 

 about a quarter of a mile long, and of a horse- shoe shape, open to 

 the east. The limestone of which this island is composed is used 

 for the shafts of chimneys ; its east and south-east beach is formed 

 of sandstone. No water has been found in it, and its vegetation has, 

 within the last few years, almost disappeared, owing to a colony of 

 rabbits which, having destroyed every thing edible, have now them- 

 selves perished. It is reported that in 1793 this island was only a 

 boat's length distant from Norfolk Island, but that in 1797 two 

 severe earthquake shocks were experienced, by the second of which 

 the nearer point of the Nepean was submerged, and the channel 

 altered to its present form. The rocks which pave the channel be- 

 tween these island are almost all limestones, whilst elsewhere they 

 are porphyritic. The Bird Islands are rocks of porphyry distributed 

 along the north shore of Norfolk Island ; they are of no economic 

 value, and are tenanted only by sea-birds. [See Statistical Section, 

 Monday.] 





