738 SUBANTARCTIC ISLANDS OF NEW ZEALAND. [Physiography, Geology. 



the Musgrave Peninsula older basic series. Tliey are all dark-coloured in the hand- 

 specimen, some appearing coarse and others very fine grained, the latter containing 

 at times a marked amount of pyrites. The microscope shows that one of the coarser 

 specimens is principally composed of even-sized deeply kaolinized feldspar (labra- 

 dorite) laths, with biotite and chlorite probably derived from interstitial augite ; 

 there is no apparent sign of ophitic structure. Much apatite occurs in the form of 

 needles, and much ilmenite in quadratic grains and skeleton crystals. Another 

 specimen of rock is a diabase-porphyrite, with large phenocrysts of labradorite and 

 purplish augite. The other rocks appear to be melaphyres, and are much decomposed. 

 The dykes are white, very hard, and somewhat flinty in appearance. Under 

 the microscope they prove to be much more acid than any rock found on the main- 

 land, and must be classified as rhyolites. The phenocrysts are of highly weathered 

 untwinned feldspar and much quartz in irregular fragments. These are contained 

 in a felsitic groundmass. A small amount of glass is visible under higher powers, 

 but the sections are obscured by the alteration of the constituents. Minute zircons 

 are occasionally seen, and, in addition, very small flakes of a mineral pleochroic in 

 greenish-yellow tints and small extinction-angle measured from the cleavage, in all 

 probability an alkaline hornblende. The iron- ores assume mossy forms, and are 

 no doubt wholly or partly derived from a hornblende or related mineral. These 

 dykes may be connected genetically with the mainland trachytes, which contain 

 a small quantity of free quartz, and are of a distinctly acid variety ; but the textures 

 of the rocks are quite diflerent. 



BOUNTY ISLANDS. 



The Bounty Islands are a group of small islets lying in long. 179° E. and lat. 

 47° 43' S. The group takes the shape of a rough semicircle facing north, the length 

 from east to west being about three miles and the width from north to south about 

 two miles. There are nine islets of appreciable size, the largest being about three- 

 quarters of a mile long by half a mile wide. Besides these there are numbers of rocks, 

 and also outlying reefs belonging to the group. The highest point is only 290 ft. 

 above the sea, but several of the islets rise to over 200 ft. They show the marks 

 of severe marine erosion, and are cut into in a remarkable manner ; but there is no 

 bay or indentation which affords even moderate shelter in stormy weather. During 

 calm weather the sea breaks heavily on the weather shore, but in severe storms 

 no part, is beyond the reach of the waves and flying spray. 



The rocks near sea-level are worn smooth not only by the action of the breakers, 

 but also by the polishing-action of the feet of the seals and millions of penguins and 

 other sea-birds which make the islands their breeding-place. The general rock- 

 surface is as slippery as glass, and exceedingly difiicult for man to travel over. 

 Immense quantities of guano are deposited on the islands during the breeding season, 

 but during winter storms it is swept ofl^, with the exception of that which accumulates 

 between the boulders. A little brackish water impregnated with guano collects 

 in hollows on the higher parts of the islands, but there is no regular supply of fresh 

 water, except that afliorded by the almost constant rains. Owing to the unfavourable 

 conditions, only one species of fresh-water alga has managed to maintain a foot- 

 hold on the exposed surface of the rocks, but great masses of seaweed are torn away 



