52 'ri'ansactions. 



Art. XII. — On a Hornblende-andesite from the Solander Islands. 

 By R. Speight, M.A., B.Sc, F.G.S. 



\Read before the Philosophical InMitute of Canterburu, 4ih November, 1908.] 



The Solanders consist of a very small group of rocky islets, the largest 

 about a mile long and 1,100 ft. high, lying to the south-west of New Zealand. 

 Their exact position is twenty nautical miles south of the south coast of 

 western Otago and thirty-five miles W. 15° N. of the north-west point of 

 Stewart Island. They are thus to the south of the western entrance of 

 Foveaux Strait. Between them and the mainland the sea reaches a depth 

 of nearly 100 fathoms, as appears from soundings on the Admiralty charts. 

 There are a number of " rises " in the western floor of the strait, but these 

 do not reach so near the surface as to be any danger to navigation ; how- 

 ever, a wide fringe of submerged and partially submerged reefs extends 

 along the northern shore of the strait. The depth of the sea between the 

 Solanders and Stewart Island is over 70 fathoms. The islands, therefore, 

 lie in somewhat deep water. From their isolated position, and on account 

 of the danger of landing except in the very calmest weather, they are rarely 

 visited now, although sealers frequently landed on them in the early days. 

 They have usually been supposed to be a remnant formed of rocks similar 

 to those on the neighbouring coasts, but this turns out not to be the 

 case. 



When the G.S.S. " Hinemoa " was searching for traces of the lost ship 

 " Loch Lomond," Captain BoUons landed and brought back two small 

 specimens of the rock of the islands, and transmitted them to me through 

 Dr. Cockayne. 



Instead of being a granite or gneissic rock, they are a hornblende- 

 andesite, quite different from any hornblende-andesite that I have met 

 with in New Zealand. Macroscopically they show fairly large crystals of 

 feldspar up to 1 cm. in length, and smaller hornblendes 0'75 cm. in length, 

 in a dark groundmass. The rock is somewhat vesicular, and has a low 

 specific gravity — viz., 2*42. When examined under the microscope the 

 phenocrysts are feldspar (labradorite), with the usual characters. There 

 is also an occasional mica flake and much brown hornblende in large 

 crystals, with marked pleochroism. These crystals show idiomorphic out- 

 lines at times, but they grade down through all sizes till they form irregular 

 fragments and flakes. Some take lath- and needle-shaped forms, with 

 parting across. The groundmass is of the hyalopilitic type, with much 

 brownish glass and extremely small microlites of feldspar (ohgoclase-ande- 

 sine), and of hornblende similar to the phenocrysts, the whole showing 

 at times rough flow-structure. The rock shows little sign of alteration, 

 and is as fresh in appearance as many of the Ruapehu andesites. It is 

 therefore likely to be of fairly recent date, most probably late Tertiary. 



This occurrence of volcanic rocks is also interesting because no similar 

 type has been recorded from Otago. Dr. Marshall tells me that he saw 

 loose blocks of hornblende-andesite at Cuttle Cove, Preservation Inlet, 

 but was unable to trace them to their source. Dr. Cockayne also says 

 that fragments of scoriaceous and pumiceous rocks are frequently picked 



