REPORT ON THE PETROLOGY OF OCEANIC ISLANDS. 107 



mass. Some crystals of plagioclase are Carlsbad twins : two individuals, tabular 

 following M, are elongated following the edge PjM, the outlines of these sections being 

 the traces of the faces of p and x. The two individuals are joined on the face M, the 

 trace of p of one of the individuals coinciding with the trace of x of the other. Other 

 sections show at the same time twinning following the albite, Carlsbad, and Baveno 

 laws. The extinction, measured from the trace of the polysynthetic lamellae, is about 

 45°, — thus this felspar is a mixture very near anorthite. Large sections of augite are 

 slightly greenish ; they do not present any noteworthy peculiarity. Olivine has rarely 

 crystallographic outlines ; in some cases the sections of this mineral show the traces of 

 a very obtuse and large dome, and the outlines are very like regular hexagons, but 

 generally the sections present a very corroded aspect. 



IX.— NOTES ON THE ROCKS OF KERGUELEN ISLAND. 



These notes on the rocks of Kerguelen Island are intended to be essentially litho- 

 logical, but geological and topographical features will require notice in so far as they 

 throw light upon the lithological description of this volcanic island. We do not 

 require to touch upon the history of early explorations of the island, a history 

 centring round the name of the illustrious navigator Cook, to whom we owe the most 

 exact, but by no means complete, data regarding the island up to the visit of Sir James 

 C. Ross in 1840. The numerous visits of the South Sea whalers added nothing to definite 

 knowledge, and to Ross we owe the first geological observations on the island. 

 MacCormick at the same time devoted himself to the natural history of the region, while 

 the flora was studied by Hooker. 



Sir James Ross landed at Christmas Harbour, explored the neighbouring region, and 

 greatly increased our knowledge of it. On the north-west coast also Hooker and 

 MacCormick made their observations. After this memorable cruise many years passed 

 away before another expedition landed on the island. The Challenger touched there in 

 1874 in order to make arrangements for the British astronomers who were to establish 

 themselves in that locality to observe the transit of Venus. Almost at the same time 

 the " Gazelle " landed the German observing party, who were stationed there for three and 

 a half months for the same purpose. Shortly afterwards the " Volage " arrived with the 

 party of British astronomers under the charge of Father S. J. Perry. 



To this party we owe some observations on the south coast, but to the present day 

 the west coast is unexplored, and the centre of the island almost unknown. This 

 ignorance is due to the difficulties of exploration in the marshes and peat-bogs of the 

 interior, amongst the fogs and snows, the torrents and ice-fields, and the terrible storms 



