128 THE VOYAGE OF H.M.S. CHALLENGER. 



were collected by the German expedition and examined by Professor Roth. He reports 

 amygdaloidal doleritic basalts with geodes of quartz as the chief rocks of this island, which 

 rises about 400 feet above the sea. Trachytic rocks covered with a brownish altered layer 

 occur on the summit. In the ground-mass of this trachyte there are crystals of sanidine 

 reaching 1 5 millimetres in diameter ; crystals of a shining triclinic felspar also occur, but 

 these are rarer, and, finally, there is augite, without crystallographic outlines. The 

 microscope also shows magnetic iron and some lamellae of mica. A trachyte resembling 

 that of Kiihlsbrunn is found in the same island. It is a greyish rock, of a scaly grain 

 and slightly slaty. Under the microscope, isolated brown crystals of hornblende appear. 

 Professor Roth could not recognise with certainty the presence of triclinic felspar. 



Mr. Buchanan collected from the rocks cropping out near the shores of Royal Sound 

 several specimens of amygdaloidal dolerites, the vesicles being filled with zeolites. One 

 of these much altered rocks has large crystalline grains, and is penetrated with a great 

 number of fibro-radial zeolites, and with limonite. Microscopically it shows the 

 doleritic structure, but this is not developed here in a very characteristic manner. The 

 crystals of olivine have too sharp crystallographic outlines ; they lead rather to a transi- 

 tion of the doleritic structure to that of the basalts, properly so called. In thin slices 

 of this rock the microscope shows large plagioclastic lamellae, between which grains of 

 augite are embedded. The olivine is impregnated with hematite, and sometimes trans- 

 formed in the interior into a fibrous matter like serpentine. In certain cases the augite 

 appears in large sections, generally much altered and charged with iron. There are 

 numerous rods of magnetite or ilmenite, and calcite is much developed in the vesicles, 

 where it is associated with zeolites. 



Other rocks from the same district are identical with the preceding. We may, 

 however, add to the foregoing description that microscopic examination shows the 

 regular association of plagioclase and augite, the former being united to the pyroxene 

 parallel to one of the pinacoids. Small yellowish transparent rods also appear, some- 

 times arranged in parallel series, and recalling the form and grouping of magnetite 

 trichites. These little rods are entirely transformed into limonite, but the larger sections 

 of magnetic iron have not been affected in this way except a little on the edges. 



Finally, at Royal Sound greenish yellow light scoriaceous rocks are found, almost 

 earthy from alteration. The only mineral to be seen is augite in large black crystals, 

 which stand out from the decomposed rock. Thin slices show that it is formed of 

 a green basaltic glass full of bubbles and partly decomposed into palagonite. This 

 vitreous matter is stretched out in filaments, and passes in some places from brown 

 to yellow. Its structure is sometimes as fibrous as that of pumice. The vesicles 

 are not filled with zeolites, but limonite is found almost everywhere in the pre- 

 parations. Besides the very numerous crystals of magnetite, there are sections of 



