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



we have examined, belong to the felspathic basalts ; the various specimens differ only 

 in colour, or in the more or less vesicular texture. We will describe first the rocks 

 formina- the volcanic cones near the small stream already mentioned. Amongst these, 

 red or black scorise are the most frequent. Their surface is very vesicular, the interior 

 part more compact, and having a somewhat waxy lustre. With the naked eye, crystals 

 and grains of olivine are seen scattered through the rock. Microscopical examination 

 shows a vitreous fundamental mass, with lamellae of plagioclase, the extinctions of 

 which are about 40°, indicating a mixture near anorthite. There are large sections of 

 olivine without any noteworthy peculiarity, the characteristics of this mineral being 

 those which it generally presents in the basaltic rocks. These sections show a perfect 

 cleavage following the base, and are often crowded with trichites. What seems to 

 characterise the crystals of augite is that they very often occur in groups of several 

 individuals, joined with their vertical axes ; this is one of the most striking peculiarities 

 of this mineral in the rock under description. Magnetite is present here as in all the 

 specimens from Marion Island. The base is speckled with globulites and trichites ; 

 this vitreous matter is often partially decomposed into a brownish palagonitic 

 matter. 



The black lava forming the bed of the little stream explored by Mr. Buchanan is 

 generally compact in some places, however vesicular ; its grain is that of dolerite. This 

 rock is spotted with white points, and contains macroscopic olivine. Under the microscope 

 it shows the structure and the composition of a felspathic basalt, and resembles in every 

 particular the rocks already described. Augite is present only in very small grains, 

 which are not always easily distinguished from olivine. However, the crystals of this 

 last mineral, even when very small, contain almost always vitreous inclusions of 

 hexagonal or rhombic shape, their outhnes being parallel to those of the section ; these 

 regular inclusions are not to be observed in the small sections of augite. 



A rock labelled "recent lava" has the same macroscopic characters as that just 

 described, but contains even less augite than the preceding specimen. There must be 

 some augitic microliths in the ground-mass, but it is difficult to give any definite 

 determination on account of the opacity of the base. The plagioclases are lamellar, 

 and extinguish under large angles. Very often these plagioclase crystals surround the 

 olivine sections, and are parallel to the outlines of the latter. Olivine does not show 

 the prismatic faces ; the sections are always rhombic. 



A volcanic bomb collected near the conical hills already mentioned is 10 centimetres 

 by 5, its shape being elliptical ; this bomb is reddish brown, rather compact. With the 

 naked eye crystals of olivine and augite are seen embedded in the ground-mass. 

 Microscopical examination shows that this rock is a felspathic basalt. In a brownish 

 base are embedded crystals of plagioclase, olivine, and augite. These minerals are 

 almost always porphyritic; microliths of felspar and of augite are hidden in the ground- 



