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



the chain of hills visible from the Challenger's anchorage. The specimens which we 

 examined must have been collected as fragments, as their contours are rounded. They 

 are greyish in colour, somewhat coarse-grained, contain augite and felspar visible to the 

 naked eye, also many zeolites, and greenish specks of a secondary substance, probably 

 delessite. Microscopical examination confirms the macroscopical determination of this 

 rock as a coarse-grained dolerite. The plagioclase is transformed into chalcedony and 

 micaceous matter. The augite is purplish and without crystallographic outlines. 

 Titaniferous iron is very abundant, appearing in the preparations as elongated or 

 irregular rods. Olivine seems to have almost entirely disappeared, hardly any trace 

 of it remaining. In the cracks of the rock colourless patches are to be seen which give 

 scarcely sensible chromatic polarisation, and are obviously of zeolitic nature. These 

 zeolites are usually framed by a zone of delessite which lines the cavities with a 

 mammillated coating. Hematite is also a somewhat common mineral. 



Fine-grained basalts were also found on the summits of these hills. These are black 

 and compact, and crystals of augite, plagioclase, and olivine may be distinguished by 

 the lens. Microscopically the rock appears to be a felspathic basalt, the ground-mass 

 being made up of microliths of felspar, grains of augite, and magnetite. In this 

 there appear large sections of olivine and augite, and broad lamellae of much altered 

 plagioclase. A second specimen of fine-grained basalt from the crest of the hills of 

 Howe's Island shows a composition analagous to that described, only the microporphyritic 

 element is almost exclusively plagioclase. 



The basalts just enumerated are traversed by a dyke of bluish black rock, in parts 

 vesicular, and of medium grain. Examined with a lens it shows augite, plagioclase, 

 and olivine entirely transformed into an almost earthy serpentinous mass with a 

 slightly greasy lustre. The microscope shows the dyke to be composed of a felspathic 

 basalt, resembling all those of the island which we have examined. The ground-mass 

 is made up of small plagioclastic lamellae, microliths of augite, and crystallites of 

 magnetite. Large sections of plagioclase, giving the extinctions of anorthite, appear 

 in the mass. This plagioclase is finely striated, and is sometimes twinned according to 

 the Baveno or pericline law ; at other times it is zonary, and very rich in brownish 

 vitreous inclusions. The sections of magnetite sometimes attain pretty large dimensions, 

 and, with the augite, determine the microporphyritic structure. 



We have mentioned that the summits of the hills of Howe's Island are strewed with 

 geodes of agate. Mr. Buchanan observed that these nodules, derived from decomposed 

 amygdaloidal rocks, are often worn on a part of their surface, as if they had been 

 planed, while in other cases they are covered with very sharp striae. The planing of 

 part of the surface may be looked upon as the result of glacial action. As we shall see 

 farther on, this action must have been formerly exerted at Kerguelen on a far larger 

 scale than is the case at present. 



