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



A volcanic bomb from Table Mountain is formed of a medium-grained, greenish black 

 rock, reddened on the surface, and furrowed with long hollows full of large crystals of 

 chabasite. Under the microscope it is seen to be formed of a slightly transparent 

 greyish mass speckled with grains of magnetite. This ground-mass, which cannot well 

 be analysed even under the highest powers, has an indistinct structure which may be 

 compared to marbling. Skeletons of felspar forked at both extremities appear in this 

 ground-mass. These plagioclastic sections are sometimes larger, and in that case are 

 almost always cracked in every direction, and appear in parts converted into chalcedony. 

 The olivine is decomposed into serpentine, and augite does not appear to be 

 present. 



A fragment of altered vitreous basalt may be mentioned, finally, amongst the 

 specimens from this locality. The rock has a reddish brown colour, is scoriaceous, and 

 very much decomposed, some parts passing into palagonite, others being almost earthy. 

 The rock is entirely impregnated with iron, and is transformed into palagonitic matter 

 in the last stage of decomposition. The ground-mass is brownish and opaque, filled 

 with colourless microliths of felspar which are aggregated in star-like groups. Like 

 the larger crystals to be described, the microliths are entirely converted into zeolites. 

 The larger sections of plagioclase have retained their form only, and give the optical 

 reactions of zeolites. Some small and very distinct sections of olivine also appear, 

 filled with zeolitic crystals, and the latter are developed in the cracks of the rock as 

 well. Other sections of olivine are less profoundly decomposed, being only impregnated 

 by ferruginous matter and products of alteration along the fissures. If augite exist in 

 this rock, it must be entirely disguised by the products of its alteration. A few 

 crystals of apatite have been observed. 



A specimen from Arch Rock may be described before considering the rocks of 

 Cumberland Bay. This natural arcade forms the extremity of the southern headland 

 enclosing Christmas Harbour. The specimen examined is a black dolerite, coloured 

 greenish by alteration, of moderately fine grain, and breaking with an unequal fracture. 

 Its microscopic structure is that of a characteristic dolerite ; lamellas of plagioclase are 

 enclosed in reddish grains of augite, which constitute, so to speak, the cement of the 

 rock. Large crystals of olivine, retaining their crystallographic form, but largely 

 altered into serpentine, are observable. Delessite has been developed at many points ; 

 its sections appear generally triangular, or with straight lines, the outlines of this 

 green secondary matter being usually defined by the intercrossed lamellas of plagioclase, 

 which themselves are more or less penetrated by delessite. The latter mineral also 

 lines the geodes, in the centre of which calcite has crystallised. Arch Rock has also 

 yielded amygdaloidal specimens with fibre-radial zeolites closely resembling those of 

 Christmas Harbour. 



