132 



THE VOYAGE OF H.M.S. CHALLENGER. 



talus of debris as it disintegrated ; this the columnar arrangement effectually prevents. 

 The upper part of the most remote phonolitic eminence, which crowns the summit of 

 this chain of hills, rises to more than 50 feet. An accumulation of blocks covering the 

 lower wall is scattered over the steeply inclined slope. 



The basaltic rocks, which form the principal mass of the hills, and extend in horizontal 

 layers at Greenland Harbour, as in all other parts of the island, will be described 

 first. The rocks where the Challenger made a landing are altered felspathic basalts, 

 black and massive, displaying no mineral's to the unaided eye. The fracture is almost 

 plane. With the lens one sees that they are formed of crystalline grains, amongst 

 which triclinic felspars appear. In the ground-mass composed of microliths of plagio- 

 clase and augite are embedded larger crystals of plagioclase, and olivine which has been 

 completely decomposed, only the form remaining ; this mineral is replaced by limonite, 

 which also penetrates the whole rock. 



The horizontal beds extending to the south-west of Greenland Harbour are formed 

 of a basaltic rock, the porphyritic structure of which is caused by the presence of 

 large crystals of augite, felspar, and olivine. The mass is compact, but the whole 

 rock is penetrated by oxide of iron. Large sections of plagioclase, cracked in all direc- 

 tions, are seen under the microscope. The cracks are filled with opal, and the whole 

 appearance of these felspars resembles those we shall describe in the augite-andesites 

 of Kandavu. Sections of augite and some small crystals of olivine are also seen, the 

 larger ones being so much altered that they are destroyed in polishing the preparations. 

 The ground-mass is formed of a network of small microliths of plagioclase and augite, 

 with some magnetite. 



The rock forming the greater part of the hills west of the bay is also spread out in 



horizontal beds, and, like the preceding, is a basalt, possess- 

 ing the usual macroscopical character of this rock. Under 

 the microscope this basalt appears with a ground-mass made 

 up of small crystals of plagioclase and augite, and numerous 

 grains of olivine. The most striking feature in these pre- 

 parations is the great number of large crystals of olivine, 

 which usually are formed of several individuals by direct 

 grouping. These sections are sometimes bounded by curved 

 lines showing the corrosive action of the magma. The olivine 

 is usually decomposed on the edges, where the alteration is 

 indicated by a slightly fibrous yellow border. Augite is less 

 common than olivine, and shows as irregular colourless or 

 fig. 23. -Basalt of Greenland Harbour, pj^ sec ti ns in the preparation. On the edges it takes the 



Section of augite showing the external __ , - - 1 



same green colour as the small augites ot the ground-mass 



green zone. 3*5 crossed nicols. 



23). These microliths surrounding the microporphyritic sections of augite 



(see fig, 



