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



With the lens some glassy white felspathic grains are to be seen ; others are dark and 

 ought to be ascribed to olivine, augite, or magnetite ; the rock is slightly stained with 

 little spots of limonite. 



Under the microscope these rocks appear to be entirely composed of crystalline 

 elements, the structure is that of dolerites ; between the felspathic lamellae the augite 

 has crystallised ; little sections of magnetite and some skeleton crystals of olivine are 

 scattered amongst these minerals. The lengthened sections of plagioclase are twinned 

 according to the albite law. It has been possible in one case to measure the extinction 

 on a section almost parallel to the face M, clearly ended by the traces of the faces x 

 and P; the value of the extinction was —17°. This plagioclase is consequently very 

 closely related to labradorite. The olivine is to be seen, like the augite, in the shape 

 of grains without distinct crystallographic outlines ; it is rather difficult at first to 

 distinguish these two minerals, but, besides the optical properties, it is observed that 

 whilst olivine is colourless, the augite is slightly tinged with pink. The cleavages of 

 the latter mineral are also more distinct, the olivine being more decomposed, and its 

 grains often rounder than those of augite. The sections of olivine offer no noteworthy 

 characteristics. We will only mention that the alteration undergone by the olivine is 

 shown by a certain fibrosity, and that the grains of this mineral are often surrounded 

 by a zone of small augitic microliths belonging, most probably, to a second generation. 

 The pyroxenic element of this dolerite is, as we have just said, generally granular ; more 

 or less lengthened sections are sometimes visible, as also sections perpendicular to the 

 vertical axis, showing the characteristic cleavage of augite. The colour of this mineral 

 is here light pink, without perceptible pleochroism ; sections parallel to oo P oo divided 

 in four parts, showing the hour-glass structure, are to be observed. Some of the 

 augite is twinned, the two individuals having for composition-plane the dome —Poo. 

 This mineral is also to be found in small granulations scattered between all the con- 

 stituent elements. The magnetite occupies an important place in this dolerite ; its 

 sections are often lengthened, it frequently presents groups of small crystals, and it is 

 found, as inclusions, in plagioclase and olivine. 



Other specimens of the rocks which, together with those just described, form the 

 central mass of the island, are not of doleritic structure ; they are common felspathic 

 basalts. They are not so dark in colour as the dolerite, their grain is finer, and the 

 fracture is large and even ; with the naked eye or with the lens, olivine alone is seen 

 in large crystals 3 to 5 mm. in diameter. This mineral gives the rock a porphyritic 

 structure, and is embedded in a fundamental mass of homogeneous appearance. The 

 altered specimens show on the surface projecting peridotic crystals ; the rock weathers 

 into balls with concentric layers. The microscopic preparations show that this rock 

 possesses the common basaltic texture ; fine plagioclastic lamellae with few polysynthetic 

 twins are interwoven with grains of augite with indistinct outlines. Quantities of small 



