20 THE VOYAGE OF H.M.S. CHALLENGER 



Between these large crystals of augite may be seen grains of olivine often partially 

 serpentinised, and pretty common lamellae of biotite and magnetite ; the plagioclase 

 is partially transformed into saussurite, and almost always presents itself in the form 

 of elongated lamellae with large extinctions similar to those of labradorite. These 

 felspars, which are generally small, form almost alone the ground-mass enclosing the 

 other crystals. 



A lava from the same locahty is slightly scoriaceous, of a reddish grey colour, with 

 an irregular fracture. Olivine reddened by oxide of iron may be seen with the lens. 

 Amongst the microporphyritic minerals, which are perceived under the microscope, 

 may be specially mentioned olivine and magnetite with subordinate felspar. These 

 larger minerals are enclosed in a ground-mass formed of a base, devitrified by globulites, 

 microliths of augite, and of felspar and secondary minerals, such as hematite, etc. 



The large sections of olivine, perfectly crystallised, are magnificent examples of 

 pseudomorphs of hematite. This last mineral appears opaque with transmitted 

 light ; with reflected light its dark-red colour is clearly seen. In this nearly perfect 

 transformation of the hyalosiderite into hematite, certain parts of the primitive crystal 

 have preserved their transparency and all their optical properties. This apparent 

 anomaly is explained if it be remembered that this alteration of the mass of olivine 

 does not take place in a uniform manner ; the trichites or the small veins of hematite 

 advance in directions determined by microscopical cracks ; they afterwards enlarge, 

 sometimes leaving small patches where the alteration has not yet commenced, and these 

 preserve all their properties. By the form of these pseudomorphosed sections, it seems 

 that it is really obvine which formerly occupied all the space invaded by the hematoid 

 substance. When the little colourless patches of these sections are examined in 

 polarised light, they all darken at the same time ; this in its turn proves that they 

 form the last remains of a single crystal. 



The felspars show themselves in an abnormal manner. Usually, in the basalts, 

 plagioclase presents itself with a considerable clearness of outline. Here, on the 

 contrary, they have the appearance of an intercalated mass of which the crystallisa- 

 tion has been impeded by the surrounding minerals. They are grains without regular 

 outlines, and the strise of the plagioclase are scarcely marked ; they present in places 

 an undulating extinction, produced perhaps by alteration. If an analogy to these 

 felspathic grains were to be sought for in other rocks, they might be compared with 

 the plagioclases as they exist in meteorites of the type of chrondrites. 



Small augites, yellow by alteration, form almost the whole of the ground-mass ; 

 they are found together with grains of magnetite, and transparent reddish brown 

 sections extinguishing parallel to the edges. This mineral cannot be precisely determined. 

 If the form, almost always quadratic, which it presents be taken into account, it might 

 perhaps be classed with perowskite, but the colour is too. red, it is not sufficiently 



