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



occur, unsynimetrical like the former ; these are perpendicular to the plane of symmetry, 

 and extinguish parallel to the line of their length. The extinctions measured on the 

 face oo J? oo exceed 40°. Sections more or less perpendicular to the axis c are fairly 

 regular octagons, in which the traces of the pinacoids are more developed than those of 

 the prisms. The augite is filled with vitreous inclusions, which are accumulated at the 

 centre of the crystals ; round this non - homogeneous nucleus is a zone of slightly 

 reddish tint, the nucleus being usually not so dark. The optical phenomena are 

 disturbed by an incipient alteration, which shows itself in the formation of chloritic 

 material. The prismatic cleavages are not distinct ; they are, rather, irregular cracks ; 

 the cause of this anomaly lies in the presence of so great a quantity of vitreous 

 inclusions. Besides the twin following the ordinary law, some sections of augite seem 

 to be twinned with a composition plane parallel to a face of a pyramid, as has already 

 been observed in augite. The vitreous inclusions are irregularly shaped ; their colour is 

 generally faint, but sometimes they assume the colour of the augite which contains them. 

 One is led to suppose that there might have been a partial refusion of the pyroxenic 

 element, but what renders this interpretation hypothetical is that the external zone 

 which surrounds these nuclei, and which is entirely homogeneous, has not been altered 

 at all. This fact appears to dispel all idea of a later caustic action exerting itself on the 

 crystal. Besides the vitreous inclusions, some are to be seen consisting of grains of 

 magnetite or of greenish patches of secondary origin, and probably of a chloritic matter. 

 The olivine shows under the microscope some interesting peculiarities in regard 

 to its decomposition. This mineral occurs in grains or in sections of the ordinary form, 

 and with the trace of the pinacoid OP, hence some sections have octagonal forms. 

 When the olivine is not altered it is colourless, its surface rugose, its chromatic j)olari- 

 sation vivid ; still, it is somewhat rare to see this mineral undecomposed. Sections are 

 often observed having the outlines of olivine, but showing that this mineral is invaded by 

 alteration products. At first the olivine is changed into a yellow pleochroic substance 

 possessing the characters of biotite. It shows a lamellar cleavage, along the traces 

 of which absorption is more marked. The colour is brownish yellow along this 

 direction, yellowish in the line perpendicular to these lamellas. They extinguish follow- 

 ing the direction of the joined lamellae. The sections parallel to the lamellae remain dark 

 during a complete rotation between crossed nicols ; with convergent light these latter 

 sections exhibit a black cross. The double refraction is negative. All these characters 

 completely justify the determination as biotite. In certain cases the olivine presents a 

 less advanced decomposition. The yellowish matter which tends to encroach upon it 

 bears less distinctly the characters of black mica ; it is not lamellar, and it is difficult, 

 even when it is possible, to detect any absorption ; perhaps this is mica in the course 

 of formation. In some cases one sees around the biotite certain more or less capillary 

 accumulations, presenting sometimes a vaguely radial arrangement, and probably 



