THE VOYAGE OF H.M.S. CHALLENGER. 



with difficulty be detected. The augite is decomposed ; it is yellowish on its edges, 

 the centre still remaining rather violet. The olivine is also altered, being reduced to 

 an external zone, where only the outline of the mineral can be seen. The interior of 

 the section is filled with trichites having a pretty regular disposition and showing 

 rectangular forms ; they are associated with small reddish particles. Certain parts only 

 of the olivine polarise, but the colours are not very brilliant. Perhaps we have here an 

 hyalosiderite ; this at least seems to be indicated by the great number of trichites, 

 which are also developed in the vitreous decomposed mass forming the ground-mass. 

 Some other specimens collected in the Cafiadas have a waxy appearance when broken, 

 a black colour shading into yellowish brown, with an irregular fracture and 

 containing rather large crystals of sanidine. With the microscope a 

 ground-mass is seen formed by small plagioclases, perhaps also by micro- 

 liths of sanidine, and by some subordinate vitreous matter. Large 

 sections of plagioclase and of sanidine stand out in this mass. The 

 former show nearly always both the Carlsbad twinning and that of 

 the albite law. They are elongated, with very small extinctions, and 

 ought to be classed in the plagioclastic series near oligoclase and andesine. 

 As frequently occurs with andesine, the hemitropic striae are exceedingly 

 close and thin. The sections are crossed by two series of polysynthetic 

 striae ; these two systems cut one another under sensibly straight 

 angles, and in polarised light give the section the aspect of microcline, 

 as can be seen in the adjoining figure (fig. 2), only the small veinules of 

 albite are here missing. In some cases these striae are so faintly visible, 

 that the section might be mistaken for sanidine, but the plagioclastic 

 striae, be they ever so feebly marked, ought to set aside that interpreta- 

 tion. Sanidine is found in the rock in the form of irregular sections, of 



Fig. 2. — Augite-ande 

 site, Cafladas. 



section of piagiociase rather large size, and twinned according to the Carlsbad law, with the 



crossed by two series 



of polysynthetic stria?, characteristic fissures of that felspar. It can be seen by the undulating 

 extinction that these crystals, like those of plagioclase, have been subjected to 

 mechanical deformation, which altered the optical properties, and renders any subsequent 

 determination difficult. The presence of augite as crystals of the first generation is also 

 ascertained ; its pleochroic sections have often indistinct outlines ; they are corroded 

 and invaded by the ground-mass. Some small prisms of apatite are also observed, 

 which show, in addition to the usual faces of the prism, a truncation on the edge 

 OPjcoP; they are terminated by the pinacoid OP. Magnetic iron is rather frequent; 

 small hexagonal hardly transparent lamellae are also seen, which may be referred to 

 titanic iron. We have stated that the ground-mass is formed by the accumulation of 

 small felspar crystals ; these are of two types. Those of the first type are tabular, 

 with less distinct outlines, and larger sections ; those belonging to the second type are 



