REPORT ON THE PETROLOGY OF OCEANIC ISLANDS. 175 



and presenting a metallic lustre like bastite, stand out from the ground - mass. 

 Microscopic examination shows that this serpentine is an alteration product of 

 pyroxenic peridotite, with granitoid texture. The olivine sections are often found 

 altered, the mineral being almost always invaded by pale yellow or colourless 

 serpentinous matter. The alteration has not affected enstatite so seriously ; some 

 fibrous sections of this mineral are to be seen, and the optical properties, although 

 already somewhat uncertain, indicate a non-pleochroic rhombic pyroxene. 



The serpentine in another specimen is clothed with a coating of chalcedony ; the 

 yellowish green serpentinous matter is brecciated, and the fragments cemented together 

 by filaments of chalcedony. Under the microscope sharp angular splinters of serpentine 

 are seen presenting the usual characteristics of this substance. There is no trace of a 

 primary mineral remaining. The chalcedony appears either as a fibro-radial aggregate, 

 showing the black cross of spherulites, or as a fibrous structure, composed of extremely 

 fine needles. The association of these veins of chalcedony with serpentine may be 

 explained by the silica eliminated, when the latter mineral was formed from the 

 original rock. 



Serpentine is not the only substance formed by the decomposition of magnesium 

 silicates ; another mineral produced in a similar way appears at Malanipa in a state of 

 remarkable purity. The fragments in question are white and close grained, hardly to 

 be scratched by steel, and breaking with a sub-conchoidal fracture. The surface is 

 covered with irregular mammillations showing its concretionary nature. Chemical 

 analysis shows that this substance is almost exclusively carbonate of magnesia, and 

 the specimens represent a type, which is remarkable for its purity, and which possesses 

 the mineralogical characters of magnesite. This mineral is frequently associated with 

 altered rocks containing silicate of magnesia. Thin slices of magnesite when examined 

 by the microscope are found to be made up of an aggregate of very small crystalline 

 grains melting into each other, and not defined by crystallographic outlines. This 

 greyish basis is grooved by microscopic fissures, along which larger grains of magnesite 

 appear, with more distinct contours, and even surrounded by a slight irisation like 

 the calcite grains in limestone. The fissures are lined by a yellowish brown fibrous 

 coating of serpentine. 



Finally, one of the specimens from Malanipa is a piece of calcareous tufa, similar to 

 that found on many other islands, and described particularly when speaking of Fer- 

 nando Noronha. The naked eye only distinguishes greenish black rounded grains of 

 serpentine amongst the constituents of this pale yellow tufa, but the microscope shows 

 the rock to consist almost entirely of fragments of the shells of calcareous organisms, 

 the interiors being often lined with fibro-radial calcite. Little crystals of calcite, formed 

 in situ and of indefinite outline, may be seen sparkling on the edges of fragments 

 of shell. 



