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



of the others. The largest crystals of plagioclase are corroded ; they are sometimes 

 zonary, and show the twins of albite and pericline ; from its extinctions the felspar may 

 be classed as labradorite. Sanidine, which is frequently associated with the former, 

 is distinguished by the absence of hemitropic lamellae, and by the very small angles of 

 extinction in almost all the sections examined. These are sometimes twinned according 

 to the Carlsbad law, and in one case that of Baveno was observed ; the extinction is 

 almost always undulating. 



The grains of augite are corroded like the felspar, and when little altered their colour 

 is green without pleochroism ; their structure is zonary ; the centre, which is darker 

 in tint, extinguishes at 36°, the outer zone only at about 45°. Augite is sometimes 

 entangled in brown hornblende sections, the two uniting with parallel axes, and it 

 often forms irregular inclusions in the hornblende along with apatite. Hornblende is 

 a much more important constituent than augite ; its sections, which are always brown 

 and strongly pleochroic, are surrounded by an altered zone where magnetite has 

 accumulated. The only other constituent of any size appears in irregular, dirty-brown 

 patches, scarcely transparent, and standing out in marked relief ; it is evidently titanite, 

 and is sometimes transformed into calcite. 



The paste enclosing the minerals mentioned above is formed of a network of nearly 

 colourless microliths showing fluidal structure. Amongst these may be seen very 

 minute sections of sanidine with indistinct outlines fibrous in appearance, and with 

 straight extinction ; they exhibit the Carlsbad twinning, and in ordinary light appear 

 almost as a homogeneous mass. Equally minute microliths of augite occur amongst the 

 foregoing, and may be distinguished by their colour, the chromatic polarisation, and the 

 angles of extinction. Magnetite is present in the ground-mass, but to a very unim- 

 portant extent. Finally, there are small, clear, colourless splinters of quartz. The 

 preparation is traversed by veins in which ferric oxide has been deposited. 



Thin slices of this tufa show the true characters of a microscopic breccia. Along- 

 side the fragments of the trachytic rock just described, and the splinters of which play 

 the most important part in this tufa, there are small lapilli of an entirely different 

 lithological nature, rich in plagioclase and similar to basalt. Other fragments of rock 

 related to vitreous masses of the same family are frequently changed into palagonite. 



