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



than the intermediate links. In the present case we cannot explain this rarity of 

 polysynthetic twins by the fact of the sections being cut parallel to the face M ; they 

 are usually cut, on the contrary, perpendicular to the edge P/k, for cleavages following 

 P and M may be observed. In some sections following P extiuctions have been 

 measured, their value varying from 38° to 42° ; this felspar is thus akin to anorthite. 

 The microliths of the ground-mass, on the contrary, must be referred to labradorite. 



It is unnecessary to do more than allude to some partly decomposed basaltic rocks 

 which exhibit the usual alteration of basalts ; it may simply be noticed that the 

 formation of zeolites often goes on simultaneously with a considerable deposition of 

 siliceous matter, and that the latter, in some cases, takes the place of the plagioclase. 



A volcanic conglomerate from the summit of a hill at the south of Christmas 

 Harbour is formed of palagonitic tufa. The black, compact, shining splinters of basalt, 

 varying from 1 to 2 centimetres in diameter, are enclosed in a brownish mass ; small 

 whitish layers of zeolites have formed around the lapilli. The brown material has the 

 well-known resinoid character of palagonitic tufas. Opal is sometimes deposited on the 

 rock, and often passes into cascholong. Microscopic examination shows that this tufa is 

 formed of an aggregation of brown vitreous granules. These fragments frequently 

 change to a yellow colour at the edges, without showing any alteration to red, or the 

 characteristic fractures and the phenomena of polarisation, which often accompany the 

 most advanced decomposition of the vitreous matter of these tufas. These amorphous 

 patches are always isotropic. Plagioclase and olivine have crystallised from the 

 magma ; no augite is to be seen, the rapid cooling of the paste accounting for the 

 absence of this mineral. The sections of felspar are often prismatic, showing the striae 

 of the albite twin, but usually this mineral crystallises in little lamellae with rhombic 

 outlines, and so thin that several of them are superimposed in the thickness of the 

 preparation. These small rhombic tables show traces of the faces P and x ; sometimes 

 they appear as disymmetric hexagons ; in this case the face y is added to the preceding. 

 Olivine is generally well crystallised, and its sections usually appear w T ith rhombic out- 

 lines and inclusions of vitreous matter at the centre. This species sometimes shows 

 crystals joined with parallel axes so as to form groups of several individuals. Magnetite 

 is rather rare, appearing as inclusion in olivine. Vesicles in the vitreous mass contain 

 delessite. The zeolitic substance, cementing the lapilli, forms fibro-radiating layers, 

 which might be classed as natrolite, but the brightness of the polarisation colours 

 seems to indicate the presence of chalcedony penetrating this zeolite. 



The rocks forming hills about Christmas Harbour are traversed by dykes, from 

 which Mr. Buchanan collected several specimens. One of these represents a compact 

 basalt in which the naked eye can only distinguish olivine in a blackish shining 

 crystalline mass. Near its contact with the adjoining rock the texture becomes closer, 

 and the basalt passes into the vitreous variety ; to this portion of the rock are joined 



