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



crystals. These zeolites are also frequently found in small veins in the rock. "We 

 may say that generally the vesicles are filled with analcime, while a prismatic zeolite 

 predominates in the fissures. 



The chain of hills on the south side of Christmas Harbour is higher than that to the 

 north, and as the southern coast is much indented the stratification is clearly shown, 

 and the superposition of basaltic layers in successive terraces becomes very apparent, 

 especially in the promontories. 



It is noticeable that all the hills are about the same height, and the general 

 impression left is that the whole formerly consisted of a great plateau which has been 

 deeply trenched by valleys descending towards the sea. This plateau is surmounted 

 by high peaks, so closely resembling recent volcanoes in form that Mr. Buchanan 

 thought they were volcanic cones until a closer examination showed them to be 

 formed of horizontal strata like the plateau on which they stand. This seems to 

 indicate that these peaks are nothing but portions of a higher plateau which have 

 escaped the erosive action of the ice. 



The greater number of basaltic rock specimens from Christmas Harbour are 

 characterised by a doleritic structure. To the naked eye they are black, with crystalline 

 grains, homogeneous in appearance, and with a plane fracture. The lens shows felspar. 

 Sometimes they are a little scoriaceous, and show a tendency to assume an amygdaloidal 

 texture, half-formed crystals of olivine and augite standing out. When the vesicular 

 texture is more pronounced, the ground-mass retains the same appearance, its very 

 numerous geodes being filled entirely with compact zeolitic matter of which the species 

 cannot be clearly distinguished. The globules of zeolites generally vary from some 

 millimetres to half a centimetre ; they sometimes attain the size of 1 or 2 centimetres, 

 but in this case they form true geodes, and the crystals lining the cavity are generally 

 fibro-radial or prismatic. 



Microscopic examination shows that these dolerites are formed of plagioclase and 

 olivine enclosed in grains of augite, which are moulded upon the other constituent 

 elements. These rather large crystals of olivine are often serpentinised, and sometimes 

 give rise to a microporphyritic structure. The crystals of plagioclase are twinned 

 according to the albite law, less often to that of pericline, and more rarely still 

 they show the twin of Baveno. Extinctions of about 30° have been measured on 

 sections which clearly present the strise of the pericline and albite twins. The augite 

 sections interposed between the felspathic lamellae are large, but very seldom bounded 

 by crystallographic contours, and usually very pleochroic. When the colour is less 

 deep, the augite at first sight is difficult to distinguish from olivine, but as the latter 

 mineral is usually altered, it is easy to distinguish it from the intact augite. Magnetic 

 iron is represented by small sections derived from the octohedron, or by little rods. 



