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



the disappearance of sections of peridotite, which crumble and are swept away during 

 the polishing. The basalts of the island are often scoriaceous ; a basaltic lava from 

 Biding School which we have examined is particularly so : it is a reddish scoriae, very 

 alveolar and rough, and containing heterogeneous half-fused fragments. 



The microscope shows a ground-mass of a very fine grain and pitted with pores. 

 Olivine and plagioclase appear in microporphyritic sections ; the former predominates 

 and is often fragmentary, although when the crystals are very small they are sharply 

 defined. Augite is found in the form of microliths in the ground-mass, which contains 

 very little vitreous matter. Little granules of hematite occur throughout the mass, 

 penetrating all the felspathic sections where they appear in zones. 



Finally, there are basaltic rocks of the dolerite type. These are greyish, almost 

 saccharoid in texture, with pretty large grains ; plagioclase crystals are visible to the 

 naked eye, and the lens shows grains of augite between them. With the microscope it 

 is seen that these basalts do not possess what can, properly speaking, be termed a 

 ground-mass. The lamellae of plagioclase felspar are twinned according to the Carlsbad 

 and albite laws ; they are comparatively little striated, and thus resemble the felspars 

 of those basalts we have just described. The extinctions show that the plagioclase 

 approaches labradorite. The augite intercalated in the felspar lamellae occurs as greenish 

 violet grains associated with magnetite, the sections of which, generally irregular, are 

 surrounded with hematite. The olivine has corroded outlines, and is coloured red or 

 green by alteration. The greenish secondary matter is sometimes more or less fibrous ; 

 it is dichroic, and to a certain point resembles hornblende. This transformation into 

 amphibole would explain the oblique extinction which has been observed in olivine 

 sections that have undergone the same alteration. 



Akdesites. 



Certain rocks, much resembling basalts, which may be classed as andesites, are met 

 with in various parts of the island, particularly on Red Mountain. 



Some specimens of andesite from Eed Mountain are bluish black or iron-grey in 

 colour, pretty compact, breaking with a plane fracture, and resembling basalt externally. 

 No constituent minerals can be detected by the naked eye. Other specimens of andesite 

 are more earthy ; they have a reddish colour, are impregnated with oxide of iron, and 

 surrounded by a rather thick crust of sublimed specular iron, which is often covered 

 with beautiful little crystals of the same mineral. 1 



Microscopic examination shows that this rock must be classed with the pyroxenic 



1 The Island of Ascension is a well-known locality for fine crystals of hematite, which probably come from Red 

 Hill. Vom Rath found octahedral crystals of magnoferrite on a specimen of Ascension hematite. This association 

 indicates a formation by fumaroles (see Vom Rath, Zeitschr. d. dtutsch. geol. Geselkch., Bd. xxv\ p. 108, 1873). 



