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



Thiii slices show a magma impregnated with quartz (perhaps of secondary origin), and 

 containing sections of felspar, augite, quartz, and biotite. The felspars are both sani- 

 dine and plagioclase ; these two felspars are to be seen in the same section, as is often 

 the case in transitional rocks such as that under consideration. The centre is, in these 

 cases, finely striated like an oligoclase or andesine, and surrounded by a zone in which 

 plagioclastic lamellae no longer appear. These lamella? in the nucleus extinguish at a 

 very low angle, which confirms the determination as a triclinic felspar approaching the 

 oligoclase series. The felspatnic microliths of the ground-mass are often Carlsbad 

 twins, and frequently appear almost rectangular in section. This leads to the con- 

 clusion that their prevailing form is determined by the lengthening of the edge PjM. 

 The crystals of augite present only indistinct or irregular outlines ; this mineral is 

 little, if at all, pleochroic. The biotite is in the form of corroded lamellae, which some- 

 times take a greenish tint, indicating an incipient alteration into chlorite. Some 

 colourless sections show the properties of quartz, giving the cross of monaxial crystals 

 in convergent light. This mineral is, very probably, also represented in the ground- 

 mass of the rock. It is noteworthy that all the older constituents, especially the 

 felspars, have been very much corroded, as if they had been subjected to the energetic 

 solvent action of an acid magma. 



More distinctly rhyolitic rocks occur in Ascension, especially in the interior of the 

 crater-like orifice of Eiding School. A specimen of this type is compact — in some 

 places a little scoriaceous — with a nearly plane fracture, and of a brick-red colour. The 

 naked eye can only detect some crystals of felspar. The microscope shows that the red 

 colour is due to an amorphous powder of hematite, which has penetrated all the fissures 

 and vesicles of the rock. The colourless ground-mass is spherulitic and impregnated 

 with quartz ; large sections of sanidine appear in it. This mineral is crystallised in a 

 tabular form, sometimes in shortened prisms, and the Carlsbad twin is common. A 

 section in the zone P : M, in which cleavages corresponding to P and to the prism with 

 traces of T or I are clearly shown, has made it possible to measure the angle of extinc- 

 tion on M. It was found to be positive and 10°, which confirms the determination of the 

 felspar as sanidine. Some colourless homogeneous sections with irregular outlines must 

 be referred to quartz, as in convergent light they show the cross of monaxial crystals 

 and the usual properties of thin slices of that mineral. The presence of quartz as a 

 microporphyritic constituent leads us to refer to the same species certain much smaller 

 sections, which present the same appearance and the usual optical properties of this 

 mineral in parallel polarised light. These little sections are, as it were, drowned 

 in the ground-mass ; they are associated with numerous sharply defined felspathic 

 microliths. The ground-mass is thus essentially quartzose, and is characterised besides 

 by the presence of spherulites, which resemble pseudo-spherulites, the cross being 

 vaguely indicated, and its arms not at right angles. Probably this is a fibro-radial 



