402 DR ROBERT CAMPBELL ON 



right angles to 010, and estimation of mean refractive indices by comparison with 

 oils of known refraction, prove that for the most part it ranges from medium labra- 

 dorite to bytownite. Along with the basic plagioclase, however, occurs in fair 

 amount, partly in interstitial patches and partly as an outer zone round the broad 

 laths, felspar with refractive indices lower than that of the Canada Balsam of the 

 slides. This felspar was found to be mostly albite and albite-oligoclase (as in the 

 trachydolerites), orthoclase occurring very sparingly. 



The rock is rich also in titaniferous magnetite in elongated lath-like crystals, most 

 of which have crystallised later than the augite and are occasionally moulded on the 

 felspar laths. Apatite in elongated needles is widely disseminated, particularly in 

 the patches of alkali felspar. 



The augite phenocrysts are much in excess of the groundmass, which is made 

 up of olivine, augite, felspar, and iron oxides in approximately equal amounts. 

 (PI. XXXVI. fig. 6.) 



The Gough Island essexite resembles the pyroxene-rich varieties of Norwegian 

 essexite described by Brogger ; * it is, however, poorer in augite and richer in olivine, 

 and it is free from biotite. In the relative proportions of augite and olivine it 

 approaches very closely the porphyritic essexite of Lennoxtown,f but nepheline has 

 not been observed in the Gough Island rock. 



Tuffs. 



Pyroclastic rocks are represented in the collection by trachytic tuffs varying 

 in colour from yellow to greyish-brown. One of the most compact specimens was 

 sectioned. It was found to consist largely of fragments of colourless spongy glass, 

 along with numerous lapilli of yellow obsidian and vesicular glassy basalt, and broken 

 crystals of olivine, augite, sanidine, and basic plagioclase. 



A large slaggy lapillus of glassy basalt obtained from one of the tuffaceous 

 deposits has some of its vesicles infilled with chalcedony. 



Limestone. 



The only sedimentary rock is an arenaceous limestone containing remains of various 

 calcareous organisms, conspicuous among which are spines and plates of echinoderms, 

 together with subangular grains of plutonic quartz and occasional flakes of muscovite 

 embedded in a calcareous cement. The specimen, unfortunately, is merely part of a 

 beach boulder, and may have been carried to the island either by human agency or 

 by floating ice. Had it been found in place, one would have been tempted to speculate 

 on its probable bearing on the existence of a former land connection between South 

 Africa and South America. 



* Rosenbusch, Mikroskopische Physiographie, 189G, ii. p. 250. 

 t Mem. (Jeol. Survey : Geology of Glasgow District, 1911, p. 129. 



