Notes on the Petrology of Gouyh Island. 263 



XXII. Notes on the Fetrologif of Gough Island. 

 By E. Campbell, M.A., B.Sc. 



(Read 26th March 1906.) 



The material collected by Dr Pirie includes volcanic rocks, 

 tuffs, and a boulder of limestone. The volcanic rocks, which 

 are all remarkably fresh, comprise several varieties of 

 trachyte, trachy-dolerite, and basalt. 



I. Trachytes. 



Trachytes are represented by two types. One of these is 

 a rock which varies in colour from very pale brown to 

 reddish brown (Gr. 5, 12, 19).i Small phenocrysts of felspar, 

 biotite, and green augite are embedded in a compact, fels- 

 pathic ground-mass. Examined microscopically, the felspar 

 phenocrysts, which occur in tabular sections, often showing 

 a marginal zone of inclusions with a narrow band of later 

 felspar substance, appear to be typical sanidines. Some of 

 them, however, exhibit the characteristic fine striation of 

 anorthoclase. They enclose fairly large idiomorphic crystals 

 of biotite and augite. Next in abundance to the felspar 

 comes biotite, showing the usual evidences of magmatic 

 resorption. The augite occurs rather sparingly, and is pale 

 green in thin section. The ground-mass is holocrystalline, 

 and consists mainly of alkali felspar. Augite occurs spar- 

 ingly in a second generation. Zircon, apatite, and magnetite 

 are also present. The structure is typically trachytic. 



The specimens examined were obtained in situ. 



The second type (G. 14) is of a pale grey colour, compact 

 in texture, and non-porphyritic. In thin section the rock is 

 seen to be holocrystalline, consisting of lath-shaped crystals 

 of orthoclase, with scattered, irregular granules and ophitic 

 patches of bright green, dichroic, soda-augite, and particles 

 of magnetite. Small zircons and apatite needles also occur. 



1 Specimens of the various rocks may be seen in the Geological Museum, 

 University of Edinburgh. 



