Notes on the Petrology of Gough Island. 265 



the felspar substance. As in G. 2, the plagioclase is a basic 

 labradorite. Olivine is abundant in rounded, often much 

 corroded, phenocrysts. These show a marginal decomposi- 

 tion zone of brown limonite. An aggregation of olivine, 

 apatite, and magnetite in one part of the slide is rather 

 suggestive of an " olivine nodule." Augite phenocrysts, pale 

 green in section, occur sparingly. 



The ground-mass is andesitic in habit. It consists largely 

 of a network of interwoven laths of oligoclase, through which 

 are scattered small augites, magnetite, and many thin needles 

 of apatite. The felspathic base, which is so abundant in 

 G. 2, here plays a very subordinate part in the composition 

 of the rock. 



(G. 4.) Macroscopically this is a grey, slaggy, vesicular 

 lava, with phenocrysts of olivine and felspar. Examined 

 microscopically it is seen to be exceedingly rich in olivine. 

 The felspar phenocrysts are again basic labradorite. The 

 ground-mass consists mainly of laths of labradorite, with 

 green augite, apatite, and magnetite in relatively small 

 proportions. 



(G. 7.) In hand specimen this rock shows large pheno- 

 crysts of felspar, augite, and olivine in a pale grey ground - 

 mass. It weathers to a brown crust. The felspar pheno- 

 crysts are labradorite, and usually occur in groups. Most 

 of the crystals are surrounded by a narrow zone of more 

 acid plagioclase. Microscopic examination reveals the pre- 

 sence of numerous idiomorphic prisms of a rhombic pyroxene, 

 which shows the characteristic pleochroism of hypersthene. 

 The rock is also rich in olivine. Along with these are pheno- 

 crysts of a pale green monoclinic pyroxene. 



The ground -mass resembles that of G. 4, but contains a 

 fair amount of the felspathic base which is so abundant in 

 G. 2. 



The most interesting feature in this rock is the occurrence 

 together of hypersthene and olivine. 



(G. 11.) This a dark grey rock, mottled all over with 

 small phenocrysts of felspar. Microscopic examination 

 reveals the presence of scattered phenocrysts of olivine and 

 brown augite. One of the latter encloses ophitically a large 



