ROCKS FROM GOUGH ISLAND, SOUTH ATLANTIC. 401 



containing microlites of plagioclase and augite. This rock resembles the vitrophyric 

 basalts from Tristan d'Acunha described by Renard.* 



A bright red specimen [G. 10], with conspicuous phenocrysts of plagioclase, proves 

 on microscopic examination to be a highly decomposed glassy basalt. 



Intrusions. 



Intrusive rocks are represented by specimens of (a) tachylite, (b) basalt, and (c) 

 essexite. 



(a) Tachylite.- — A tachylite [G. 9] was collected from the chilled margin of one 

 of the narrow dykes which traverse the lava series. The hand specimen shows 

 small scattered phenocrysts of hornblende and plagioclase in a very fine-grained 

 groundmass, which is crowded with narrow slit-like vesicles orientated parallel to a 

 selvage of black glass. In thin section the rock is seen to consist of a pale brown 

 glass containing crystals of basic plagioclase, brown hornblende, ilmenite, and apatite, 

 along with skeletal growth forms of olivine and plagioclase, and minute microlites of 

 a mineral with strong double refraction, probably a pyroxene. 



(b) Basalt. — A specimen [G. 11] collected from another small dyke is a fine- 

 grained ashy-grey basalt, conspicuously porphyritic with basic labradorite and 

 bytownite. Phenocrysts of corroded olivine, brownish euhedral augite, and iron 

 oxides are also present. The groundmass consists of laths of labradorite, prismatic 

 crystals of brown augite, iron oxides, and apatite, together with some interstitial 

 alkali felspar. Although it differs from these in the preponderance of basic 

 plagioclase, the presence of the characteristic "cement" of alkali felspar suggests 

 that this dyke rock is related in origin to the trachydolerite lavas. 



(c) Essexite. — Perhaps the most interesting intrusive rock collected by Dr Pirie 

 is a coarsely crystalline essexite, strongly porphyritic with augite in euhedral and 

 broken crystals averaging ^--inch in diameter. The large augites enclose numerous 

 yellow rounded olivines and occasional laths of plagioclase. The groundmass is a 

 greyish-brown, even-grained, holocrystalline aggregate, chiefly of olivine, augite, 

 and felspar. 



In thin section the augite has the purplish-violet colour with distinct pleochroism 

 characteristic of titanaugite. The porphyritic crystals are sometimes deeper in tint 

 towards their margins, but the zonal structure is not well marked. In addition to 

 enclosures of olivine and plagioclase, they contain gas and liquid inclusions, often 

 arranged in bands. Simple twinning on 100 is common, and lamellar and cruciform 

 twins are also found. The groundmass augite is also a titanaugite occurring in 

 euhedral and subhedral crystals. 



The predominating felspar is plagioclase, in broad laths occasionally showing 

 zonary structure. The values of symmetrical extinction angles in sections cut at 



* Bull. Acad. Roy. de Belgique, 1885 (3), ix., No. 6. 



