ROCKS FROM GOUGH ISLAND, SOUTH ATLANTIC. 399 



a thick cream-coloured crust of decomposition products, streaked in places with red 

 iron oxides. 



The rock consists essentially of a holocrystalline trachytic aggregate of anortho- 

 clase, segerine-augite, and an undetermined mineral which in some respects resembles 

 lavenite. It is sparingly microporphyritic with titaniferous magnetite, zircon, and 

 brown dichroic apatite, and an occasional phenocryst of the undetermined mineral 

 has also been noted (PL XXXVI. figs. 3 and 4). The rock is remarkably rich in 

 zircon, and it contains as additional accessory constituents magnetite, colourless 

 needles of apatite, and a little interstitial sodalite. 



The predominating mineral is anorthoclase in simply-twinned crystals with 

 characteristic denticulate edges. Next in abundance is segerine-augite, pleochroic in 

 grass-green and yellowish-green tints, in elongated prismatic crystals and irregular 

 ophitic intergrowths with felspar. In almost equal amount is a mineral which has 

 not been determined. In its pleochroism, crystal form, strength of double refraction, 

 and optically negative character it comes near lavenite ; it differs from the latter, 

 however, in refractive index (a = 178), in the absence of twinning, and in its ready 

 solubility in HC1. A detailed description of this interesting, and probably new, 

 mineral is deferred until a complete chemical analysis has been made. 



B. Tracliydolevites. 



Although differing from one another in some respects, the specimens obtained 

 from four basic lava flows possess in common certain characters which justify their 

 classification as trachydolerites, that term being used in its restricted sense for the 

 effusive representatives of an essexitic magma. 



The most acid variety [G. 2] is a porous ashy-grey rock with scattered pheno- 

 crysts of basic plagioclase, augite, olivine, skeletal ilmenite, and apatite. The 

 plagioclase crystals contain inclusions of the groundmass, and are twinned on the 

 Carlsbad, albite, and pericline laws. Their mean refractive index is 1'563, a value 

 which indicates that they belong to labradorite-bytownite (Ab 2 An 3 ). Symmetrical 

 extinction angles in sections cut at right angles to 010 confirm that determina- 

 tion. The augites, greenish-brown in thin section, are euhedral and occasionally 

 exhibit lamellar twinning. The olivines are in rounded crystals showing incipient 

 serpentinisation. 



The porous fine-grained groundmass consists of plagioclase, granular augite, and 

 iron oxides. Minute laths of oligoclase in fluxional arrangement form the most 

 conspicuous feature ; but between the laths there is a very considerable development 

 of albite and alhite-oligoclase, the last product of crystallisation, partly in euhedral 

 crystals, but for the most part as an anhedral crystalline cement in which the other 

 constituents are embedded. 



The most basic example [G. 4] is a grey vesicular lava sparingly porphyritic with 



