404 ROCKS FROM GOUGH ISLAND, SOUTH ATLANTIC. 



EXPLANATION OF PLATE XXXVI. 

 Photomicrographs of Rocks. 



Fig. L Biotite trachyte [G. 12], magnified 35 diameters, nicols crossed. The groundmass of the rock 

 is a holocrystalline aggregate consisting chiefly of soda sanidine with subordinate augite and magnetite. The 

 large phenocryst is anorthoclase, partially enclosing a crystal of biotite. 



Fig. 2. Sodalite trachyte [G. 14], magnified 35 diameters, nicols crossed. A holocrystalline aggregate 

 of anorthoclase, segerine-augite, magnetite, and interstitial sodalite. The dark area to the right of the 

 centre is sodalite enclosing small felspars micropoecilitically. 



Fig. 3. JEgcrine-augite trachyte [G. 8], magnified 23 diameters. The dark crystals are mostly an 

 undetermined mineral resembling lavenite. The rest of the rock is chiefly anorthoclase and segerine-augite. 



Fig. 4. jEgerine-augite trachyte [G. 8], magnified 35 diameters, nicols crossed. From the same slide as 

 fig. 3. The trachytic texture is well seen. The small phenocryst belongs to the same undetermined mineral 

 as the dark crystals in fig. 3. 



Fig. 5. Trachydolerite [G. 7], magnified 35 diameters, nicols crossed. Phenocrysts of olivine, 

 labradorite-bytownite, and ilmenite in a holocrystalline groundmass consisting of laths of oligoclase and 

 oligoclase-andesine, granules of augite, iron oxides, and apatite needles, together with a " cement" of albite 

 and albite-oligoclase. 



Fig. 6. Essexite [G. 13], magnified 20 diameters. The large crystal in the lower right-hand quadrant is 

 titanaugite. On either side are colourless olivines. The rest of the photograph shows a holocrystalline 

 aggregate of olivine, titanaugite, iron oxides, and basic plagioclase, with subordinate interstitial alkali felspar. 



