480 PROF. W. S. BOTJLTON ON THE [Nov. I904, 



A typical specimen of the granulitic dolerite (28) is from the 

 northern slope of the hill. It contains laths of felspar measuring 

 up to O05 inch in length, showing simple, but generally lamellar 

 twinning, and a tendency to a parallel arrangement, with extinction- 

 angles as high as 40°, indicating a felspar of the basic labradorite- 

 series ; plates of yellowish-brown augite measuring 0*04 inch across, 

 sometimes enclosing felspar-laths, and in one or two places sur- 

 rounding phenocrysts of olivine. Numerous large crystals of olivine, 

 0*06 inch long, showing the t} T pical crystal-outlines and cracking, 

 are now converted into pale-green, faintly-polarizing serpentine, 

 with the characteristic haematite-rods and plates along the edges 

 and cracks. Frequently the phenocrysts are aggregated, so as to 

 approach the ' glomeroporphyritic ' structure of Prof. Judd. The 

 groundmass is made up of a fine mesh of felspar-laths having very 

 low extinction-angles, surrounded b}' much light-brown and greenish 

 augite in a finely-granular condition, and possibly some minute 

 olivine-crystals, together with grains of magnetite and ilmenite, 

 forming the pilotaxitic structure of Rosenbusch. 



The rock appears to be a typical example of a granulitic 

 augite-olivine-dolerite with two generations of felspar, the 

 earlier consisting of phenocrysts of labradorite, the later, forming the 

 matrix, a more acid felspar allied to oligoclase (PI. XLIII, fig. 6). 

 For the full analysis and the specific gravity see p. 481. 



No. 60, at the extreme southern end of the hill, may be taken as a 

 type of the ophitic dolerite. It is made up of plates of yellowish- 

 brown, almost colourless augite nearly 0*1 inch across, enclosing laths 

 of labradorite measuring up to 0*08 inch in length. Olivine -pheno- 

 crysts are abundant, but are all converted into pale-green serpentine, 

 in some cases with a distinct spherulitic or fibrous structure, and 

 with the usual separation of magnetite and haematite along the 

 borders and cracks. The felspars have been largely replaced 

 by secondary substances, while the augite is relatively fresh ; but 

 this, too, in places has degenerated into a serpentine-product. Silica - 

 percentage=45'64 ; specific gravity = 2-84. 



No. 569, from the edge of the dolerite-mass on the western 

 slope, shows pronounced ophitic structure, and the olivine, which is 

 plentiful, has completely degenerated ; much green serpentine, with 

 pale-yellow granules, white by reflected light, and plates and rods 

 of haematite and magnetite resulting from its decomposition. 

 Specific gravity = 2*85. 



]S"o. 514, from the north-western slope, near the gritty tufts of 

 the Andesite-Group, shows the ophitic structure to the naked eye, 

 with much green material, little laths of felspar, and occasional 

 round vesicles filled with a dark-green substance. Microscopic 

 examination reveals large plates of augite, of a deeper brown than 

 usual, enclosing felspar-laths measuring up to nearly 0'1 inch in 

 length. No recognizable olivine occurs, but much greenish-yellow 

 serpentine-material, often minutely spherulitic, and containing small 

 pale-brown granules, white by reflected light, as well as grains 

 iron-ore. A good deal of ilmenite with leucoxene occurs in this 

 rock. Specific gravity = 2-86. 



