BY W. N. BENSON. 529 



Microscopical. — Fabric hypidiomorphic granular. The min- 

 erals composing the rock arranged in descending order of 

 abundance are : — quartz in fairly large grains with inclusions of 

 apatite. Plagioclase, in large and small crystals, fairly idio- 

 morpliic, and sometimes zoned, probably oligoclase. Orlhoclase 

 in rather large, slightly decomposed crystals, fairly idiomorphic; 

 twinning is infrequent. Hornblende in green pleochroic and 

 idiomorphic prisms, occasionally twinned; charade teristic six-sided 

 basal sections occur. Biotite in large brown pleochroic flakes, 

 frequently intergrown with the hornblende; in one instance a 

 biotite flake appears in the centre of a perfect basal section of 

 hornblende. Sphene in brown to red-yellow pleochroic grains, 

 often approximating to the idiomorphic lozenge-form. Magnetite 

 in cubes and irregular grains. Apatite and zircon. 



Besides these granite.s, there are other rocks more satisfactorily 

 referable to the granite-porphyries. Such a rock is that in a 

 vein crossing the railway at the 162-mile peg. The vein is 

 decomposed, and contains large, spheroidal boulders, segregations 

 of a pink granite of the following description : — 



Macroscopically coarse-grained, with quartz, pinkish felspar, 

 and hexagonal biotites, easily recognisable. Large ortlioclase 

 phenocrysts are present. 



Microscopically, the fabric is microporphyritic, showing the 

 above minerals, and also twinned and zoned oligoclase and micro- 

 cline. The biotite is very subordinate and is passing into 

 chlorite; apatite occurs. 



Another, and perhaps a more typical granite-porphyry, is that 

 which forms the boulders marking the outcrop of the largest 

 granite vein shown on the map (Plate xxii.); it is near the 164|- 

 raile peg, and is about 70 yards wide, and half a mile long; and 

 forks at the southern extremity. The main mass of the vein is 

 decomposed, the fresh boulders onl}^ being capable of sectioning. 

 The precise description of the rock is : — 



MacrosGopical. — Fine- to medium-grained brownish granite, 

 showing quartz, felspar and biotite. 



